Takes two validators, the condition validator and the fn validator, then returns a new function receiving a value v (of any type) and execute the fn validator only when the condition(v) returns true.
It is largely used within the library itself to evaluate certain conditions:
contains.js
/* Part of the contains validator */
const contains = (value) => compose(
when(isString, (string) => string.includes(value)),
when(Array.isArray, arrayContains(value)),
when(not(or(isString, Array.isArray)), stubFalse),
);