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[![Build Status](https://travis-ci.com/Kaiepi/p6-Kind-Subset-Parametric.svg?branch=master)](https://travis-ci.com/Kaiepi/p6-Kind-Subset-Parametric) NAME ==== Kind::Subset::Parametric - Support for generic subsets SYNOPSIS ======== ```perl6 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: ```perl6 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 ----------------- ```perl6 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: ```perl6 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: ```perl6 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.