Documentation for syntax Coercion type assembled from the following types:

class Signature

From Signature

(Signature) Coercion type

To accept one type but coerce it automatically to another, use the accepted type as an argument to the target type. If the accepted type is Any it can be omitted.

sub f(Int(Str) $want-int, Str() $want-str) {
    say $want-int.^name ~ ' ' ~ $want-str.^name
}
f '10', 10;
# OUTPUT: «Int Str␤» 
 
use MONKEY;
augment class Str { method Date() { Date.new(self) } };
sub foo(Date(Str) $d) { say $d.^name; say $d };
foo "2016-12-01";
# OUTPUT: «Date␤2016-12-01␤»

The coercion is performed by calling the method with the name of the type to coerce to, if it exists (e.g. Foo(Bar) coercer, would call method Foo). The method is assumed to return the correct type—no additional checks on the result are currently performed.

Coercion can also be performed on return types:

sub square-str (Int $x --> Str(Int)) {
    $x²
}
 
for 2,4, *²  … 256 -> $a {
    say $a, "² is ", square-str( $a ).chars, " figures long";
}
 
# OUTPUT: «2² is 1 figures long␤ 
#          4² is 2 figures long␤ 
#          16² is 3 figures long␤ 
#          256² is 5 figures long␤» 

In this example, coercing the return type to String allows us to directly apply string methods, such as the number of characters.