Module `Relude_Eq`

type eq('a) = 'a => 'a => bool;

let by: a b. ('b => 'a) => eq('a) => eq('b);

Eq.by creates an eq('b) function from an eq('a) and a function from 'b => 'a. Effectively, this allows you to say "my complex type can be compared for equality because I'm already able to compare these simpler parts."

This is the contravariant map for the eq('a) function.

module User = {
  type t = { id: string, name: string };

  // create an equality function for our user type by comparing its id.
  let equals: eq(t) = Eq.by(user => user.id, String.eq);
}

let cmap: ('a => 'b) => eq('b) => eq('a);: Eq.cmap is the contravariant map for the equals function. It is an alias for Eq.by.

module Contravariant: BsBastet.Interface.CONTRAVARIANT with type Contravariant.t('a) = eq('a);

let invert: a. eq('a) => eq('a);

Eq.invert produces an equality function from an existing equality function, but with the returned boolean switched.

let stringNotEq = Eq.invert(String.eq);
stringNotEq("hello", "world") == true;