Joseph Ferano
d43c372efe
`flan-inspect-edit' turns off read-only and line truncation, and `--show' is the one funnel every drawing goes through — but the mode body that set them in the first place does not run a second time on a buffer already in the mode. So a listing drawn after a commit or an abandon presented as a listing and was plain editable text: `C-k' on one of its lines did what `C-k' does. Put back beside the flag that says which of the two the buffer is. And that flag is now the marker it should have been. The header above the value is two lines or three depending on whether the daemon named a type, and the count that skipped it was written once in the writer and once in the reader that parses the buffer back — two places to keep in step for nothing. The MANUAL's key table lists the two keys that commit and abandon, which were in the prose and not in the table anybody scans.
1094 lines
52 KiB
Markdown
1094 lines
52 KiB
Markdown
# Flan in Emacs
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A manual for the editor side. It assumes you know Emacs and nothing about how
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Flan's dev loop is built — if you want that, `docs/BUILT.md` has it.
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The short version: you start a program, you keep it running, and you change it
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while it runs. Everything below is a variation on that.
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---
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## Setting up
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Put `emacs/` on your load path and require the mode. Nothing else is needed —
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`flan-mode` pulls in the rest as you use it.
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```elisp
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(add-to-list 'load-path "~/Development/flan/emacs")
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(require 'flan-mode)
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```
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`.flan` files open in `flan-mode` after that. The client, the REPL, the
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inspector and the conditions buffer all load on first use, so requiring the mode
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does not drag them in.
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One optional extra: `flan-dape.el` gives you lldb through
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[dape](https://github.com/svaante/dape). It is separate on purpose — `flan-mode`
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works without dape installed, and `C-c C-g` only exists once you load it.
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```elisp
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(require 'flan-dape) ; only if you have dape
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```
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Or install it as a package, which gets you autoloads and byte-compiled files
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without a `load-path` line. `package-install-file` names the package after the
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directory, so the directory has to be called `flan-mode` — symlink it once and
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point Emacs at the link:
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```sh
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ln -s ~/Development/flan/emacs ~/.emacs.d/flan-mode
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```
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```
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M-x package-install-file RET ~/.emacs.d/flan-mode/ RET
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```
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Every file carries `Version:` and `Package-Requires: ((emacs "29.1"))`, which is
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what package.el reads. dape is deliberately not in that list: `flan-dape.el`
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reaches it through `declare-function`, so everything else installs and works
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without it.
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You also need the `flan` binary on your `PATH`. If it is somewhere else, set
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`flan-command`.
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Two settings worth knowing about before you need them. `flan-start-timeout`
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(60s) bounds the wait for a daemon to come up, and `flan-reply-timeout`
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(30s) bounds one request once it has. The second is the one a long first
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compile can exhaust: the message says so and names `*flan-dev*`, where the
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daemon's own build log is, so you can see whether it is still working before
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you raise it.
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---
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## Starting a program
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Two ways in, and they are different.
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**`M-x flan`** starts the program for you. It runs `flan dev` on a file,
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waits for it to come up, and connects. This is the normal way.
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From a buffer already visiting a `.flan` file it starts *that* file and asks
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nothing — the buffer has answered the question. From anywhere else it reads the
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file from the minibuffer, and `C-u M-x flan` reads it even from a `.flan`
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buffer, which is how you start some other program without leaving the one you
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are looking at.
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**`C-c C-z`** (`flan-connect`) attaches to a program that is *already* running —
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one you started in a terminal, say. It looks for a `.flan-dev.sock` file in the
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current directory and upward, so from anywhere in the project it finds the one
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program you have running.
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Either way, when you are connected the modeline says so and Emacs tells you what
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it found:
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```
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flan dev: connected to ~/Development/flan/.flan-dev.sock (47 functions, 2 globals)
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```
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**`C-c C-q`** disconnects without stopping the program. **`M-x flan-quit`**
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stops the program too — but only one this Emacs started. A daemon you launched
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in a terminal is not Emacs' to kill, and it will say so rather than do something
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surprising.
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### Which backend the session uses, and what it costs
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`flan dev` compiles the session with the hand-written x86-64 backend. That is
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the default, and it is the reason `C-c C-c` is fast: about 28ms at the socket
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against about 63ms through LLVM, because `as` does in 8ms what `llc` does in
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45ms. On a project you are iterating in, that is the difference you feel.
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It is not the whole compiler. Three things a session built by it cannot do:
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- **No backtrace, no locals, no globals.** The inspector — `C-c C-b`, and
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everything the break loop shows you about *where* a stopped program is — walks
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a shadow stack of frames, and this backend does not push them. A program built
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by it still stops on an unhandled error, still takes a restart, still answers
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`C-x C-e` at the break. It just cannot tell you where it stopped.
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- **No `C-u C-c C-a`.** That view shows the LLVM IR a body was built from, and
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there is none; the listing this backend produced is assembly. Plain `C-c C-a`
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disassembles the object and works exactly as before.
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- **Some programs it refuses by name.** It covers a subset of the IR and says
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so rather than guessing — a `signal` or a `restart-case` inside a global's
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initialiser, a `declare` that returns a struct by value, a value crossing the
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C boundary in a shape it has no classifier for. A refusal names the form
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*and* names `--llvm`, in both places you can meet one: the daemon's own build
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log when it starts, and the error overlay when `C-c C-c` hits one
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mid-session. It is never a bare "unsupported".
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**`--debug` is not on this list**, because it picks LLVM for you. `flan dev
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--debug` — the breakpoint and `flan-dape` path — is an LLVM session, since a
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redefinition module compiled by the x86 backend carries no line table and a
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breakpoint set in one would stop firing at the first `C-c C-c`. Writing
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`--x86 --debug` together is still refused, and now says which one to drop.
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### Asking for LLVM
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Set `flan-daemon-args`. It is a list of strings, spliced into the daemon's
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command line after the file, and it is how anything that is not the file or the
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socket reaches `flan dev` from Emacs:
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```elisp
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(setq flan-daemon-args '("--llvm")) ; the inspector, the IR view, every form
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(setq flan-daemon-args '("--debug")) ; breakpoints, through flan-dape
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```
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The first line of `*flan-dev*` is the command that actually ran, so you can
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always check what a session was started with.
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There is no prefix argument for this and that is deliberate: `C-u M-x flan`
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already means "ask me which file", and a second level that meant "ask me which
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flags" would be two unrelated questions on one key. A setting is also the
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honest shape of the thing — which backend your sessions use is a property of
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the project you are working on, not of the keystroke that started this one.
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**If you were relying on the old default:** every `flan dev` before this one
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was an LLVM session, so a workflow built around `C-c C-b` or `C-u C-c C-a` will
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find them refused now. One line in your init puts it back.
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---
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## The loop
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This is the part the whole project exists for.
