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zaetrik/applicative-of-example.ts

Last active May 9, 2020 09:11
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Applicative of() example
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applicative-of-example.ts
// This is how fp-ts implements this for Option
// Taken from https://github.com/gcanti/fp-ts/blob/master/src/Option.ts
interface None {
readonly _tag: "None";
}
interface Some<A> {
readonly _tag: "Some";
readonly value: A;
}
type Option<A> = None | Some<A>;
function some<A>(a: A): Option<A> {
return { _tag: "Some", value: a };
}
const OptionApplicative = {
map: (ma, f) => (isNone(ma) ? none : some(f(ma.value))),
of: some,
ap: (mab, ma) => (isNone(mab) ? none : isNone(ma) ? none : some(mab.value(ma.value))),
}
// Now we can create a new instance like this =>
const MyApplicative = OptionApplicative.of(5) // { _tag: "Some", value: 5 }
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