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.

2026-09-11 09:22:44 +07:00

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Conditions and restarts — cheatsheet

Works
Not yet
Gotchas
Stops the program, exit 134

Why it is shaped this way: spec-conditions.md. Something to poke at: conditions-play.flan — flan dev conditions-play.flan, then C-c C-c.

Works

(signal c)                  ; Unit. Handler returns -> carry on. No handler -> no-op.
(error  c)                  ; Never. Only a transfer gets past; else the program stops.

(handler-bind [(Type [c] body ...) ...] body ...)     ; match by type, no hierarchy

(restart-case BODY          ; BODY and every clause have the same type = the form's
  (name [] CLAUSE) ...)

(invoke-restart 'name)      ; Never. Innermost frame offering the name wins.

(defn fetch [n i32] i32
  (restart-case (middle n)              ; its value if nothing transfers
    (use-placeholder [] -1)
    (retry           [] 7)))

(handler-bind [(AssetMissing [c] (invoke-restart 'use-placeholder))]
  (fetch 2))                            ; -1

defer between the invoke and the target runs, innermost first, before the clause body. errdefer does not.

Not yet

handler-case · find-restart · compute-restarts · restarts with parameters · the dev-build break loop. Each refused by name with its reason.

Gotchas

A handler closes over nothing. It is lifted into its own function. Accumulate into a global, or put the value on the condition.
A restart re-runs whatever sits between it and the target. Control resumes at the restart-case, so a retry repeats side effects after it. Put the restart-case where re-entry is safe.
An unknown restart name is a hard stop. No find-restart to test with.
No supertype, so nothing can say "any condition".
signal cannot hand a value back. Deliberate (§1).
A condition must be a struct. return is refused inside either form.

Stops the program, exit 134

unhandled AssetMissing
file.flan:3:25: no restart named nope is active
file.flan:4:7: a defer invoked a restart, which a defer may not do — ...