In Parameter§
See primary documentation in context for method type captures.
method type_captures(Parameter:D: --> List:D)
Returns a list of variable names of type captures associated with this parameter. Type captures define a type name within the attached code, which is an alias to the type gleaned from the argument during a call.
sub a(::T $x) { T.say } a(8); # OUTPUT: «(Int)» say &a.signature.params[0].type_captures; # OUTPUT: «(T)» sub b($x) { $x.^name.say } a(8); # OUTPUT: «Int»
The type used may change from call to call. Once they are defined, type captures can be used wherever you would use a type, even later in the same signature:
sub c(::T $x, T $y, $z) { my T $zz = $z }; c(4, 5, 6); # OK try c(4, 5, "six"); given $! { .message.say }; # OUTPUT: «Type check failed in assignment to $zz; expected Int but got Str ("six")» try c("four", 5, "six"); given $! { .message.say }; # OUTPUT: «Type check failed in binding to parameter '$y'; expected Str but got Int (5)»
Type captures may be used at the same time as type constraints.
sub d(::T Numeric $x, T $y) {}; d(4, 5); # OK d(4e0, 5e0); # OK try d(4e0, 5); given $! { .message.say }; # OUTPUT: «Type check failed in binding to parameter '$y'; expected Num but got Int (5)» try d("four", "five"); given $! { .message.say }; # OUTPUT: «Type check failed in binding to parameter '$x'; expected Numeric but got Str ("four")»