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Kind::Subset::Parametric cpan:KAIEPI last updated on 2021-10-03

=begin pod

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=head1 NAME

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


=begin code :lang<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
=end code


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:

=begin code :lang<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)                            { }
=end code

Kind::Subset::Parametric is documented. You can refer to the documentation for
its trait and C<MetamodelX::ParametricSubset> at any time using C<WHY>.

=head1 TRAITS

=head2 will parameterize

=for code :lang<perl6>
multi sub trait_mod:<will>(Mu \T where Subset, &body_block, :$parameterize!)

This trait mixes the C<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 C<&body_block>. This makes it possible
to use the subset as a parametric type.

The main metamethod of interest this metarole provides is C<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.

C<MetamodelX::ParametricSubset> also provides a C<body_block> metamethod, which
returns the body block it was parameterized with given a subset type.

What all this means that the C<TypedArray[Int:D]> parameterization from the
synopsis generates a subset functionally equivalent to the one this type
declaration creates:

=for code :lang<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 C<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:

=for code :lang<perl6>
subset Identity will parameterize -> Mu \T {
} where { ... };

Refer to C<t/02-will.t> for more examples of how to use this trait.

=head1 AUTHOR

Ben Davies (Kaiepi)


Copyright 2019 Ben Davies

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

=end pod