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### `C-c C-c` — change one function
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Put point anywhere in a top-level form and press it. The form is recompiled and
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installed into the running program, which does not stop, restart, or lose
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anything. The next time that function is called, the new one runs.
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It works on the *buffer text*, not the saved file, so you do not have to save
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first.
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If the form point is in is not a declaration — a bare `(+ 1 1)` at column 1,
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say — it is evaluated as an expression instead, and its value comes back the
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way `C-x C-e`'s does. `C-M-x` is the same command on the binding SLIME and
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CIDER use, and does the same thing: whatever is under it, the obvious thing.
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Point *inside* a `defn` body still means the `defn` — the form chosen is the
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one point is in, and only then is it asked whether it is a definition.
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### `C-x C-e` — evaluate an expression
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The expression before point is compiled, run **inside the running program**, and
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its value shown beside it, as `=> 2` after the form you evaluated. Not a copy of
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the program, not a simulation — the actual process, with its actual state.
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So in a game you can type `(len enemies)` and get the real number.
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The value goes away on your next keystroke, like every other bit of feedback
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about a command that just ran. Set `flan-inline-result` to `nil` and it goes to
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the echo area instead; that is also where it goes when there is nowhere in a
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buffer to draw it, as at the REPL.
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The two are different questions and the settings are not two spellings of one
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thing. `flan-echo-result` is about the sentence an *install* reports — which
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names landed and what it cost — and an install has no value to draw. Where they
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would overlap, on an expression, the inline value wins and the echo is not also
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written: the same number twice is how a reader learns to skip both.
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The form before point can also be a declaration — a `defvar` typed at the top of
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a file — in which case `C-x C-e` installs it rather than refusing it, and says
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which names changed instead of printing a value.
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### `C-u C-c C-c` — stop there
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The same key with a prefix argument **marks a form as a breakpoint**. `C-u C-c
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C-c` marks the innermost form point is inside — with the cursor in `(+ ticks 1)`
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the program stops at that `(+ ...)` — and `C-u C-u C-c C-c` marks the top-level
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form itself, which means stopping on entry to it.
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The buffer is not edited. The position goes to the daemon beside the code, and
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the `(pause)` call is put into the tree after parsing, so every location in the
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file stays exactly where it was and error markers, `M-.` and the debugger keep
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pointing at the right place. The marked form is underlined while the mark is on
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it.
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When the program reaches it, it stops in the ordinary break loop: `C-c C-b`
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shows the stack, and `continue` resumes at the call. Nothing special-cases this
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— `pause` is a prelude function that signals a `Pause` condition, so a
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breakpoint is just a condition nobody handled.
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**The mark sticks** until you evaluate that form plainly. Mark it, run the game,
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hit it as many times as you like; an ordinary `C-c C-c` over the same form (or
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`C-c C-k` over the buffer) takes it off.
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`C-u C-x C-e` does the same for the expression before point: it stops *at* the
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expression instead of printing its value. That one does not stick, because there
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is no definition for it to stick to. `C-u C-c C-c` on a top-level form that is
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an expression rather than a declaration means exactly this — there is no inside
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for a position to point at, so `C-u` and `C-u C-u` say the one thing there is to
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say, and it does not stick either.
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### `C-c C-m` — what a macro call expands to
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Put point on a macro call and press it. `*flan-macroexpansion*` opens with the
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code that call turns into, as Flan source, indented and coloured like any other.
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**One step by default, `C-u C-c C-m` for all the way.** The two genuinely
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differ: a macro may produce a call to another macro, so `(mac/quad 3)` is
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`(mac/twice (mac/twice 3))` after one step and `(+ (+ 3 3) (+ 3 3))` at the end.
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One step is the default because it is the one that can say *which* macro
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produced what — the full expansion is tagged with the name you wrote and
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nothing else.
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Inside the buffer, `m` expands the form at point one more step **in place**,
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`a` takes it to the end, `g` asks again — useful after you have re-evaluated
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the `defmacro` — and `q` closes it.
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This matters most for a macro you cannot read beside the call: a package's
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macros arrive qualified, so `rl/with-drawing` is defined in another directory
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and there is nothing next to the call site to look at.
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It expands against the macros **this session holds** — the prelude's, the ones
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its imports brought in, and every `defmacro` you have evaluated since it
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started — not against a fresh read of the file. So it agrees with what an
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evaluation does, and it sees the macro you have typed rather than the one you
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last saved.
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Two things the buffer says out loud. Nothing in it has a location of its own:
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every node carries the *call site's* file, line and column with the macro's
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name stamped on it, which is how an error inside expanded code points at the
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call you wrote. And it is not the text of any file, so `C-c C-c`, `C-c C-k` and
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`C-x C-e` in that buffer refuse rather than installing code nobody wrote.
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A macro that never settles is refused at a bound and named, with the refusal
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drawn on the call — it does not hang.
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### `C-c C-k` — the whole buffer
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The whole buffer, sent as **one** module rather than as a form at a time. That
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matters: a `defvar` and the function that uses it have to arrive together, or the
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function refers to storage that does not exist yet.
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Use this when you have changed several things at once, or when you have added a
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new global.
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### When it lands
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Changes install at a frame boundary — the program finishes what it is doing and
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picks up the new code at a clean point. You do not have to think about this
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except to know that a change is not necessarily live the same *millisecond* you
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press the key.
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---
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## The REPL
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**`C-c C-r`** opens `*flan-repl*`. It is a comint buffer; every line goes through
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the same machinery `C-x C-e` uses, so anything you can evaluate there you can
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evaluate here.
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One thing to know: it is **program-scoped**, not buffer-scoped. Names are the
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running program's names. In sand you write `sim/settle`, not `settle`, because
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that is what the program calls it.
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**`C-c C-o`** shows `*flan-output*` — whatever the program itself has printed.
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That is separate from the REPL, because the program's stdout belongs to the
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program.
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**History.** `<up>` and `<down>` walk it while you are on the line you are
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typing, filtered by whatever you have typed so far — `(sim` then `<up>` reaches
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only the forms that start that way. Anywhere above the prompt the same keys move
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by line, so the transcript is still something you can scroll back through.
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`M-p`, `M-n` and `M-r` are comint's own and unchanged.
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It survives quitting Emacs. The file is `flan-repl-history` under
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`user-emacs-directory`, so `no-littering` and a moved state directory both take
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it with them; set `flan-repl-history-file` to nil to keep history for the
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session only, or `flan-repl-history-size` to keep more or less than 500 entries.
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A form typed over several lines is saved whole and comes back whole, and RET on
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one recalled sends all of it.
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---
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## When the program stops
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If the program hits an error nobody handled, it does not die. It stops, on the
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frame where the error happened, and waits.
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The modeline says `stopped`. Everything else in Emacs behaves normally — a
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stopped program looks like a running one from anywhere else.
