compile/try

Symbol: compile/try

Syntax: (try BODY CLAUSE...)

where each CLAUSE is (EXCEPTMATCH BODY...) and EXCEPTMATCH is one of:

[TYPEEXPR], [SYM TYPEEXPR], or [TYPEEXPR as: SYM] – USE form; type-only or binding.
except: – bare catch-all; catches any exception without a type check or binding

In addition, two keyword-head clauses are accepted:

(else: BODY...) – run when the try body completed without exception
(finally: BODY...) – always run; must be last

Use-case: Exception handling. Runs BODY; on exception, matches against clauses in order and runs the first matching handler. Optional else: runs when no exception occurred. Optional finally: runs on every exit from the form: normal completion, an exception, a return, break or continue, and a continuation transfer that leaves it (a call/cc escape, or a shift handler that abandons the captured slice), in order with the __exit__ of any with and the after thunk of any dyn-wind on the same path, innermost first. A slice re-entered through its continuation runs the finally: again when that run leaves. An escape issued from inside a running finally: body abandons the rest of that body. At least one except-style clause or finally: is required.

Behaviour: Uses PUSH_TRY, CLEAR_TRY, POP_TRY, PUSH_FINALLY, POP_FINALLY. Exception handlers use EXCEPTION_MATCH, EXCEPTION_CLEAR, EXCEPTION opcodes. The bare except: form is lowered to an else: clause in the internal compile/cond dispatch so no EXCEPTION_MATCH is emitted; uncaught exceptions re-raise via compile/raise unless a bare handler is present. else: runs between CLEAR_TRY and POP_TRY; finally: runs after cleanup. Expressions in finally: are compiled for side effects only; they do not replace the value of the try form (including a pending compile/return from the try or except paths).

Examples:

;; Type-only catch
(try (risky-call)
  [[Exception] (handle-exception)])

;; Named binding
(try (parse input)
  [[e ValueError] (log e)])

;; Bare catch-all
(try (risky-call)
  [except: (handle-anything)])

;; Try/else/finally
(try (do-work)
  [else: (success)]
  [finally: (cleanup)])

;; Full form
(try (do-work)
  [[Exception] (handle-exc)]
  [else: (on-success)]
  [finally: (cleanup)])

Also: layout handler try.