# NAME

Kind::Subset::Parametric - Support for generic subsets

# SYNOPSIS

use Kind::Subset::Parametric; # Arrays don't type their elements by default: my @untyped = 1, 2, 3; say @untyped.^name; # OUTPUT: Array # You can make it so they do by parameterizing Array: my Int:D @typed = 1, 2, 3; say @typed.^name; # OUTPUT: Array[Int:D] # But you can't typecheck untyped arrays using these parameterizations: say @typed ~~ Array[Int:D]; # OUTPUT: True say @untyped ~~ Array[Int:D]; # OUTPUT: False # So let's make our own array type that handles this using a parametric subset: subset TypedArray of Array will parameterize -> Mu ::T { Array:_ \array { array ~~ Array[T] || (array ~~ Array:D && so array.all ~~ T) } } where { ... }; # Now they can both be typechecked: given TypedArray[Int:D] -> \IntArray { say @typed ~~ IntArray; # OUTPUT: True say @untyped ~~ IntArray; # OUTPUT: True say <foo bar baz> ~~ IntArray; # OUTPUT: False }

# DESCRIPTION

Kind::Subset::Parametric is a library that enhances subsets with support for parameterization. This allows you to easily implement your own generic subsets.

Note: while you can make generic subsets using this library, subsets cannot be parameterized with generics yet. Parameterizations using type captures, such as this, may not work as expected:

subset TypeIdentity will parameterize -> Mu ::T { T }; proto sub is-type-id(Mu --> Bool:D) {*} multi sub is-type-id(Mu ::T $ where TypeIdentity[T] --> True) { } multi sub is-type-id(Mu --> False) { }

Kind::Subset::Parametric is documented. You can refer to the documentation for its trait and `MetamodelX::ParametricSubset`

at any time using `WHY`

.

# TRAITS

## will parameterize

multi sub trait_mod:<will>(Mu \T where Subset, &body_block, :$parameterize!)

This trait mixes the `MetamodelX::ParametricSubset`

metarole into the metaobject of the type this trait is used with (which may only be a subset of some sort) after parameterizing it with `&body_block`

. This makes it possible to use the subset as a parametric type.

The main metamethod of interest this metarole provides is `parameterize`

, which handles creating a new subset type upon parameterization given an arbitrary list of parameters. This is created using a name (generated similarly to how the name of a parametric role is generated), the refinee of the parametric subset, and the return value of the body block when invoked with the list of parameters as its refinement.

`MetamodelX::ParametricSubset`

also provides a `body_block`

metamethod, which returns the body block it was parameterized with given a subset type.

What all this means that the `TypedArray[Int:D]`

parameterization from the synopsis generates a subset functionally equivalent to the one this type declaration creates:

subset :: of Array where { $_ ~~ Array[Int:D] || ($_ ~~ Array:D && so $_.all ~~ Int:D) };

Parametric subsets can still be given a refinement (the value given to `where`

in a subset declaration) when this trait is used. This gets used to handle typechecking against the subset when it has not been parameterized. If it's not desirable for a parametric subset to be possible to use without being parameterized, one way you can prevent this from happening is to give it a stubbed refinement:

subset Identity will parameterize -> Mu \T { T } where { ... };

Refer to `t/02-will.t`

for more examples of how to use this trait.

# AUTHOR

Ben Davies (Kaiepi)

# COPYRIGHT AND LICENSE

Copyright 2019 Ben Davies

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.