filter

If the Maybe is a Just, passes its value to the predicate. If the predicate returns true, then the Maybe is returned unchanged. In every other case, a Nothing gets returned.

This feature is experimental!

This API is still experimental, so it may change or be removed in future versions. You should not rely on it for production applications.

Signature

Type(what is this?)

forall a: (Maybe a).((a) => Boolean) => Maybe a

Documentation

If the Maybe is a Just, passes its value to the predicate. If the predicate returns true, then the Maybe is returned unchanged. In every other case, a Nothing gets returned.

Example:

const Maybe = require('folktale/maybe');

// This line evaluates to true.
Maybe.Just.hasInstance(Maybe.Just(3).filter(n => n === 3));

// These lines evaluate to false.
Maybe.Just.hasInstance(Maybe.Just(2).filter(n => n === 3));
Maybe.Just.hasInstance(Maybe.Nothing().filter(n => n !== 3));
Maybe.Just.hasInstance(Maybe.Nothing().filter(n => n === 3));

Properties

Source Code

Defined in source/maybe/maybe.js at line 57, column 18

{
    /*~*/
    Nothing: function filter(predicate) {
      assertFunction('Maybe.Nothing#filter', predicate);
      return this;
    },

    /*~*/
    Just: function filter(predicate) {
      assertFunction('Maybe.Just#filter', predicate);
      return predicate(this.value) ? this : Nothing();
    }
  }