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### `C-c C-b` — the conditions buffer
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This is where you decide what to do. It shows three things, in this order:
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1. **the condition** — what went wrong
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2. **the restarts** — your choices
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3. **the stack** — the explanation
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That order is deliberate. The decision in front of you is which restart to take;
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the stack is why. A debugger that opens with forty frames has buried the decision
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under the explanation.
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Keys in that buffer:
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| Key | Does |
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|---|---|
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| `RET` | take the restart at point |
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| `0`–`9` | take that restart by number |
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| `TAB` / `n` | next restart |
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| `S-TAB` / `p` | previous |
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| `f` | fold a stack frame open or closed |
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| `i` | inspect the local or global at point |
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| `a` | abort |
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| `g` | read the program again |
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| `q` | close the buffer |
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**Why restarts are numbered.** A restart is taken by *position*, not by name.
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Two frames can offer a restart with the same name — `retry` is common — and a
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name resolves to the innermost one. So an outer `retry` is real, is on the list,
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and cannot be reached by name. The numbers are how you reach it. Restarts that
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genuinely cannot be taken are shown and refused with a reason rather than
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silently omitted.
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**`C-c C-M-b`** is the same choice as a quick one-key prompt, when you already
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know which restart you want and do not need the buffer.
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**The condition's fields are named and typed, and have no values.** Under the
|
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condition you get the struct it is — `:path string`, `:tried i32` — because the
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daemon compiled the program and knows what that type looks like without asking
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the program anything. What is beside each field is a note saying the value is
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not available, not a blank: a value lives in the stopped frame, and nothing yet
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||
hands the break loop's condition pointer back. Knowing the shape is still worth
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having — it tells you whether the field you were about to blame is a field of
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this condition at all.
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||
If that section says it could not resolve the name, read it: a package
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||
qualifies what it declares, so two packages' `Missing` are `a/Missing` and
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||
`b/Missing`. The daemon refuses a bare name and says what it could have meant
|
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rather than picking one.
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After you choose, the program carries on from the restart. It never unwound, so
|
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everything it had is still there.
|
||
|
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---
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||
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## Looking at values
|
||
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**`C-c C-i`** inspects a value. Give it an expression; you get its fields, one
|
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per line.
|
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| Key | Does |
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|---|---|
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| `RET` | go into the field at point |
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| `l` | back out one level |
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| `g` | read it again |
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| `TAB` / `n` | next field |
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| `S-TAB` / `p` | previous field |
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| `e` | set the field at point |
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| `C-c C-e` | open the value for editing |
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| `C-c C-c` | commit what you changed in it |
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| `C-c C-k` | abandon the edit |
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| `q` | close |
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Two things worth knowing, because they are unlike other inspectors.
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||
|
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**The view is never stale.** Every step reads the program as it is *now*. Most
|
||
inspectors show you the object as it was when you opened it.
|
||
|
||
**The root runs again on every step.** Going into a field sends a new request,
|
||
not a lookup in something remembered. Appending a field name to an expression
|
||
is harmless, but the expression itself need not be: if you inspect
|
||
`(spawn-enemy)`, you spawn one per keystroke. That is why there is no
|
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auto-refresh and why `g` is a key you press rather than a timer.
|
||
|
||
### Three ways to root a walk
|
||
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There are three, they are not equally capable, and the top line of the buffer
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says which one you are on.
|
||
|
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**An expression** — `C-c C-i`, and `i` on a global line in the break buffer.
|
||
It works on a **running** program and starts from anything you can write,
|
||
including a call. It cannot name a frame: an expression is evaluated where the
|
||
evaluator stands, so a local's name reaches that local only when its frame is
|
||
the innermost one. And it cannot reach an option's payload, because nothing in
|
||
the language names it.
|
||
|
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**A frame and a slot** — `i` on a local line in the break buffer. It is exact
|
||
to one frame and one slot, so the value you get is the one the listing above it
|
||
drew. It reaches an **option's payload** and a **union case's fields**, which
|
||
have offsets but no accessor form to write. In exchange it needs a **stopped**
|
||
program, and it is refused — by name, with the reason — once the program
|
||
resumes or if the frame's body was redefined since the frame was entered. It
|
||
cannot start from an expression at all.
|
||
|
||
**An address** — `M-x flan-inspect-address`. A number, `#x7f…` or decimal, of
|
||
the kind a debugger, a valgrind report or a C shim's `printf` hands you. There
|
||
is no frame in it and no expression: what it shows is the `(Ptr T)` at that
|
||
address, followed if the storage is still live and an epitaph naming what died
|
||
there if it is not.
|
||
|
||
**The type is optional, and leaving it out is the point.** A dev build records
|
||
the type at every allocation — the allocator's caller knew it, and a Flan value
|
||
carries no header, so that is the only moment anything could — and this command
|
||
is the one place that recorded name is read back and turned into a type again.
|
||
Naming a type overrides it, for reading half a struct or an element the table
|
||
recorded under a container's name; the reply still carries what the allocator
|
||
wrote down, so you are never shown one type while the program believes another.
|
||
|
||
It needs a **stopped** program, for a reason of its own: whether an address is
|
||
still live is exactly what a running program is changing. And it takes **no
|
||
path** — `RET` does not go into it — because the whole answer is the pointer
|
||
arm's branch, and stepping in would step from the pointee, which is the deref
|
||
the registry has only just been asked to bless.
|
||
|
||
An address the registry has never seen is refused by name rather than
|
||
rendered. That is a stack local, a global, or a pointer from C; the first two
|
||
are answered by name in the stack and globals sections already.
|
||
|
||
None of the three subsumes the others, which is why all three are here. The
|
||
one you get is chosen for you by the line you press `i` on, or by starting an
|
||
address root by hand.
|
||
|
||
**`l` never crosses between them**, and that is structural rather than a rule
|
||
someone has to remember. Every entry on the buffer's stack carries its own
|
||
root; `RET` only ever lengthens the path under the root already in hand; and
|
||
starting a new root starts an empty stack. So a stack with both kinds in it
|
||
cannot be built, and `l` has nothing to cross into.
|
||
|
||
### Changing one — `e`, and `C-c C-e`
|
||
|
||
The inspector writes as well as reads, and the loop it is for is the one the
|
||
whole dev story is about: "that field is wrong" and "is it *this* value that
|
||
fixes it" are two keystrokes apart, where editing the source and reloading
|
||
answers a different question — it answers what the **next** run does.
|
||
|
||
**`e`** sets the field or element at point. You are prompted with what is there
|
||
now, and what you type is a Flan **expression**, evaluated in the program and
|
||
checked against the type of the place it is going into. `(+ 1 2)`, a string
|
||
literal, a whole struct literal all work; one that does not fit comes back with
|
||
the checker's own sentence — *expected f32, found string* — and nothing is
|
||
stored. A literal arrives at the place's width, so `3` into an `f32` field is
|
||
three point oh and not a refusal you would have had to write `3.0` to avoid.
