@ts-common/fun.ts

fun.ts

Purely functional subset of JavaScripts/TypeScript.

There are a lot of pure functional languages that can be compiled to JavaScript. Usually, the biggest problem with these libraries is interoperability. For example, if you have a big project written on JavaScript, it's very challenging to rewrite parts of this project step-by-step using another language. https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS#static-typing.

The project is not another functional language that can be compiled into JavaScript. The project tries to define a subset of JavaScript that can be formally verified.

The subset can be used as a safe script or as a target platform for other programming languages.

Roadmap

Potential Targets and Applications

• Markdown safe script that can be run in a browser (for example http://madoko.org/reference.html),
• query languages,
• distributed systems, like ALIQ, machine learning, AI,
• as a target platform for other functional languages.
• the subset can be recognized by browser and compiled into more optimal code (similar to asm.js).

Wish List

• All data is immutable.

• Pure functions, without side-effects.

• Strong structural typing.

• Type inference.

• Compatibility with JavaScript and TypeScript.

  • write/read .d.ts files.
  • the subset should be valid JavaScript or TypeScript. So no need for additional transpilers, we only need a validator.

• The language validator should be written on JavaScript/TypeScript so it can run in a browser.

• no implicit type conversions. For example ?: should only accept bool type.

• Type system should allow to describe monads. Example on pseudo-TypeScript

  type MonadStrategy = {
      type Monad<T>; // this line can't be compiled in TypeScript.
      readonly just: <T>(v: T) => Monad<T>
      readonly join: <T>(m: Monad<Monad<T>>) => Monad<T>
  }
  
  // Or
  type MonadStrategy = {
      type Monad { type T }; // this line can't be compiled in TypeScript.
      readonly just: <T>(v: T) => Monad { T }
      readonly join: <T>(m: Monad { T: Monad { T } }) => Monad { T }
  }

• Type system should be able to reflect JSON-Schema.

Typing

Typing requires a languages extension. Several safe options are

• embed typing in comments.
• embed typing in a separate file.
• typing is based on special run-time definitions, similar to Json-Schema. For example const MyType = { type: 'string', ... }.

Possible typing languages are

• TypeScript,
• JS docs,
• JSON-Schema,
• Some kind of Haskell type notations?

Proposed Typing

• JavaScript. Starts with //: or /*:

  const myFunc
      //: (_: number) => string
      = v => v.toString()

  const myFunc /*: (_: number) => string */ = v => v.toString()

  Simplified types (incompatable with TypeScript).

  const myFunc
      //: number => string
      = v = v.toString()

  //type MyType = ...

  /*type MyType = {
  
  }*/

• TypeScript

  const myFunc
      : (_: number) => string
      = v => v.toString()

Notes

Use hasOwnProperty() to check if we can read such properties as constructor. Incorrect code:

const m = x.constructor
// or
const { constructor } = x

Correct code:

const m = Object.prototype.hasOwnProperty.call(x, 'constructor') ? x.constructor : undefined

References

• https://en.wikipedia.org/wiki/Hindley–Milner_type_system
• https://en.wikipedia.org/wiki/Type_class
• https://en.wikipedia.org/wiki/Structural_type_system
• https://webkit.org/blog/6240/ecmascript-6-proper-tail-calls-in-webkit/
• http://dippl.org/chapters/02-webppl.html
• https://github.com/gcanti/fp-ts
• https://github.com/jonaskello/tslint-immutable

Languages

• ELM
• Koka
• PureScript

ECMAScript Proposals

• https://github.com/tc39/proposal-pattern-matching
• https://github.com/tc39/proposal-pipeline-operator
• https://github.com/tc39/proposal-partial-application
• https://github.com/tc39/proposal-decorators