Multiple Dispatch

Candidates are ordered by narrowness: more specific types and constraints win. Ties raise an "Ambiguous call" error. where clauses are checked in declaration order among equally narrow candidates.

# Dispatch by type
multi sub describe(Int $x)  { "Int: $x" }
multi sub describe(Str $x)  { "Str: '$x'" }
multi sub describe(Rat $x)  { "Rat: $x" }
multi sub describe(@x)      { "Positional with {@x.elems} elements" }
multi sub describe(%x)      { "Associative with keys {%x.keys.sort}" }
multi sub describe($x)      { "Something else: {$x.^name}" }
say describe($_) for 42, 'hi', 1.5, [1, 2], {a => 1}, 3e0;

# Dispatch by value and constraint
multi fib(0) { 0 }
multi fib(1) { 1 }
multi fib(Int $n where * > 1) { fib($n - 1) + fib($n - 2) }
say fib(20);                     # 6765

multi factorial(0) { 1 }
multi factorial(Int $n where * > 0) { $n * factorial($n - 1) }

# Dispatch by arity and named arguments
multi area(Numeric $side)                 { $side ** 2 }
multi area(Numeric $w, Numeric $h)        { $w * $h }
multi area(:$radius!)                     { π * $radius ** 2 }
say area(3), ' ', area(2, 5), ' ', area(radius => 1);

# proto: define the shared interface and wrap the candidates
proto sub process(|) {
    say "before";
    {*}                         # dispatch to the best candidate
}
multi process(Int $x) { say "int $x" }
multi process(Str $x) { say "str $x" }
process(5);

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