|
||
Point on no field line at all sets the whole value, which is how a data type's
|
||
case is changed.
|
||
|
||
**`C-c C-e`** turns the buffer into the value. What you get is the Flan literal
|
||
the program wrote — the value's own spelling, with no second notation to learn
|
||
— and you edit it as text. **`C-c C-c`** commits; **`C-c C-k`** throws it away.
|
||
|
||
A commit is a **diff**, not a blast. The buffer is read back as a value,
|
||
compared leaf by leaf with what was drawn, and one write is sent per leaf that
|
||
changed — so editing one field of a struct does not rewrite the others with
|
||
what happened to be on your screen. Every write in a commit goes in **one**
|
||
module: either all of them happened at this stop, or none did.
|
||
|
||
What it refuses, and each by name with the reason:
|
||
|
||
- **A shape that changed.** Adding or removing an element, adding or renaming a
|
||
field, putting a number where a struct was, changing which case a data type
|
||
holds. Those are changes to the container or to the tag, not stores into
|
||
storage. `e` on the value sets it whole, which is the operation that *is*
|
||
available.
|
||
- **A value the renderer did not write.** `<ptr>`, `<Name>`, and the `...` the
|
||
walk writes where it stopped at its depth or span bound. A buffer with one of
|
||
those in it is a buffer missing part of the value, and a commit read off it
|
||
could not tell a field that was never written from one you deleted — so the
|
||
value refuses to be opened for editing at all.
|
||
- **A pointer.** An address typed into a prompt is one the editor made up.
|
||
Nothing blessed it and the program would dereference it at a moment nobody
|
||
chose; the read half does not follow pointers either.
|
||
- **An option's payload on its own.** The tag is what says whether there is
|
||
one. Set the option.
|
||
|
||
### What makes writing safe
|
||
|
||
Writing is refused outright on the expression and address roots. Only the
|
||
**frame and slot** root can be written to, and the reasons are what the whole
|
||
feature rests on.
|
||
|
||
The program must be **stopped**, because a poke into storage the program is
|
||
mutating is corruption with a plausible shape. But "stopped" is not enough on
|
||
its own: a write is built by a compiler, and a third of a second of `llc` is
|
||
long enough for a game to resume, run a frame, and stop again. The same slot
|
||
index of a different stack is not the place you were looking at.
|
||
|
||
So a write names the **stop** it was addressed to, not the state. Every
|
||
inspection carries the number of the stop it was read at; a write hands that
|
||
number back; and it is checked twice — here, before anything is built, so a
|
||
stale buffer refuses in a tenth of the time and names the buffer rather than
|
||
the module, and again inside the program, on the game thread, at the moment the
|
||
module is claimed. A program that went round its loop between the drawing and
|
||
the commit refuses with *look again and re-do the edit*, which is the whole of
|
||
the fix.
|
||
|
||
The read-only view stays the default. `e` and `C-c C-e` are there when the root
|
||
can carry them, and the key legend at the foot of the buffer lists them only
|
||
then — a legend offering a key that would refuse is advertising a refusal.
|
||
|
||
A **global** is not writable from here yet: the break buffer roots globals at an
|
||
expression, and an expression is evaluated wherever the evaluator stands and
|
||
whenever it next reaches a frame boundary, which for a write means possibly
|
||
into a running program. `C-x C-e` on `(set the-global …)` does that write
|
||
knowingly; the inspector will not do it behind a key that looks the same as the
|
||
safe one.
|
||
|
||
### Where the memory went — `M-x flan-allocations` and `M-x flan-leaks`
|
||
|
||
The same registry, read as a table rather than at one address. **`M-x
|
||
flan-allocations`** is every block it recorded, live and dead both, grouped by
|
||
the type the allocator's caller named and ordered biggest first by bytes. The
|
||
dead are in it on purpose: in a long-running program they are the bulk of it,
|
||
and they are what says where the allocation *went* rather than only where it
|
||
stayed.
|
||
|
||
**`M-x flan-leaks`** is the same walk with the dead left out — what the program
|
||
is still holding.
|
||
|
||
**"Still holding" means at the moment you ask, and there is no exit report to
|
||
wait for.** A program killed by a signal, which is how a program under this
|
||
editor usually ends, runs no exit handler at all, so nothing written inside it
|
||
could report anything. Asking is the answer, and you can ask at any time
|
||
including just before you quit. A program that returns from `main` on its own
|
||
can print the same breakdown to stderr by being run with `FLAN_DEV_LEAKS` set
|
||
— off by default, because a dev build's output belongs to the program.
|
||
|
||
Both are dev-build only. A release build records nothing and says so.
|
||
|
||
### The watch buffer — values while the program runs
|
||
|
||
Everything above is for a program you have stopped, or one you interrupt with a
|
||
keystroke. **`M-x flan-watch`** is the other thing: a small buffer that shows
|
||
values *while the game runs*, updating at frame rate. `M-x flan-watch-stop`
|
||
closes it down, and killing the buffer does the same.
|
||
|
||
**The program decides what is shown.** There is no watch list to maintain, no
|
||
per-variable registration, no place to type an expression. The program says
|
||
what it wants seen, from inside its own loop:
|
||
|
||
```flan
|
||
(declare-c watch-i64 [name string x i64] i32 "flan_dev_watch_i64")
|
||
(declare-c watch-f64 [name string x f64] i32 "flan_dev_watch_f64")
|
||
(declare-c watch-str [name string s string] i32 "flan_dev_watch_str")
|
||
|
||
(defn step [] i64
|
||
(set ticks (+ ticks 1))
|
||
(watch-i64 "ticks" ticks)
|
||
ticks)
|
||
```
|
||
|
||
That is the whole of it. `C-c C-c` on `step` adds or removes a watched value
|
||
the same way it changes anything else, so the watch list is edited in the place
|
||
you were already looking.
|
||
|
||
**Why it is pushed rather than polled.** Every other listing in this manual is
|
||
a question the editor asks, which the daemon answers by compiling a small
|
||
module and handing it to the program. That is fine at the rate you press a key
|
||
and ruinous at the rate a HUD refreshes — each one is a new `.so`. So the
|
||
direction is inverted: the program writes into a table, and the editor reads
|
||
the table, which is memory. Two things follow that a poll could not have given.
|
||
The values are as fresh as the last frame, whatever the repaint interval is set
|
||
to. And **they are still there while the program is stopped** — a break loop is
|
||
exactly when nothing can be run at a frame boundary, and exactly when you want
|
||
to see what the last frame held.
