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Static Land

This is a specification for common algebraic structures in JavaScript based on Fantasy Land.

Difference from Fantasy Land

Fantasy Land uses methods to define interfaces that a type must implement in order to support a particular Algebra. For example values of a type that implements the Monoid algebra must have fantasy-land/empty and fantasy-land/concat methods on them.

Static Land takes a different approach. Instead of methods, we use static functions, that are grouped together in modules.

For example, here is an Addition module that uses numbers as values and satisfies the Monoid algebra requirements:

const Addition = {

  empty() {
    return 0
  },

  concat(a, b) {
    return a + b
  },

}

Pros

  • No name clashes. Since a module is just a collection of functions that don't share any namespace we don't have problems with name clashes.
  • We can implement many modules for one type, therefore we can have more than one instance of the same Algebra for a single type. For example, we can implement two Monoids for numbers: Addition and Multiplication.
  • We can implement modules that work with built-in types as values (Number, Boolean, Array, etc).

Cons

  • We have to pass around modules when we write generic code. In Fantasy Land most of generic code can be written using only methods, only if we need methods like of or empty we might need to pass the type representative. (This can be fixed!)

How to add compatibility with Static Land to your library

Simply expose a module that works with types that your library provides or with types defined in another library or with native types like Array.

Modules don't have to be simple JavaScript objects; they can also be constructors if desired. The only requirements are:

  • this object contains some static methods from Static Land; and
  • if it contains a method with one of the names that Static Land reserves, that method must be the Static Land method (obey laws etc).

Example 1. Static Land module for Array

const SArray = {

  of(x) {
    return [x]
  },

  map(fn, arr) {
    return arr.map(fn)
  },

  chain(fn, arr) {
    // ...
  },

}

export {SArray}

Example 2. Static Land module as a Class

class MyType {

  constructor() {
    // ...
  }

  someInstanceMethod() {
    // ...
  }

  static someNonStaticLandStaticMethod() {
    // ...
  }


  // Static Land methods

  static of(x) {
    // ...
  }

  static map(fn, value) {
    // ...
  }

}

export {MyType}

Example 3. Static Land module as ECMAScript modules

// mytype.js

// Static Land methods

export function of(x) {
  // ...
}

export function map(fn, value) {
  // ...
}

Import as

import * as MyType from "./mytype" // MyType is now a Static Land module

Compatible libraries

We have a list in the wiki. Feel free to add your library there.