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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Allow classes to be parametric in other parametric classes","articleBody":"This is a proposal for allowing generics as type parameters.  It's currently possible to write specific examples of monads, but in order to write the interface that all monads satisfy, I propose writing\n\n``` javascript\ninterface Monad\u003cT\u003c~\u003e\u003e {\n  map\u003cA, B\u003e(f: (a: A) =\u003e B): T\u003cA\u003e =\u003e T\u003cB\u003e;\n  lift\u003cA\u003e(a: A): T\u003cA\u003e;\n  join\u003cA\u003e(tta: T\u003cT\u003cA\u003e\u003e): T\u003cA\u003e;\n}\n```\n\nSimilarly, it's possible to write specific examples of cartesian functors, but in order to write the interface that all cartesian functors satisfy, I propose writing\n\n``` javascript\ninterface Cartesian\u003cT\u003c~\u003e\u003e {\n  all\u003cA\u003e(a: Array\u003cT\u003cA\u003e\u003e): T\u003cArray\u003cA\u003e\u003e;\n}\n```\n\nParametric type parameters can take any number of arguments:\n\n``` javascript\ninterface Foo\u003cT\u003c~,~\u003e\u003e {\n  bar\u003cA, B\u003e(f: (a: A) =\u003e B): T\u003cA, B\u003e;\n}\n```\n\nThat is, when a type parameter is followed by a tilde and a natural arity, the type parameter should be allowed to be used as a generic type with the given arity in the rest of the declaration.\n\nJust as is the case now, when implementing such an interface, the generic type parameters should be filled in:\n\n``` javascript\nclass ArrayMonad\u003cA\u003e implements Monad\u003cArray\u003e {\n  map\u003cA, B\u003e(f: (a:A) =\u003e B): Array\u003cA\u003e =\u003e Array\u003cB\u003e {\n    return (arr: Array\u003cA\u003e) =\u003e  arr.map(f);\n  }\n  lift\u003cA\u003e(a: A): Array\u003cA\u003e { return [a]; }\n  join\u003cA\u003e(tta: Array\u003cArray\u003cA\u003e\u003e): Array\u003cA\u003e {\n    return tta.reduce((prev, cur) =\u003e prev.concat(cur));\n  }\n}\n```\n\nIn addition to directly allowing compositions of generic types in the arguments, I propose that typedefs also support defining generics in this way (see [issue 308](https://github.com/Microsoft/TypeScript/issues/308)):\n\n``` javascript\ntypedef Maybe\u003cArray\u003c~\u003e\u003e Composite\u003c~\u003e ;\nclass Foo implements Monad\u003cComposite\u003c~\u003e\u003e { ... }\n```\n\nThe arities of the definition and the alias must match for the typedef to be valid.\n","author":{"url":"https://github.com/metaweta","@type":"Person","name":"metaweta"},"datePublished":"2014-11-19T18:46:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":189},"url":"https://github.com/1213/TypeScript/issues/1213"}