|
||
|
||
**What it costs when you are not watching.** Nothing writes the table until a
|
||
watch buffer is open; `M-x flan-watch` tells the program somebody is looking
|
||
and closing it tells the program to stop. So a `watch-i64` call in a program
|
||
nobody is debugging is a load and a branch that is not taken, in a release
|
||
build as in a dev one.
|
||
|
||
**The limits.** The table holds **64 names**, and names past that are dropped
|
||
rather than being fatal — the buffer says how many, because a value that simply
|
||
never appeared would send you looking for a bug in the program. A name is
|
||
truncated at 31 bytes and a rendered value at 192, with an ellipsis where a
|
||
value was clipped.
|
||
|
||
**A number sampled thousands of times a frame — `watch-num-i64`,
|
||
`watch-num-f64`.** The scalar entry points keep one value per name, and from a
|
||
hot inner loop that is nearly useless: you see whichever of the 91,200 cells ran
|
||
last. These two are the other half. A slot keeps **count, min, max, last and
|
||
mean**, and the buffer renders them as one line:
|
||
|
||
```flan
|
||
(declare-c watch-num-i64 [name string x i64] i32 "flan_dev_watch_num_i64")
|
||
(declare-c watch-num-f64 [name string x f64] i32 "flan_dev_watch_num_f64")
|
||
|
||
(watch-num-i64 "cell" (at grid i))
|
||
```
|
||
|
||
The slot renders as `n=… min=… max=… last=… mean=…`, one line per name.
|
||
|
||
Two entry points rather than one so a program need not cast at the call site.
|
||
The write path does **no formatting** — a sample is a load, five compares and
|
||
the slot's seqlock — and the listener thread renders once per editor tick, which
|
||
is the whole reason this exists rather than a second `watch-i64`. **The window is
|
||
since the editor's last tick**, not since the program started: a min and a max
|
||
over a whole session reach the session's extremes within seconds and then never
|
||
move again, so the two most useful of the five would go dead exactly when you
|
||
start playing. A whole number prints as one, because a spy on an array index
|
||
reading `66.0000` sends you looking for a rounding bug that is not there.
|
||
|
||
**Scalars only, so far.** `i64`, `u64`, `f64` and `string` have entry points; a
|
||
struct or a slice does not. That is not an oversight in the runtime — a Flan
|
||
value carries no header, so rendering one is a walk over its *type* at compile
|
||
time, and a `(watch "hp" hp)` form in the compiler is what would do that walk.
|
||
It is not built. `flan-watch.el` says what it would need.
|
||
|
||
### Ghost text — the same values, inline
|
||
|
||
**`M-x flan-watch-ghost-mode`** shows each watched value *at the call that wrote
|
||
it*, as faint text after the line, instead of in a buffer of its own:
|
||
|
||
```flan
|
||
(defn step [] i64
|
||
(set ticks (+ ticks 1))
|
||
(watch-i64 "ticks" ticks) => 4812
|
||
ticks)
|
||
```
|
||
|
||
It is an **addition, not a replacement**, and the two can be on together. The
|
||
buffer is what you want when you want everything at once; inline is what you
|
||
want when you are reading one function and the value belongs beside the code
|
||
rather than three windows away. `M-x flan-watch-ghost-mode` again turns it off,
|
||
and turning either one off leaves the other running.
|
||
|
||
**How it finds the place.** The table carries a name and a rendered string and
|
||
no source location — that would need the `(watch ...)` form above. It does not
|
||
have to: `(watch-i64 "ticks" ticks)` is *in the buffer you are looking at*, and
|
||
the name in the table is the string literal in it, so the call site is found by
|
||
searching the text. Nothing new is asked of the running program. If you bound
|
||
your own entry points to names that do not start with `watch`, set
|
||
`flan-watch-ghost-call-regexp` — the Flan name is yours, and only the C symbol
|
||
behind it is fixed.
|
||
|
||
**When it updates.** On the same timer as the buffer, from the same reply, so
|
||
the two cannot disagree. As with the buffer, the interval decides how often the
|
||
picture is repainted and not how fresh it is. Overlays are replaced wholesale
|
||
each repaint rather than followed through your edits, so a line you moved never
|
||
leaves one stranded, and a file you scroll to gets its values within a tick.
|
||
Only buffers **shown in a window** are scanned, which is what keeps that cheap.
|
||
|
||
**One name, two call sites.** Both show it, and both say `one slot, 2 sites`.
|
||
That is the truth rather than a failure to choose: the table has one slot per
|
||
name and the last call in the frame wins, so the value really is the same at
|
||
both, and it is whichever ran last. In a **loop**, you see the last value
|
||
written — the same answer the buffer gives. "The last of 4000 iterations" is
|
||
often not the one you wanted, but every better answer is a UI for building a
|
||
query, which is the thing this design exists not to have.
|
||
|
||
**While the program is stopped**, the values are the last frame's — that is the
|
||
point of pushing rather than polling — and each one says `last frame` and changes
|
||
colour. The watch buffer does not need to, because you opened it on purpose and
|
||
`flan:stopped(...)` is already in the modeline; inline, the value sits in the
|
||
middle of code that looks perfectly live.
|
||
|
||
**What gets no ghost text.** A watch call produced by a macro, or one whose name
|
||
is not a literal: there is nothing in the source to find, and the editor is
|
||
reading your file rather than debug info. Those still appear in the watch
|
||
buffer. A site you have typed but not yet installed with `C-c C-c` has no value
|
||
yet either — that is the ordinary stale-caller situation the modeline already
|
||
tracks. And when the table is **full**, a site that found no slot says so rather
|
||
than showing nothing, which is the one case where silence would send you looking
|
||
for a bug in your program.
|
||
|
||
---
|
||
|
||
## When a change is refused
|
||
|
||
Two different things wear the same refusal today, and only one of them is the
|
||
design.
|
||
|
||
### A changed signature — a placeholder, not a rule
|
||
|
||
The intended behaviour, and what plan.org specifies, is that changing a
|
||
function's signature makes a **new version** of it: new callers resolve the new
|
||
one, existing callers and any stored `Fn` value stay safely on the old one, and
|
||
the session **warns** at each tracked stale caller site so you know what to
|
||
re-evaluate. Nothing should have to restart.
|
||
|
||
That needs function versions, trampolines and caller tracking, none of which are
|
||
built yet. Until they are, the session refuses rather than letting an
|
||
indirection cell hand old arguments to a new body — a wrong answer would be
|
||
worse than a refusal. `lib/session.ml` says so at the refusal itself, and
|
||
plan.org tracks it as open decision #6.
|
||
|
||
So if you hit this: it is a limitation with a date on it, not how the language
|
||
is meant to work.
|
||
|
||
### A changed struct layout — the genuinely hard one
|
||
|
||
Rejected while live values of that struct exist, and this one plan.org does
|
||
still specify as a rejection. Storage already allocated has the old shape; a new
|
||
body would read its fields at the wrong offsets and nothing at run time would
|
||
say so. Managed classes are the planned way through — an explicit migration at a
|
||
frame boundary — and they are not built either.
