throw

Throws a recoverable error.

Syntax: throw <expression>

Category#

errors

Related#

• catch
• panic
• if
• ⇥

Examples#

radix/corpus/iace/iace-si-guard.fab (canonical · keyword)#

Conditional throw with if guard.

# =============================================================================
# throw — Conditional throw with if guard
# =============================================================================
#
# What this teaches:
#   • Conditional throw with if guard.
#   • Related keywords: catch, panic, if, ⇥
#
# Common mistakes:
#   • Using `throw if` without a `⇥` on the enclosing function or a `do/cape` wrapper — the guard sugar expands to `if cond { throw val }`, which still needs an error context.
#
# See also: catch, panic, if, ⇥
# =============================================================================

# throw with optional if guard
#
# throw <value> [if <condicio>]
#
# GRAMMAR:
#   throwStmt :← 'throw' expr ['if' expr]
#
# The optional `if` guard is parser sugar:
#   throw val if cond  →  if cond { throw val }
#
# Without the guard, throw is unconditional (unchanged).
#
# EXPECTED OUTPUT:
#   "ok: valid input"
#   "caught: empty input"
#   "caught: negative"

fn validata(string input) → string ⇥ string {
    # Guard: throw if input is empty
    throw "empty input" if input.length() ≡ 0

    return "valid: §"(input)
}

main {
    # Happy path — guard not triggered
    do {
        const string a ← validata("hello")
        print "ok: §"(a)
    }
    catch err {
        print "unexpected: §"(err)
    }

    # Empty input — throw if guard triggers
    do {
        const string b ← validata("")
        print "should not reach"
    }
    catch err {
        print "caught: §"(err)
    }

    # throw with if on same line
    do {
        throw "force throw" if true
        print "should not reach"
    }
    catch err {
        print "caught: §"(err)
    }
}

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

Throws a recoverable error.

# =============================================================================
# throw — Throws a recoverable error
# =============================================================================
#
# What this teaches:
#   • Throws a recoverable error.
#   • Related keywords: ⇥, catch, panic, return
#
# Common mistakes:
#   • Using `throw` without a `⇥` on the enclosing function or a `do { ... } catch err { ... }` wrapper — `throw` requires an error-handling context (SEM010).
#
# See also: ⇥, catch, panic, return
# =============================================================================

# throw — recoverable throw and catch handler
#
# throw <value>
# do { <body> }
# catch err { <handler> }
#
# GRAMMAR:
#   throwStmt :← 'throw' expr
#   facStmt   :← 'do' block catchClause?
#
# EXPECTED OUTPUT:
#   Caught error messages from do/cape recovery paths.
#
# BACKEND:
#   Rust/Go lowering does not emit throw/cape yet — compile-only smoke.

main {
    # Bare throw with string payload
    do {
        throw "Something went wrong"
    }
    catch err {
        print "Caught:", err
    }

    # throw with interpolated message
    const int code ← 404
    do {
        throw "Error code: §"(code)
    }
    catch err {
        print "Caught:", err
    }

    # Conditional throw inside do — validation guard
    const int value ← -5
    do {
        if value ≺ 0 {
            throw "Value must be non-negative"
        }
        print "Value is valid"
    }
    catch err {
        print "Validation failed:", err
    }
}

Expected output:

Caught: Something went wrong
Caught: Error code: 404
Validation failed: Value must be non-negative