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Common Functional Programming Algebraic data types for JavaScript that is compatible with most modern browsers and Deno.
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Functional

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Type factory

The Type factory can be used to build complex data structure.

import { factorizeType } from "https://deno.land/x/functional/SumType.js"

const Coordinates = factorizeType("Coordinates", [ "x", "y" ]);
const vector = Coordinates(150, 200);
// vector.x === 150
// vector.y === 200

Type.from

from :: Type ~> Object -> t

Create an instance of Type using an object representation.

const vector = Coordinates.from({ x: 150, y: 200 });
// vector.x === 150
// vector.y === 200

Type.is

is :: Type ~> Type t -> Boolean

Assert that an instance is of the same Type.

Coordinates.is(vector);
// true

Type.toString

toString :: Type ~> () -> String

Serialize the Type Representation into a string.

Coordinates.toString();
// "Coordinates"

Type(a).toString

toString :: Type t => t ~> () -> String

Serialize the instance into a string.

vector.toString();
// "Coordinates(150, 200)"

Type Sum factory

import { factorizeSumType } from "https://deno.land/x/functional/SumType.js"

const Shape = factorizeSumType(
  "Shape",
  {
    // Square :: (Coord, Coord) -> Shape
    Square: [ "topLeft", "bottomRight" ],
    // Circle :: (Coord, Number) -> Shape
    Circle: [ "center", "radius" ]
  }
);

SumType.from

from :: SumType ~> Object -> t

Create an instance of Type using an object representation.

const oval = Shape.Circle.from(
  {
    center: Coordinates.from({ x: 150, y: 200 }),
    radius: 200
  }
);
// oval.center === Coordinates(150, 200)
// oval.radius === 200

SumType.is

is :: SumType ~> SumType t -> Boolean

Assert that an instance is of the same Sum Type.

Shape.Circle.is(oval);
// true

SumType.fold

Shape.prototype.translate =
  function (x, y, z) {
    return this.fold({
      Square: (topleft, bottomright) =>
        Shape.Square(
          topLeft.translate(x, y, z),
          bottomRight.translate(x, y, z)
        ),

      Circle: (centre, radius) =>
        Shape.Circle(
          centre.translate(x, y, z),
          radius
        )
    })
  };

SumType(a).toString

toString :: SumType t => t ~> () -> String

Serialize the instance into a string.

oval.toString();
// "Shape.Circle(Coordinates(150, 200), 200)"

Example of writing a binary tree with Sum Types

import { factorizeSumType } from "https://deno.land/x/functional/SumType.js"

const BinaryTree = factorizeSumType('BinaryTree', {
  Node: ['left', 'x', 'right'],
  Leaf: []
});

BinaryTree.prototype.reduce = function (f, accumulator) {

  return this.fold(
    {
      Node: (l, x, r) => {
        const left = l.reduce(f, accumulator);
        const leftAndMiddle = f(left, x);

        return r.reduce(f, leftAndMiddle);
      },
      Leaf: () => accumulator
    }
  );
};

const tree =
  BinaryTree.Node(
    BinaryTree.Node(
      BinaryTree.Leaf,
      1,
      BinaryTree.Node(
        BinaryTree.Leaf,
        2,
        BinaryTree.Leaf
      )
    ),
    3,
    BinaryTree.Node(
      BinaryTree.Node(
        BinaryTree.Leaf,
        4,
        BinaryTree.Leaf
      ),
      5,
      BinaryTree.Leaf
    )
  );

// tree.reduce((x, y) => x + y, 0) === 15

Maybe type

The Maybe type represents potentially Just a value or Nothing.

import Maybe from "https://deno.land/x/functional/Maybe.js"

const container = Maybe.Just(42);

const serialize = (container) =>
  container.fold({
    Nothing: () => "There is no value.",
    Just: (value) => `The value is ${value}.`
  });

// serialize(container) === "The value is 42."

Deno

This codebase uses the assertion library from Deno.

MIT License

Copyright © 2020 - Sebastien Filion

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.