|
||
|
||
### The way out, for now
|
||
|
||
`C-c C-x` stops the program, rebuilds from source, starts it again and
|
||
reconnects. It costs the program's state, which is why it is a key you press
|
||
rather than something `C-c C-c` quietly falls back to.
|
||
|
||
## Under the debugger
|
||
|
||
**`C-c C-g`** (`flan-debug`, needs `flan-dape.el`) builds the current file with
|
||
DWARF and stops it at `main` under lldb.
|
||
|
||
Breakpoints are ordinary dape breakpoints set in the `.flan` buffer — the line
|
||
table names your Flan file, not the generated LLVM IR. `dape-breakpoint-toggle`
|
||
on a line, or `dape-breakpoint-global` to break on a function without hunting for
|
||
its first line.
|
||
|
||
lldb needs no plugin to read Flan values. A Flan struct *is* a C struct, a local
|
||
is an ordinary stack slot, and there are no tag words or object headers anywhere,
|
||
so lldb's own C support prints them correctly with nothing taught to it.
|
||
|
||
Local variables show under their real names. One caveat: if you shadow a name —
|
||
a `let` inside a `let`, both called `v` — both appear, the inner one as `v~2`,
|
||
but plain `v` still answers with the *outer* one. Read `v~2` when you are inside
|
||
the inner binding.
|
||
|
||
---
|
||
|
||
## Writing it
|
||
|
||
`TAB` indents the line, and `C-M-q` the form under point. The rules are ported
|
||
from `clojure-mode`'s, because Clojure has the shapes Flan has and Emacs Lisp
|
||
does not — vectors that bind, bracket variety, and keys inside braces.
|
||
|
||
**A binding vector lines up name under name.** The second and later bindings of a
|
||
`let` sit under the first one's *name*, not under its value:
|
||
|
||
```
|
||
(let [vel (+ gravity (at velocity row col))
|
||
y (min (- rows 1) (+ row (i32 vel)))]
|
||
…)
|
||
```
|
||
|
||
The same rule draws `defn` parameter lists, `restart-case` and `handler-bind`
|
||
clause parameters, and both spellings of a struct literal — they are all a vector
|
||
or a brace read in pairs, so they are all indented as one.
|
||
|
||
**A body indents two.** `let`, `if`, `when`, `while`, `match`, `restart-case`,
|
||
`handler-bind`, `defn` and the `def…` forms all put their body two columns in
|
||
from the head. What differs between them is only how many forms come *before*
|
||
the body and stay on the head's line — a `let`'s binding vector, an `if`'s test,
|
||
a `restart-case`'s protected form — and the indenter knows that count per form.
|
||
|
||
A form it has no entry for is treated as a call: the arguments line up under the
|
||
first argument, not two in. That is the fallback, and it is what you want for
|
||
`(rl/draw-rectangle x y w h)` and for every function you write.
|
||
|
||
`defn` carries a return type between the parameter vector and the body, and it is
|
||
optional. The indenter does not need to know which — everything after the head
|
||
indents two, which is the right answer for the name, the parameters, a return type
|
||
if one is written, and the body alike.
|
||
|
||
**A field is drawn as a constant**, in the accessor `(.x v)` and as a label in
|
||
`{.x 1.0}`. The colon is still a constant too; it means an enum member, `:green`,
|
||
and a key in a map.
|
||
|
||
Nothing here needs a running program. Indentation and colouring are the major
|
||
mode's, so they work in a file you have only opened.
|
||
|
||
---
|
||
|
||
## Getting around
|
||
|
||
| Key | Does |
|
||
|---|---|
|
||
| `M-.` | jump to where a name is written |
|
||
| `M-,` | jump back |
|
||
| `C-c C-d` | what the running program currently defines |
|
||
| `C-c C-v` | help on the name at point |
|
||
| `C-c C-a` | disassemble a function; `C-u` first for its LLVM IR |
|
||
| `C-c C-l` | every lowering of a function: IR, `-O0`, `-O2`, x86 backend |
|
||
| `C-c C-m` | what the macro call at point expands to; `C-u` for all the way |
|
||
|
||
Completion, eldoc and `M-.` all read one cached answer rather than asking the
|
||
program per keystroke. It refreshes at the two moments the answer can have
|
||
changed: when you connect, and after an evaluation the daemon accepted.
|
||
|
||
The answer covers the compiler's builtins as well as the program's own names,
|
||
so `C-c C-v` on `arena-new` or `map-next` gives you its signature and a line
|
||
about what it does. They are marked `builtin`, and they come after the
|
||
program's names in a completion list. `M-.` on one refuses rather than jumping:
|
||
it is written in the compiler, so there is no file to open.
|
||
|
||
---
|
||
|
||
## Every lowering of one function — `C-c C-l`
|
||
|
||
`C-c C-l` on a name opens `*flan-lowering*`: the LLVM IR the frontend emits for
|
||
that function, what `llc` makes of it at `-O0` and at `-O2`, and what the
|
||
hand-written x86 backend emits, all narrowed to the one function. It is
|
||
`spike/x86/dump.sh` with a buffer around it. Reading one against another is the
|
||
only way to check a lowering by eye, and the reason the second backend is
|
||
trustworthy is that the two agree.
|
||
|
||
The four are folding sections, so the buffer is a four-line summary when
|
||
everything is shut:
|
||
|
||
```
|
||
; every lowering of step
|
||
; file ~/src/game.flan
|
||
; compiler /home/joe/src/flan/_build/default/bin/main.exe
|
||
; flags none
|
||
; showing what this file compiles to, not what a running program is
|
||
; calling for this name -- for that, C-c C-a
|
||
|
||
▸ LLVM IR 8 lines
|
||
▸ LLVM -O0 13 lines
|
||
▸ LLVM -O2 10 lines
|
||
▾ x86 backend 19 lines
|
||
0000000000012d94 <flan.step>:
|
||
12d94: push %rbp
|
||
...
|
||
```
|
||
|
||
Which sections are open is remembered for the rest of the Emacs session, and it
|
||
is remembered **by section, not by function**: if you are working on the backend
|
||
this week, `C-c C-l` on a different name leaves the backend open and the other
|
||
three shut. It is not written to disk — four booleans are not worth a
|
||
preference file.
|
||
|
||
| Key | Does |
|
||
|---|---|
|
||
| `TAB` | open or close the section point is in |
|
||
| `S-TAB` | close everything, or open everything if it is all shut |
|
||
| `c` | close every section |
|
||
| `e` | open every section |
|
||
| `n` / `p` | to the next or previous section heading |
|
||
| `g` | compile the file again and redraw all four |
|
||
| `r` | compile and redraw only the section point is in |
|
||
| `q` | bury the buffer |
|
||
|
||
`r` is worth the separate key because the four are not the same price: the IR
|
||
is the frontend alone, and `llc -O2` over a whole program's worth of it is the
|
||
one that is felt. Refreshing one section reuses the IR the other three were
|
||
made from. Refreshing the IR drops everything, since everything is downstream
|
||
of it.
