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An experimental implementation of first class functional types using pure ES at runtime, inspired by Haskell, PureScript and Idris.

npm: npm install typified --save
cdn: https://cdn.jsdelivr.net/npm/typified@latest/src/index.js

With Typified you introduce JavaScript to the world of strong typing in a functional fashion. Instead of the Java-style typing that TypeScript and Flow provide, Typified takes the Haskell-inspired approach where types are kept entirely separate from their associated values.

import ꓽ from 'typified';

const sayHello =
    t`sayHello ∷ String → String` (name => `Hello ${name}!`);

By invoking sayHello with a String we're guaranteed to be returned a String otherwise Typified will throw a TypeMismatchError that will notify the developer of such an occurrence. Passing in anything other than String will throw the mismatch error.

Typified tries to stay close to the JavaScript world, and thus allows specifying the union operator (|) to accept multiple types. For instance our sayHello function could take a Number type as well and be perfectly fine, as it's not performing any String-specific operations – thus we could augment the types to allow Number to be passed too.

const sayHello =
    t`sayHello ∷ String|Number → String` (name => `Hello ${name}!`);

Invoking sayHello with a Number type would be just as fine as invoking it with a String.

Typified also supports to the concept of generics. Below we have modified the function a little to accept two name parameters, which yields either one or two greetings depending on whether the names are the same.

const sayHello =
    t`sayHello ∷ String → String → String` ((first, second) => {
        return first === second
            ? `Hello ${first}!`
            : `Hello ${first} & ${second}!`;
    });

We've removed the Number type constraint as we always want to compare String types with each other. Invoking sayHello as sayHello('Adam', 100) would be somewhat pointless as a String and Number can never match with strict equality. However we're still performing a non-String exclusive operation, and therefore being able to pass a Number would still be rather useful.

You might be tempted to define the type as String|Number → String|Number → String however that would still allow the passing of a String and a Number at the same time. What we need is a way to enforce any type as long as both match – generics suddenly become a useful tool.

const sayHello =
    t`sayHello ∷ ∀ a. a → a → String` ((first, second) => {
        return first === second
            ? `Hello ${first}!`
            : `Hello ${first} & ${second}!`;
    });

Using generics we have the above type annotation which takes a generic a – the a doesn't yet have a concrete type, and will instead assume a concrete type when the function is invoked. Passing a Number as firstName would cause a to be of type Number, and therefore ensure that secondName is also a Number. Likewise passing String as the first parameter would give a a type of String.

You can define as many generics as you wish in the type annotation, but it's crucial that you define them using ∀ or forall as otherwise the types would be assumed to be concrete types. In Haskell it's perfectly valid to not define generics, whereas in PureScript the defining of generics is mandatory – Typified also takes this approach.