Created
May 10, 2020 08:13
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Applicative: Compose pure n-ary functions with effectful functions
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| // We import the the Option effect from fp-ts | |
| // We could also implement our own Applicative instance | |
| import * as O from "fp-ts/lib/Option"; | |
| // lift function for the Option effect for pure functions with arity 2 | |
| function liftA2<A, B, C>(f: (a: A) => (b: B) => C): (a: O.Option<A>) => (b: O.Option<B>) => O.Option<C> { | |
| return (fa) => (fb) => (O.isNone(fa) || O.isNone(fb) ? O.none : O.ap(fb)(O.map(f)(fa))); | |
| } | |
| const effectfulDouble = (x: number) => O.some(x * 2); | |
| // Binary pure function | |
| const pureBinarySay = (text: string) => (x: number) => `${text} ${x}`; | |
| // We lift pureBinarySay to a function of type: | |
| // (text: O.Option<string>) => (x: O.Option<number>) => O.Option<string> | |
| const liftedPureBinarySay = liftA2(pureBinarySay); | |
| liftedPureBinarySay(O.some("The result is"))(effectfulDouble(5)); // { _tag: 'Some', value: 'The result is 10' } | |
| // We can do the same for pure ternary functions | |
| // lift function for the Option effect for pure functions with arity 3 | |
| function liftA3<A, B, C, D>( | |
| f: (a: A) => (b: B) => (c: C) => D | |
| ): (a: O.Option<A>) => (b: O.Option<B>) => (c: O.Option<C>) => O.Option<D> { | |
| return (fa) => (fb) => (fc) => | |
| O.isNone(fa) || O.isNone(fb) || O.isNone(fc) ? O.none : O.ap(fc)(O.ap(fb)(O.map(f)(fa))); | |
| } | |
| // Ternary pure function | |
| const add = (x: number) => (y: number) => (z: number) => x + y + z; | |
| // We lift add to a function of type: | |
| // (x: O.Option<number>) => (y: O.Option<number>) => (z: O.Option<number>) => O.Option<number> | |
| const addLifted = liftA3(add); | |
| addLifted(effectfulDouble(2))(effectfulDouble(5))(O.some(10)); // { _tag: 'Some', value: 24 } | |
| // We could now create lift functions for any arity |
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