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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Higher order functions lose type parameters","articleBody":"When making higher order functions that return functions, type parameters are not preserved in return type. Notice in the following example how `identityM` has the generic type parameter `A` changed into `{}`, resulting in a loss of type safety in the rest of the code.\n\nIdeally `identityM` would have the type `\u003cA\u003e(a: A) =\u003e A`. If this isn't possible, issuing a compiler warning (or error) about loss of genericity would be very helpful in tracking down errors like this.\n\n``` typescript\nfunction mirror\u003cA, B\u003e(f: (a: A) =\u003e B): (a: A) =\u003e B { return f; }\nfunction identity\u003cA\u003e(a: A): A { return a; }\nvar identityM = mirror(identity); // type: (a: {}) =\u003e {}\n\nvar x = 1;\nvar y = identity(x); // type: number\nvar z = identityM(x); // type: {}\n```\n","author":{"url":"https://github.com/wavebeem","@type":"Person","name":"wavebeem"},"datePublished":"2014-08-10T05:49:45.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/417/TypeScript/issues/417"}