fantasyland/fantasy-land

★ 10,235JavaScriptAudience · developerComplexity · 4/5Setup · hard

Mindmap

mindmap
  root((fantasy-land))
    What it is
      Written specification
      Not a runtime library
      Compliance test suite
    Key structures
      Functor
      Monad
      Applicative
      Semigroup
    Rules defined
      Required method names
      Algebraic laws
      Associativity checks
    Ecosystem
      Ramda
      Sanctuary
      Custom data types

mindmap root((fantasy-land)) What it is Written specification Not a runtime library Compliance test suite Key structures Functor Monad Applicative Semigroup Rules defined Required method names Algebraic laws Associativity checks Ecosystem Ramda Sanctuary Custom data types

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Things people build with this

USE CASE 1

Build a custom JavaScript data type that works out of the box with functional libraries like Ramda or Sanctuary without any glue code.

USE CASE 2

Verify that your library's Functor or Monad implementation follows the correct algebraic laws using the included test suite.

USE CASE 3

Learn what methods like concat, map, and chain must do so that functional code can compose cleanly across different libraries.

Tech stack

JavaScript

Getting it running

Difficulty · hard Time to first run · 1day+

This is a specification document, not installable software, meaningful use requires solid familiarity with functional programming and abstract algebra concepts.

In plain English

Fantasy Land is not a library you install and run. It is a written specification, a document that defines how certain kinds of data structures in JavaScript should behave so that they can work together without conflicts. Libraries and developers who follow the spec can share code freely because they agree on the rules each structure must follow. The structures defined in the spec come from a branch of mathematics called abstract algebra. Names like Functor, Monad, Applicative, and Semigroup describe patterns for how data can be transformed, combined, or chained. If you have used the array map method in JavaScript, you have used something that fits the Functor pattern. Fantasy Land formalizes those patterns so that any library implementing them correctly can plug into tools built for any other Fantasy Land-compatible library. The specification works by defining what methods an object must have and what rules those methods must obey. For example, a Semigroup must have a concat method, and that method must satisfy the associativity law: combining A with the combination of B and C must give the same result as combining the combination of A and B with C. The document lists these laws precisely for each structure. The practical benefit is that functional programming libraries in JavaScript, such as Ramda, Sanctuary, and others, can interoperate when they follow the spec. If your custom data type implements the right methods correctly, it will work with any tool built around Fantasy Land without any special adapter code. The repository contains the specification text itself and a test suite for verifying compliance. It does not ship runtime code that you call in your application. The full README is longer than what was shown.

Copy-paste prompts

Prompt 1

Show me a minimal JavaScript class that implements the Fantasy Land Functor spec so it works with Ramda's map function.

Prompt 2

How do I implement the Fantasy Land Semigroup spec for a custom data type and verify it satisfies the associativity law?

Prompt 3

Write a Fantasy Land-compliant Monad in JavaScript and show how to chain two operations together using Sanctuary.

Prompt 4

What is the practical difference between Fantasy Land's Applicative and Monad, and when should I implement one instead of the other?

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