|
||
|
||
This is **not** `C-c C-a`, and no prefix argument turns one into the other.
|
||
`C-c C-a` asks the *running program* what the body it is calling for a name
|
||
actually came out as; only the daemon can answer that, because the daemon
|
||
compiled that module and still has both its `.ll` and its `.so`. `C-c C-l` asks
|
||
what the *file on disk* compiles to, by compiling it, and needs no program
|
||
running at all. The two cannot be merged: neither `llc -O2` nor the x86 backend
|
||
has ever been run over an installed body, so there is no four-way answer in the
|
||
daemon to give. The `showing` line in each buffer's header says which of the two
|
||
you are reading.
|
||
|
||
`C-u C-c C-l` asks for the file as well as the name, for the case where the
|
||
function you want to read is not in the buffer you are in.
|
||
|
||
The four outputs need `llc`, `as` and `objdump` on `PATH` — the backend writes
|
||
machine code rather than mnemonics, so its section is assembled and
|
||
disassembled to be readable, which is also a check that the bytes are well
|
||
formed. A section whose tool is missing says so in place of its listing; the
|
||
other three are unaffected.
|
||
|
||
---
|
||
|
||
## When something is wrong
|
||
|
||
**An error draws an overlay** where it happened, with the message. It clears the
|
||
next time that buffer's evaluation is accepted — so it disappears when you fix
|
||
the thing rather than when you dismiss it.
|
||
|
||
**"No .flan-dev.sock found above this buffer"** — nothing is running, or you are
|
||
outside the project. Start one with `M-x flan`.
|
||
|
||
**The modeline says nothing about a program** — you are not connected. `C-c C-z`.
|
||
|
||
**A restart you picked did nothing** — it should not happen silently any more,
|
||
but if a restart is genuinely unreachable the buffer marks it and refuses with a
|
||
reason. Read the reason.
|
||
|
||
**The stack pane says it cannot show frames** — that is a real limit, not a bug.
|
||
The conditions buffer reaches the program over a socket, and a socket cannot read
|
||
another process's stack. The program stopped itself; it is not being debugged.
|
||
Use `C-c C-g` if you need frames.
|
||
|
||
---
|
||
|
||
## Full key reference
|
||
|
||
| Key | Does |
|
||
|---|---|
|
||
| `C-c C-c` | the top-level form at point: a declaration installed, anything else evaluated |
|
||
| `C-u C-c C-c` | ...and stop at the form point is inside (`C-u C-u`: on entry) |
|
||
| `C-M-x` | the same as `C-c C-c`, on the binding SLIME and CIDER use |
|
||
| `C-c C-k` | the whole buffer, as one module |
|
||
| `C-x C-e` | the form before point, evaluated — or installed, if it is a declaration |
|
||
| `C-u C-x C-e` | ...and stop at it instead of showing its value |
|
||
| `C-c C-z` | connect (finds `.flan-dev.sock` upward) |
|
||
| `C-c C-q` | disconnect |
|
||
| `C-c C-o` | the running program's own output |
|
||
| `C-c C-r` | a prompt on the running program |
|
||
| `C-c C-b` | a stopped program: condition, restarts, stack |
|
||
| `C-c C-M-b` | the same restarts, as a one-key prompt |
|
||
| `C-c C-i` | inspect a value |
|
||
| `C-c C-m` | what the macro call at point expands to |
|
||
| `C-u C-c C-m` | ...all the way, rather than one step |
|
||
| `C-c C-a` | disassemble; `C-u` first for LLVM IR |
|
||
| `C-c C-l` | every lowering of a function, in folding sections |
|
||
| `C-c C-g` | debug under lldb, through dape |
|
||
| `C-c C-d` | what the running program defines |
|
||
| `C-c C-v` | help on the name at point |
|
||
| `C-c C-x` | rebuild, relaunch, reconnect |
|
||
| `M-.` / `M-,` | where a name is written / back |
|
||
|
||
Commands with no key: `M-x flan` (start a program), `M-x flan-quit`
|
||
(stop it), `M-x flan-watch` (the watch buffer), `M-x flan-watch-stop`,
|
||
`M-x flan-watch-ghost-mode` (the same values inline),
|
||
`M-x flan-inspect-address` (what is at an address), `M-x flan-macroexpand-all`
|
||
(the `C-u` half of `C-c C-m`, by name), and `M-x flan-allocations` /
|
||
`M-x flan-leaks` (where the memory went, and what is still held).
|
||
|
||
---
|
||
|
||
## Settings
|
||
|
||
| Variable | Default | What it is |
|
||
|---|---|---|
|
||
| `flan-command` | `"flan"` | the compiler binary |
|
||
| `flan-daemon-args` | `nil` | extra arguments for `flan dev` — `("--llvm")`, `("--debug")` |
|
||
| `flan-socket-name` | `".flan-dev.sock"` | what `C-c C-z` searches for |
|
||
| `flan-echo-result` | `t` | report an accepted evaluation in the echo area |
|
||
| `flan-inline-result` | `t` | show an expression's value beside the form, not in the echo area |
|
||
| `flan-names-shown` | `4` | how many names to list before summarising |
|
||
| `flan-output-buffer` | `"*flan-output*"` | where the program's output goes |
|
||
| `flan-poll-interval` | `1.0` | seconds between checks for whether it stopped |
|
||
| `flan-daemon-buffer` | `"*flan-dev*"` | the daemon's own log |
|
||
| `flan-start-timeout` | `60` | seconds to wait for a program to come up |
|
||
| `flan-lower-buffer` | `"*flan-lowering*"` | where `C-c C-l` writes |
|
||
| `flan-lower-program` | `"flan"` | the compiler `C-c C-l` shells out to |
|
||
| `flan-lower-flags` | `nil` | flags for `flan emit` — `("--dev")` for the dev lowerings |
|
||
| `flan-lower-llc` | `"llc"` | what the `-O0` and `-O2` sections are made with |
|
||
| `flan-watch-buffer` | `"*flan-watch*"` | where watched values are painted |
|
||
| `flan-watch-interval` | `0.2` | seconds between repaints — not the watch rate |
|
||
| `flan-watch-ghost-call-regexp` | `"watch\(?:-[[:alnum:]]+\)?"` | the head of a call ghost text anchors on |
|
||
|
||
Every one of these used to be spelled `flan-dev-…`, and so did the commands:
|
||
`M-x flan-dev` is now `M-x flan`, `flan-dev-quit` is `flan-quit`, and so on
|
||
through the file. Nothing answers to the old names — there are no aliases —
|
||
so a `setq` or a keybinding in your config that names one will break, and the
|
||
fix is to delete `-dev` from it.
