TypeCat

Introduction

TypeCat is a tiny, zero dependency TypeScript library that supplements the missing but very practical types (classes in TypeScript term) you can find in typical functional programming laugages such as Scala or Haskell:

• Option and subclasses Some and None
• Either and subclasses Left and Right

Classical functional operators fmap, applyMap ( applicative map as in Haskell ) and flatMap ( monad map as in Haskell and Scala ).

Iterator is also added so that you can use convenient for expression on the types.

The types and the operators meet the laws of Functor, Applicative and Monad.

Documentation @ Gitbook

undefined and null

It is decided explicitly that Some / Right can't be initiated with undefined and null. TypeError will throwned if tried.

The reason is to simplify and also streamline the functions used for fmap, applicativeMap and flatMap.

get getOrElse, getLeft, getLeftOrElse

TypeCat provides methods to take out value from Option/Either

• get / getOrElse : Option / Either. If None or Left, get will throw exception. Use getOrElse (defaultValue)
• getLeft / getLeftOrElse: Either. If Right, getLeft will throw exception. Use getLeftOrElse(defaultValue)

Usage

Install

yarn add typecat

or

npm install typecat

Use

import { id, Option, Some, None, Either, Left, Right } from 'typecat'

const option = new Some(3).fmap( ( i: number ) => i * 2 ) 

// option = Some( 6 )

console.log( option )

for( const i of option ) 
    console.log( i )  // Console output 6
    
const failed = new Some( 0 ).
    flatMap( ( i: number ) => i === 0 ? new None() : new Some ( 1 / i ) ).
    fmap( (w: number) => `inverse is ${w}` )
    
    // failed = None

console.log(failed)

const succeeded = new Some( 10 ).
    flatMap( ( i: number ) => i === 0 ? new None() : new Some ( 1 / i ) ).
    fmap( (w: number) => `inverse is ${w}` )
    
    // succeeded = Some(0.1)
    
console.log(succeeded)

const either = new Right(25).fmap( ( i: number ) => i > 20 ? true : false )
    .flatMap( ( t: boolean ) => t ? new Right( 'Ok' ) : new Left( 'Fail' ) )
    
    // Either = Right('Ok')
const triplePlusF = (a: number) => (b: number) => (c: number) => a + b + c
const triplePlus = new Some(triplePlusF)
    
const a = new Some(2)
const b = new Some(3)
const c = new Some(4)
const d = new None()

const plusedA = triplePlus.applyMap(a).applyMap(b).applyMap(c) // plusdA = Some( 2 + 3 + 4 )

console.log(plusedA) // plusedA = Some(9)

const ra = plusedA.get() // ra = 9


const plusedB = triplePlus.applyMap(a).applyMap(b).applyMap(d) // plusedB = None()
const rb = plusedB.get() // Exception thrown : Value not exist in None
const rc = plusedB.getOrElse(0) // rc = 0

console.log(ra)
console.log(rb)
console.log(rc)