Composition

Composition in fnts is represented through the compose and pipe functions. Both accept two functions at a time, allowing for safer type inference which is not always possible with overloads or event without them (which happened in 1.x versions of fnts).

Commonly, as well as in fnts, compose is used to apply a list of functions to an argument, passing the result of each application to the next one in a right to left order:

import compose from 'fnts/compose';

const isTwoDigits = compose(
  compose(
    (b: boolean) => b ? 'true' : 'false',
    (s: string) => s.length === 2 // argument type imposed from the next function
  ),
  (n: number) => `${n}`,
); // will accept only a number as argument

isTwoDigits(5) === 'false';
isTwoDigits(14) === 'true';

pipe, on the other hand, is executed in a more familiar left to right order:

import pipe from 'fnts/pipe';

const isTwoDigits = pipe(
  pipe(
    (n: number) => `${n}`,
    (s: string) => s.length === 2
  ),
  (b: boolean) => b ? 'true' : 'false',
);

isTwoDigits(5) === 'false';
isTwoDigits(14) === 'true';

Both compose and pipe allow for their first to-be-executed function to have multiple arguments.

import compose from 'fnts/compose';

const isEvenSum = compose(
  compose(
    (b: boolean) => b ? 'true' : 'false',
    (s: number) => s % 2 === 0
  ),
  (a: number, b: number) => a + b,
);

isEvenSum(2, 2) === 'true';
isEvenSum(17, 32) === 'false';