How to work with non-functional code using fp-ts

Sometimes you are forced to interoperate with code not written in a functional style, let’s see how to deal with it.

Sentinels

Use case
an API that may fail and returns a special value of the codomain.
Example
Array.prototype.findIndex
Solution
Option
import { Option, none, some } from 'fp-ts/lib/Option'

function findIndex<A>(as: Array<A>, predicate: (a: A) => boolean): Option<number> {
  const index = as.findIndex(predicate)
  return index === -1 ? none : some(index)
}

undefined and null

Use case
an API that may fail and returns undefined (or null).
Example
Array.prototype.find
Solution
Option, fromNullable
import { Option, fromNullable } from 'fp-ts/lib/Option'

function find<A>(as: Array<A>, predicate: (a: A) => boolean): Option<A> {
  return fromNullable(as.find(predicate))
}

Exceptions

Use case
an API that may throw.
Example
JSON.parse
Solution
Either, tryCatch
import { Either, tryCatch } from 'fp-ts/lib/Either'

function parse(s: string): Either<Error, unknown> {
  return tryCatch(() => JSON.parse(s), reason => new Error(String(reason)))
}

Random values

Use case
an API that returns a non deterministic value.
Example
Math.random
Solution
IO
import { IO } from 'fp-ts/lib/IO'

const random: IO<number> = () => Math.random()

Synchronous side effects

Use case
an API that reads and/or writes to a global state.
Example
localStorage.getItem
Solution
IO
import { Option, fromNullable } from 'fp-ts/lib/Option'
import { IO } from 'fp-ts/lib/IO'

function getItem(key: string): IO<Option<string>> {
  return () => fromNullable(localStorage.getItem(key))
}
Use case
an API that reads and/or writes to a global state and may throw.
Example
readFileSync
Solution
IOEither, tryCatch
import * as fs from 'fs'
import { IOEither, tryCatch } from 'fp-ts/lib/IOEither'

function readFileSync(path: string): IOEither<Error, string> {
  return tryCatch(() => fs.readFileSync(path, 'utf8'), reason => new Error(String(reason)))
}

Asynchronous side effects

Use case
an API that performs an asynchronous computation.
Example
reading from standard input
Solution
Task
const read: Task<string> = () =>
  new Promise<string>(resolve => {
    const rl = createInterface({
      input: process.stdin,
      output: process.stdout
    })
    rl.question('', answer => {
      rl.close()
      resolve(answer)
    })
  })
Use case
an API that performs an asynchronous computation and may reject.
Example
fetch
Solution
TaskEither, tryCatch
import { TaskEither, tryCatch } from 'fp-ts/lib/TaskEither'

function get(url: string): TaskEither<Error, string> {
  return tryCatch(() => fetch(url).then(res => res.text()), reason => new Error(String(reason)))
}

This content was originally published as a blog post on February 12, 2019.