chain-rec

Chain-rec instances and utilities for fp-ts

   

     

   

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Welcome

ChainRec is a typeclass that models tail-recursion in higher kinded types. However, fp-ts only exports instances for Either, Array, ReadonlyArray, IO, Tuple, and ReadonlyTuple. ChainRec can be particularly useful for effectful types such as Task, Reader, and State which is the purpose of this library, and to export various utilities from purescript-tailrec.

Simple Example

This example constructs a program in the StateIO monad and will print the numbers 1 through 10 to the console.

Imports

This example uses the following imports:

import * as ChnRec from '@jacob-alford/chain-rec/ChainRec'
import { ChainRec } from '@jacob-alford/chain-rec/StateIO'
import * as Cons from 'fp-ts/Console'
import { pipe } from 'fp-ts/function'
import * as O from 'fp-ts/Option'
import * as SIO from 'fp-ts-contrib/StateIO'

Construction

1. SIO.get<number>() - The program takes number as input
2. SIO.chainFirst(n => SIO.fromIO(Cons.log(n))) - Prints the current number to the console
3. SIO.map(O.fromPredicate(n => n >= 10)) - Returns none if the number is less than 10
4. SIO.chainFirst(() => SIO.modify(n => n + 1)) - Increments the number by 1

const program = pipe(
  SIO.get<number>(),
  SIO.chainFirst(n => SIO.fromIO(Cons.log(n))),
  SIO.map(O.fromPredicate(n => n >= 10)),
  SIO.chainFirst(() => SIO.modify(n => n + 1)),
)

const runProgram = ChnRec.untilSome(ChainRec)(program)

runProgram(0)()

// prints: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Installation

Yarn

yarn add @jacob-alford/chain-rec

NPM

npm install @jacob-alford/chain-rec

PNPM

pnpm add @jacob-alford/chain-rec