Joseph Ferano
ca954a47b5
The cheatsheet was a document. It should fit on a screen: the syntax, what is missing, the traps that are not in any error message, and the three ways it stops. What was cut is either in spec-conditions.md or in the compiler's own refusal, and both say it better.
58 lines
2.1 KiB
Org Mode
58 lines
2.1 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 [] CLAUSE) ...)
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(invoke-restart 'name) ; 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 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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* Not yet
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~handler-case~ · ~find-restart~ · ~compute-restarts~ · restarts with
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parameters · the dev-build break loop. Each refused by name with its reason.
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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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- *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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#+end_example
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