Function composition in Typescript

Question

I need to implement a function (flow) that makes a functional composition of its two arguments.

Requirements:

The flow function should take two functions as arguments (called fab and fbc) and return a new function that applies fab to its input parameter and then applies fbc to the resulting value.

Hints:

The first function is called fab because it maps a value from type A to type B. The same for fbc.

The problem is that I don't know what to pass as a parameter to fab function, I'm a Typescript newbie, would appreciate any help!

This is my code so far:

type Flow = <A, B, C>(
  fab: (a: A) => B, // Takes a function from A to B
  fbc: (b: B) => C // Takes a function from B to C
) => (a: A) => C; // Returns a function from A to C

export const flow: Flow = (fab, fbc) => {
  return fbc(fab());
};

return a => fbc(fab(a)); typescriptlang.org/play?#code/… — Keith
– Keith, Commented Apr 22, 2021 at 20:28
might be worth looking at how lodash does this github.com/DefinitelyTyped/DefinitelyTyped/blob/… — Frank
– Frank, Commented Apr 22, 2021 at 20:35

Moe · Accepted Answer · 2021-04-22 20:33:12Z

1

type Flow = <A, B, C>(
  fab: (a: A) => B, // Takes a function from A to B
  fbc: (b: B) => C // Takes a function from B to C
) => (a: A) => C; // Returns a function from A to C

export const flow: Flow = (fab, fbc) => (a) => fbc(fab(a));

answered Apr 22, 2021 at 20:33

Moe

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