NAME

Kind - Typechecking based on kinds

SYNOPSIS

use Kind;

my constant Class = Kind[Metamodel::ClassHOW];

proto sub is-class(Mu --> Bool:D)  {*}
multi sub is-class(Class --> True) { }
multi sub is-class(Mu --> False)   { }

say Str.&is-class;  # OUTPUT: True
say Blob.&is-class; # OUTPUT: False

DESCRIPTION

Kind is an uninstantiable parametric type that can be used to typecheck values based off their kind. A parameterization produces a type object that can process the HOW of a type in a typecheck context with ACCEPTS when available, otherwise falling back to the bare typecheck.

Kind is documented. You can view the documentation for it and its methods at any time using WHY.

METAMETHODS

method parameterize

method ^parameterize(|args) is raw

Produces a parameterization by delegating to ^kind with args.

method kind

method ^kind(Mu $obj is raw, Mu \K, Mu:U \T = Mu) is raw

Produces a cached subset with a refinement (where) built from K and a refinement (of) from T if present. This backs ^parameterize so as to allow for a different parameterizer in a subtype. This is more or less a wrapper for Metamodel::Primitives.parameterize_type.

Some useful values with which to produce a type are:

• a metaclass or metarole

# Smartmatches any class.
Kind[Metamodel::ClassHOW]

• a junction of metaclasses or metaroles

# Smartmatches any type that supports naming, versioning, and documenting.
Kind[Metamodel::Naming & Metamodel::Versioning & Metamodel::Documenting]

• a block

# Smartmatches any parametric type.
Kind[{ use nqp; nqp::hllbool(nqp::can($_, 'parameterize')) }]

• a metaobject

# This class' metamethods constrain their metaobject to itself or its subtypes.
class Configurable {
    my constant K := Kind[$?CLASS.HOW.WHAT, $?CLASS];

    my constant %configuration := hash;

    method ^configure(K $obj, %config --> Map:D) {
        %configuration{$obj.WHAT.WHICH} := %config.Map
    }

    method ^configuration(K $obj --> Map:D) {
        %configuration{$obj.WHAT.WHICH} // Map.new
    }
}

For more examples of parameterizations of Kind, refer to t/01-typecheck.t.

SYMBOLS

&set_parameterizer

our sub set_parameterizer(Mu $obj is raw, &parameterizer = &parameterize --> Nil)

Applies the parameterizer of Kind to a metaobject, providing it with a parameterization cache. A subtype needs to apply this at BEGIN-time in order to parameterize with the default metamethods, for instance:

class Kind::Instantiable is Kind {
    BEGIN Kind::set_parameterizer($?CLASS);
}

A &parameterizer may be provided, in which case that will be set instead. This should carry a compatible signature with &parameterize.

For more examples of how to subclass Kind, refer to t/02-meta.t.

&parameterize

our sub parameterize(Mu $root is raw, Any $args) is raw

Given the $root metaobject of the parameterization and its $args, produces a type object minus the caching. $args is assumed to carry K at position 0 and T, if present, at position 1.

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.