commune

Declares shared fields once on a tagged union.

Aliases: shared

Syntax: @ commune

Category#

type

Related#

• 判别
• 匹配
• 构造

Examples#

radix/corpus/commune/commune.fab (canonical · annotation)#

Declares shared fields once on a tagged union.

# =============================================================================
# commune — Declares shared fields once on a tagged union.
# =============================================================================
#
# What this teaches:
#   • Shared fields — `@ commune` (English `@ shared`) declares fields every
#     variant carries. Construction supplies them next to the variant payload.
#   • Member access — `e.message` reads the commune field on the union value.
#     Matching still binds the same field in case arms.
#
# Common mistakes:
#   • Redeclaring a commune field on a variant — the field is already on the
#     union; repeating it on a variant is rejected.
#   • Annotating `@ commune` on a genus field — the region is union-only.
#
# See also: discretio, discerne, finge
# =============================================================================
#
# BACKEND:
#   TEU3: commune fields emit on the primary lanes (Rust enum payloads, TS
#   tagged unions) and MIR lowering/stepper. Tail backends are TEU4.

union ShapeError {
    @ shared
    string message

    Incompatible,
    ElementMismatch {
        int expected
        int actual
    }
}

fn describe(ShapeError e) → string {
    match e {
        case Incompatible const message { return message }
        case ElementMismatch const message, expected, actual { return message }
    }
}

fn render(ShapeError e) → string {
    return e.message
}

main {
    const ShapeError a ← variant Incompatible { message = "incompatible" }
    const ShapeError b ← variant ElementMismatch { message = "mismatch", expected = 1, actual = 2 }
    print describe(a)
    print describe(b)
    print render(a)
}