Subterfuge

A functional Javascript library to use and learn from! The idea is two-fold. A library to make your professional life easier as well as teach you some of the concepts behind functional programming.

Subterfuge will provide you with good documentation in the comments and a documenting test-suite. This means you will be able to implement the behaviour yourself using the documentation and tests as a guide, making sure your own implementation behaves as expected.

Contents:

• Function composition
  • composeRight
  • composeLeft
  • pipeRight
  • pipeLeft
• Container types
  • Box
  • LazyBox
  • Right
  • Left
  • Either
  • fromNullable
• Functionality
  • range

Function composition

• composeRight
• composeLeft
• pipeRight
• pipeLeft

composeRight

Composes two functions from right to left into one new function. This new function takes one or more parameters.

composeLeft

Composes two functions from left to right into one new function. This new function takes one or more parameters.

pipeRight

Composes multiple functions from right to left into one new function. This new function takes one or more parameters.

pipeLeft

Composes multiple functions from left to right into one new function. This new function takes one or more parameters.

Container types

• Box
• LazyBox
• Right
• Left
• Either
• fromNullable

Box

A Box takes a value and boxes it up. On the box you'll be able to use a minimal, Box-specific API which does not care about the value inside the Box. The API is as follows:

• map:
  • takes a function
  • applies the function to the Box value
  • returns a new Box which contains the result
• fold:
  • takes a function
  • applies the function to the Box value
  • returns the result
• inspect (optional):
  • shows you the Box with the value inside

const Box = (value) => ({
  map: (f) => Box(f(value)),
  fold: (f) => f(value),
  inspect: () => `Box(${value})`
});

The Box above is just the simplest form of the concept. Based on what you want to put in and/or want to get back out of it, the implementation differs. Some examples of Boxes are: Left, Right, Maybe, Nothing, LazyBox, ...

LazyBox

Does the same as Box but does it to a function instead of a value. The function passed to the LazyBox will not be executed while mapping over it. Only when you fold the LazyBox, the chained functionality will be executed.

const LazyBox = g => ({
  map: f => LazyBox(() => f(g())),
  fold: (f = x => x) => f(g())
});

Right

Right is like a Box, it has a chain, map, fold and inspect method. Chain applies the function to the value without putting it in a new Right. The map method on Right does the same as the one in Box. Fold takes two functions, an error-handler and a success-handler. It applies the success-handler to the contained value.

const Right = value => ({
  map: func => Right(func(value)),
  fold: (errorhandler, successhandler) => successhandler(value),
  inspect: () => `Right(${value})`
});

Left

Left is like a Box, it has a chain, map, fold and inspect method. Chain on the Left returns itself like nothing happened. The map method on Left does not apply the function to the contained value. Fold takes two functions, an error-handler and a success-handler. It applies the error-handler to the contained value.

const Left = value => ({
  map: func => Left(value),
  fold: (errorhandler, successhandler) => errorhandler(value),
  inspect: () => `Left(${value})`
});

Either

Branches your code to a Right or a Left based on the value it was given. If the value is truthy it will branch to a Right, otherwise it will branch to a Left.

const Either = value => value ? Right(value) : Left(value);

fromNullable

Branches your code to a Right or a Left based on the value it was given. If the value is null or undefined it will branch to a Left passing in null, otherwise it will branch to a Right.

const fromNullable = (value) => value == null ? Left(null) : Right(value);

Functionality

• range

range

Range returns an array containing numbers starting at the first parameter all the way up to, but not including the last parameter. example: range(2, 6) --> [2, 3, 4, 5];

When only one parameter is passed in, creates an array of length equal to the parameter starting at zero. example: range(4) --> [0, 1, 2, 3];