比對

Starts an exhaustive pattern match over a value or values.

Aliases: match

Syntax: 比對 <subject> <block>

Category#

control-flow

Related#

• 分支聯集
• 分支

Examples#

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

Starts an exhaustive pattern match over a value or values.

# =============================================================================
# 比對 — Starts an exhaustive pattern match over a value or values.
# =============================================================================
#
# What this teaches:
#   • Exhaustive pattern matching — `比對 <value> { 分支 ... }` matches against all variants of a 分支聯集 (tagged union).
#   • Variant destructuring — `分支 <Variant> 定值 <field>, ...` destructures variant payload fields.
#   • Unit variants — variant without payload fields match by name alone.
#
# Common mistakes:
#   • Non-exhaustive match — omitting a variant from `比對` without a `預設` catchall; every 分支聯集 variant must be covered (SEM040).
#
# See also: 分支聯集, 分支
# =============================================================================

# 比對 — pattern matching on 分支聯集 values
#
# 比對 <value> {
#     分支 <Casus> { <body> }
#     分支 <Casus> 作為 <alias> { <body> }
#     分支 <Casus> 定值 <bindings> { <body> }
#     分支 <Casus> 定值 <field> 作為 <alias> { <body> }
# }
#
# GRAMMAR:
#   discerneStmt :← '比對' expr '{' casuClause* '}'
#
# EXPECTED OUTPUT:
#   比對 exempla parata

# 分支聯集 — tagged union / enum declaration
union Condicio {
    Agens,
    Iners,
    Pendens
}

union Nuntius {
    Pulsus {
        int x
        int y
    },
    Clavis {
        string clavis
    },
    Finis
}

# Simple 分支 matching on unit variants (no payload)
fn nomina(Condicio s) → string {
    match s {
        case Agens {
            return "agens"
        }
        case Iners {
            return "iners"
        }
        case Pendens {
            return "pendens"
        }
    }
}

# 分支 with 定值 bindings destructures variant fields
fn tracta(Nuntius e) → none {
    match e {
        case Pulsus const x, y {
            print "pulsus: § §"(x, y)
        }
        case Clavis const clavis {
            print "clavis: §"(clavis)
        }
        case Finis {
            print "finis"
        }
    }
}

main {
    # nomina/tracta are defined for downstream exempla; smoke check only
    print "discerne exempla parata"
}

Expected output:

discerne exempla parata