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Kind - Typechecking based on kinds


use Kind;

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

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

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


Kind is an uninstantiable parametric type that can be used to typecheck values based off their kind. If parameterized, it may be used in a where clause or on the right-hand side of a smartmatch to typecheck a value's HOW against its type parameter.

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

For examples of how to use Kind with any of Rakudo's kinds, see t/01-kind.t.


method parameterize

method ^parameterize(Kind:U $this is raw, Mu \K --> Kind:U) { }

Mixes in kind and ACCEPTS methods. See below.

Some useful values with which to parameterize Kind are:

# Smartmatches any class.
# Smartmatches any type that supports naming, versioning, and documenting.
Kind[Metamodel::Naming & Metamodel::Versioning & Metamodel::Documenting]
# Smartmatches any parametric type.
Kind[{ use nqp; nqp::hllbool(nqp::can($_, 'parameterize')) }]
# This class' metamethods ensure they can only be called with itself or its
# subclasses.
class Configurable {
    my Map:D %CONFIGURATIONS{ObjAt:D};

    method ^configure(Configurable:_ $this where Kind[self], %configuration --> Map:D) {
        %CONFIGURATIONS{$this.WHAT.WHICH} := %configuration.Map
    method ^configuration(Configurable:_ $this where Kind[self] --> Map:D) {
        %CONFIGURATIONS{$this.WHAT.WHICH} // Map.new


method ACCEPTS

method ACCEPTS(Kind:U: Mu $checker is raw) { }

Returns True if the HOW of $checker typechecks against Kind's type parameter, otherwise returns False.

If Kind's type parameter has an ACCEPTS method, this will smartmatch the HOW of $checker against it; otherwise, Metamodel::Primitives.is_type will be called with $checker's HOW and it. Most of the time, the former will be the case; the latter behaviour exists because it's not guaranteed K will actually have Mu's methods (this is case with Rakudo's metaroles).

method kind

method kind(Kind:U: --> Mu) { }

Returns Kind's type parameter.


Ben Davies (Kaiepi)


Copyright 2020 Ben Davies

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