捕捉

Starts a catch block.

Aliases: catch

Syntax: <structured-statement> cape <name> <block>

Category#

errors

Related#

• 執行
• 拋出
• ⇥

Examples#

radix/corpus/cape/cape.fab (canonical · keyword)#

Starts a catch block.

# =============================================================================
# cape — Starts a catch block.
# =============================================================================
#
# What this teaches:
#   • Error recovery — `fac { … } cape err { … }` attempts a block and catches
#     errors with a named handler
#   • Guarded execution — code inside the `fac` block can `iace` errors that
#     are intercepted by the `cape` handler
#
# Common mistakes:
#   • attaching `cape` to a bare `{ }` block — `cape` only attaches to `fac` blocks and structured statements; use `fac { ... } cape err { ... }`
#
# See also: fac, iace, ⇥
# =============================================================================

# cape — catch handler on fac
#
# GRAMMAR:
#   facStmt :← 'fac' block 'cape' ident block
#
# EXPECTED OUTPUT:
#   Scalar stdout smoke (see body).
#
# BACKEND:
#   Rust/Go: fac/cape emit gap (whitelist: cape/cape.fab).
#

main {
    var int attempt ← 0
    do {
        attempt ← attempt + 1
        if attempt > 1 {
            throw "simulated failure"
        }
        print "attempt §"(attempt)
    }
    catch err {
        print "caught"
    }
}

Expected output:

attempt 1

radix/corpus/cape/receptio-decline.fab (supporting · reject)#

cape AIR decline rows: whole-loop dum/itera cape stays out under the loop-spectrum causes, a rethrow from a recovery arm declines (rethrow_in_recovery), and the cura construct keeps its independent deferred-init keep-out.

# =============================================================================
# receptio decline — the Tier 0.5 keep-outs stay fail-closed.
# =============================================================================
#
# Decline rows (reject reason in each comment):
#   1. whole-loop cape on `dum` — one trailing cape wraps the whole loop
#      (never re-armed per iteration), and the loop spectrum stays an AIR
#      keep-out: SEM059 dum_loop. A loop statement has no normal-exit
#      value, so the recovery arm also fails the outer-type rule:
#      SEM059 recovery_type_mismatch
#   2. whole-loop cape on `itera` — same per-loop attachment, same
#      keep-out: SEM059 itera_loop plus recovery_type_mismatch
#   3. rethrow — a throw from inside the catch arm is a rethrow; it
#      declines to Tier 2 fail-closed and is never silently treated as a
#      recovered value: SEM059 rethrow_in_recovery
#   4. cura construct — the cape handler itself does not decline; the
#      construct's independent deferred-init ruling keeps the function out
#      of AIR: SEM059 deferred_local_init
#
# Common mistakes:
#   • expecting the trailing loop `cape` to recover per iteration — the
#      handler is per-loop by the grammar; per-iteration recovery would
#      need new syntax and its own goal
#   • expecting a rethrow to admit because the region admitted — the
#      recovery arm is checked as a catch region; throwing from it is the
#      named Tier 2 decline
#   • reading the cura row as a handler decline — the handler admits; the
#      construct's own keep-out is what fails
#
# See also: cape, dum, itera, cura, iace, receptio-air
# =============================================================================

# 1. whole-loop cape on dum — loop-spectrum keep-out
@ radix lane "air"
functio custos_dum(bivalens pergo) → vacuum {
    dum pergo {
        iace "x"
    } cape err { }
}

# 2. whole-loop cape on itera — loop-spectrum keep-out
@ radix lane "air"
functio custos_itera() → vacuum {
    itera ab 0‥4 fixum i {
        iace "x"
    } cape err { }
}

# 3. rethrow — a throw inside the catch arm declines to Tier 2
@ radix lane "air"
functio reicit(bivalens malum) → numerus ⇥ textus {
    fac {
        si malum { iace "x" }
        redde 1
    } cape err {
        iace "reicitur"
    }
    redde 2
}

# 4. cura construct — the handler admits; deferred-init keeps the function out
@ radix lane "air"
functio cura_recepta() → numerus {
    cura "arena" fixum _ res { redde 1 } cape err { redde 0 }
    redde 3
}

Expected: compilation rejects this example.