Joseph Ferano
7c1fcbff19
§4 meets §3, and the answer a reader will assume is the other one. An inner (use-value [s string] ...) shadows an outer (use-value [v i32] ...), so an i32 is refused there and the outer clause that would have taken it is never consulted. Searching outward for a frame whose signature fits would make which restart runs depend on the arguments, which is overload resolution on a dynamic stack. Also: neither of the new guards is a bounds check, so --no-bounds-checks does not remove them. A wrong index is a wrong answer; a transfer into a clause whose parameters were written to a different layout is not.
108 lines
4.5 KiB
Org Mode
108 lines
4.5 KiB
Org Mode
#+TITLE: Conditions and restarts — cheatsheet
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#+STARTUP: showeverything
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Why it is shaped this way: [[file:spec-conditions.md][spec-conditions.md]]. Something to poke at:
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=conditions-play.flan= — =flan dev conditions-play.flan=, then ~C-c C-c~.
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* Works
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#+begin_src lisp
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(signal c) ; Unit. Handler returns -> carry on. No handler -> no-op.
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(error c) ; Never. Only a transfer gets past; else the program stops.
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(handler-bind [(Type [c] body ...) ...] body ...) ; match by type, no hierarchy
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(restart-case BODY ; BODY and every clause have the same type = the form's
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(name [p T ...] CLAUSE) ...)
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(invoke-restart 'name arg ...) ; Never. Innermost frame offering the name wins.
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#+end_src
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#+begin_src lisp
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(defn supplied [n i32] i32
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(restart-case (middle n)
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(use-value [v i32] (* v 2)) ; the answer comes from outside
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(retry [] 7)))
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(handler-bind [(AssetMissing [c] (invoke-restart 'use-value 21))]
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(supplied 7)) ; 42
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#+end_src
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A clause's parameters are slots of the function that wrote it, and the invoker
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fills a buffer that function owns — by the time a clause runs, the invoking
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frame has gone. What a clause takes is compared with what was given at *run
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time*, count then spelling, because a restart is found by name on a dynamic
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stack and neither end can see the other.
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#+begin_src lisp
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(defn fetch [n i32] i32
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(restart-case (middle n) ; its value if nothing transfers
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(use-placeholder [] -1)
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(retry [] 7)))
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(handler-bind [(AssetMissing [c] (invoke-restart 'use-placeholder))]
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(fetch 2)) ; -1
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#+end_src
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~defer~ between the invoke and the target runs, innermost first, before the
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clause body. ~errdefer~ does not.
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* The break loop
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An unhandled ~error~ in a dev build stops on the frame that erred, with nothing
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unwound, and waits. ~C-c C-b~ in Emacs lists what is on offer and resumes into
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the choice; ~flan:stopped(Missing)~ in the modeline says it happened.
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The list is *numbered*, and the number is what is chosen. Two frames offering
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~retry~ both appear and §4's by-name walk can only ever reach the first, so a
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name cannot say which one is meant — ~restart-at~ can.
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A restart below the evaluation a break is inside is listed, marked, and
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refused: ~C-x C-e~ runs its thunk through a C frame that holds its own transfer
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channel, so an unwind aimed past it would stop at the thunk. Choose one offered
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above it, or ~abort~.
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* Not yet
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~handler-case~ · ~find-restart~ · ~compute-restarts~ · a clause's report
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string. Each refused by name with its reason.
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A restart *with parameters* cannot be taken from the break loop: it aims at a
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frame by position and has nothing to fill the parameters in with, so the clause
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stops the program rather than running on values no one supplied. Choose one
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that takes none, or ~abort~.
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~find-restart~ and ~compute-restarts~ are blocked on a type rather than on
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effort: §4 gives them ~(Option Restart)~ and a list, and there is no ~Restart~
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type and no list to return one in. The break loop reads the same stack through
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the agent's socket instead.
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* Gotchas
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- *A handler closes over nothing.* It is lifted into its own function.
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Accumulate into a global, or put the value on the condition.
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- *A restart re-runs whatever sits between it and the target.* Control resumes
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at the ~restart-case~, so a ~retry~ repeats side effects after it. Put the
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~restart-case~ where re-entry is safe.
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- *An unknown restart name is a hard stop.* No ~find-restart~ to test with.
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- *So are the wrong arguments*, and for the same reason: nothing static can
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know what a name will find. The message names both signatures.
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- *Lookup is by name; the signature is checked after it.* Nothing searches for
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a frame the arguments would fit. An inner ~(use-value [s string] ...)~
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shadows an outer ~(use-value [v i32] ...)~, so ~(invoke-restart 'use-value
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21)~ stops the program even though the outer clause would have taken it.
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- *No supertype*, so nothing can say "any condition".
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- *~signal~ cannot hand a value back.* Deliberate (§1).
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- A condition must be a *struct*. ~return~ is refused inside either form.
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* Stops the program, exit 134
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#+begin_example
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unhandled AssetMissing
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file.flan:3:25: no restart named nope is active
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file.flan:4:7: a defer invoked a restart, which a defer may not do — ...
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file.flan:9:12: restart use-value takes (i32), given (string)
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file.flan:6:5: restart use-value takes (i32), and whatever took it supplied
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no arguments — ...
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#+end_example
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