|
||
|
||
---
|
||
|
||
## The files
|
||
|
||
| File | What it is |
|
||
|---|---|
|
||
| `flan-mode.el` | the major mode: syntax, indentation, imenu, the keymap |
|
||
| `flan.el` | the client — the socket, evaluation, xref, eldoc, completion |
|
||
| `flan-repl.el` | the `*flan-repl*` buffer |
|
||
| `flan-watch.el` | watched values: the program pushes, this paints them in a buffer and inline |
|
||
| `flan-cnr.el` | the conditions-and-restarts buffer |
|
||
| `flan-inspect.el` | the value inspector |
|
||
| `flan-lower.el` | the lowering buffer: four outputs, folding, and which was open |
|
||
| `flan-dape.el` | lldb through dape; optional |
|
||
|
||
There is no Flan parser in any of them. The client sends text and the compiler
|
||
answers; anything that needs to know what a form means asks.
|
||
|
||
## The stack, and what a frame was holding
|
||
|
||
`C-c C-b` opens the conditions-and-restarts buffer, and its Stack section is no
|
||
longer empty. It lists the stopped program's frames, innermost first, each with
|
||
where it is and how many named slots it has.
|
||
|
||
`TAB` on a frame opens it (or `f`, which folds from anywhere on the frame) and shows what its locals hold — name, type and
|
||
value, rendered the same way the inspector renders anything else. They are
|
||
fetched the first time a frame is opened and then kept, because a stopped
|
||
program's locals cannot change underneath you and a round trip behind a key
|
||
that looks like folding would be a surprise.
|
||
|
||
Two kinds of frame are marked. A `program` frame was on the stack when the
|
||
error happened. An `eval` frame belongs to an expression you evaluated *inside*
|
||
the break loop, sitting on top of them. Those are shown rather than hidden, on
|
||
the same principle the unreachable restarts are: a frame you did not write is
|
||
better explained than silently removed.
|
||
|
||
Not everything can be shown, and what cannot is refused by name under the frame
|
||
rather than left blank:
|
||
|
||
- a slot the compiler invented, which has no name in your source — showing it
|
||
as `s4` would put a variable in front of you that is not in the file;
|
||
- a slot whose binding had not run yet when the error happened, which has no
|
||
address to read;
|
||
- a `Vec` or a pointer, which render as `<vec>` and `<ptr>` here exactly as
|
||
they do everywhere else.
|
||
|
||
**A redefined body is refused, not guessed at.** Installing a fix while the
|
||
program is stopped is deliberately allowed — it is the fix-it-and-retry loop —
|
||
so the frame on the stack and the body the daemon holds can be two bodies of
|
||
one function. A redefinition that renames a local changes neither the slot
|
||
count nor the types, and showing the new names against the old storage would be
|
||
a confident wrong answer in the one place someone is working out what went
|
||
wrong. Each frame therefore carries a fingerprint of its body's slots, and a
|
||
frame whose fingerprint no longer matches is refused by name with that reason.
|
||
Redefining one function says nothing about the others, so every untouched frame
|
||
still answers.
|
||
|
||
## The globals the stack is working on
|
||
|
||
Under the stack there is one more section: the globals this stopped stack
|
||
reaches. In this language that is often the more useful half — a game keeps
|
||
most of its state in top-level `defvar`s, and `sand.flan` holds its entire grid
|
||
that way.
|
||
|
||
It is **one section, not a fold under each frame**. A global is not part of a
|
||
frame; it is program state the frame happened to touch. Nesting it under one
|
||
would imply an ownership that is not there and would repeat the name once per
|
||
frame that reads it.
|
||
|
||
It is **scoped to the stack, not to the program**. The contents are the union
|
||
of the globals every frame on the stack references — the compiler already knows
|
||
each function's reference set, so it does the choosing. Listing all of a
|
||
program's globals instead would bury the one you want under the prelude's PRNG
|
||
state.
|
||
|
||
Each entry says **which frames touch it**, by the same number the Stack section
|
||
labels them with. That recovers what per-frame nesting would have told you —
|
||
"the whole chain is reading this" reads differently from "only the innermost
|
||
does" — at no cost in duplication. And the order is **by the innermost frame
|
||
that touches it**, because a deep stack makes the union large and proximity to
|
||
the error is what puts the likely culprit on top.
|
||
|
||
`i` works on a global line, and it roots differently there than on a local —
|
||
which is the fix rather than an inconsistency. A global really is reached by
|
||
name: the thunk the daemon loads binds to the program's own storage through the
|
||
dynamic linker, so the name means the same thing wherever it is evaluated. A
|
||
local is storage in one frame, and its name evaluated anywhere else may find a
|
||
global, another binding of the same name, or nothing. So a local goes in by
|
||
frame and slot index — the same two facts the listing was drawn from — and a
|
||
global by name. See "Two ways to root a walk" above for what each can reach.
|
||
|
||
Two things are said rather than left out. A global whose type the structural
|
||
printer has no arm for is refused by name with the reason. And a frame the
|
||
daemon could not attribute — one belonging to an expression you evaluated in
|
||
the break loop, a lifted handler clause, or a body redefined since it was
|
||
entered — is listed under "the union is incomplete", because a short list and a
|
||
complete list look identical if nothing says which it is.
|
||
|
||
**One hole worth knowing.** The redefinition check is a fingerprint over a
|
||
body's *slots*. A new body that names different globals while binding exactly
|
||
the same locals is not caught, and that frame's contribution will be the new
|
||
body's reference set. The values shown are still read from the program's own
|
||
storage and are still correct; what can be wrong is which frames an entry is
|
||
annotated with.
|
||
|
||
## Stopping on purpose
|
||
|
||
`(pause)` stops the program where it stands and hands it to the break loop.
|
||
`C-c C-b` then shows the stack, `TAB` opens a frame's locals, and taking
|
||
`continue` resumes at the call as though nothing happened.
|
||
|
||
It is spelled `pause` rather than `break` because `break` is reserved for
|
||
leaving a loop — the same word meaning "exit this loop" and "stop for
|
||
inspection" in the same position would be the worst available collision.
|
||
|
||
Nothing in the compiler implements it. It is an `error` under a `restart-case`,
|
||
written in the prelude, which is what a breakpoint *is* in a language that
|
||
already has conditions. One consequence worth knowing: a `handler-bind` above
|
||
it can intercept a `Pause` and decline to stop, so a release build can neuter
|
||
every breakpoint in the program without editing any of them.
|
||
|
||
**Untested.** It compiles and the shape is right, but nobody has run it into a
|
||
real break loop yet.
|