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[![Build Status](https://github.com/Kaiepi/ra-Kind-Subset-Parametric/actions/workflows/test.yml/badge.svg)](https://github.com/Kaiepi/ra-Kind-Subset-Parametric/actions/workflows/test.yml) NAME ==== Kind::Subset::Parametric - Support for generic subsets SYNOPSIS ======== ```raku 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 -> ::T { proto sub check(Array) {*} multi sub check(Array[T] --> True) { } multi sub check(Array:U --> False) { } multi sub check(Array:D $topic) { so $topic.all ~~ T } &check } where { !!! }; # Now they can both be typechecked: given TypedArray[Int:D] -> \IntArray { say @typed ~~ IntArray; # OUTPUT: True say @untyped ~~ IntArray; # OUTPUT: True say <foo bar baz> ~~ IntArray; # OUTPUT: False } ``` DESCRIPTION =========== `Kind::Subset::Parametric` enhances subsets with support for parameterization. This allows you to write subsets with generic `where` clauses. `Kind::Subset::Parametric` is documented. You can refer to the documentation for its trait and `MetamodelX::ParametricSubset` at any time using `WHY`. TRAITS ====== will parameterize ----------------- ```raku multi sub trait_mod:<will>(Mu \T where Subset, &body_block, :parameterize($)!) ``` This trait mixes the `MetamodelX::ParametricSubset` metarole into the HOW of the type it is applied to, which may only be a subset of some sort. Afterwards, the subset will be instantiated with its `&body_block`, which is what makes it possible to parameterize the subset. The main metamethod of interest this metarole provides is `parameterize`, which accepts arbitrary type argumentss. This is created using a name (generated similarly to how the name of a parametric role is generated), the refinee of the parametric subset (the value of `of`), and the return value of the body block when invoked with the arguments as its refinement (the value of `where`). For example, given an identity type: ```raku subset Identity will parameterize { $_ } where { !!! }; ``` `Identity[Any]` may be written to produce: ```raku subset ::('Identity[Any]') where Any; ``` Note the stubbed `where` clause. Parametric subsets can still be given a refinement when this trait is applied, which handles typechecking against the subset when it has not been parameterized. If it's not desirable for a parametric subset to typecheck without being parameterized, the `!!!` stub will throw. Historically, `...` was used, but failures don't necessarily sink when the typechecker's nqp is involved. The body block may be introspected with the `body_block` metamethod, which returns the body block it was parameterized with given a subset type. `set_body_block` must be called after being instantiated either by mixin on a HOW or by `&trait_mod:<does>` in order to replace this. Refer to `t/02-will.t` for more examples of how to use this trait. AUTHOR ====== Ben Davies (Kaiepi) COPYRIGHT AND LICENSE ===================== Copyright 2022 Ben Davies This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.