fp-ts extra functions and utilities

Array

groupBy

import { groupBy } from "fp-ts-extras/lib/Array";

assert.deepStrictEqual(groupBy(eqNumber)([1, 1, 2, 3, 3, 4]), [
  [1, 1],
  [2],
  [3, 3],
  [4],
]);

Option

partial

Drop-in replacement of t.partial

Instead of returning A | undefined, it returns Option<A>

import { partial } from "fp-ts-extras/lib/Option";

const B = partial({
  bar: t.number,
});

import * as t from "io-ts";

const User = t.type({
  userId: t.number,
  name: t.string,
});

const PartialUser = t.partial(User.props);

type PartialUser = t.TypeOf<typeof PartialUser>;

// same as
type PartialUser = {
  name: Option<string>;
  age: Option<number>;
};

Function

memPipe

Problem: I have a task and if I call

task();
task();

it fires the internal promise twice. The expected behaviour is it should only fire once. The second call returns the same result as the first one.

Use case: I have 3 tasks: t1, t2, t3. tA is called once t1 and t2 is done. tB is called once t1 and t3 is done. tC is called once t2 and t3 is done. With raw promise I could do

Promise.all([
   Promise.all([t1, t2]).then(_ => tA)
   Promise.all([t2, t3]).then(_ => tC)
   Promise.all([t1, t3]).then(_ => tB)
]).then(...)

But with Task

sequence([
   pipe(sequence([t1, t2])), chain(_ => tA)),
   pipe(sequence([t2, t3]), chain(_ => tC)),
   pipe(sequence([t1, t3]), chain(_ => tB)),
])
...

side effects of t1, t2, t3 will be fired twice each. memPipe will come in handy

import { memPipe } from "fp-ts-extras/lib/Function";

const t1 = memPipe(...)
const t2 = memPipe(...)
const t3 = memPipe(...)

sequence([
   pipe(sequence([t1, t2])), chain(_ => tA)),
   pipe(sequence([t2, t3]), chain(_ => tC)),
   pipe(sequence([t1, t3]), chain(_ => tB)),
])

Each task only performs side effect once.

JSON

encode/decode automatically when you stringify/parse

Why? You can use your preferred algebraic data types in your business logic, and can be converted to traditional json automatically when you do http request or save stuff to db, and vice versa.

const T = t.type({
  foo: optionFromNullable(t.string),
});

const result = stringify(T, {
  foo: O.of("abc"),
});
// right '{"foo":"abc"}'

const result = parse(T, '{"foo":"abc"}');
/*
right {
  foo: O.of("abc"),
}
*/

Record

union

import { union } from "fp-ts-extras/lib/Record";

assert.deepStrictEqual(
  union([
    {
      foo: "foo",
    },
    {
      bar: "bar",
    },
  ]),
  {
    foo: "foo",
    bar: "bar",
  }
);

String

import { split, join } from "fp-ts-extras/lib/String";
import { pipe } from "fp-ts/lib/Function";
import * as assert from "assert";

const result = pipe("a,b,c", split(","), join(":"));

assert.deepStrictEqual(result, "a:b:c");

TaskEither

decode

Decode with error to reduce boilerplate

import { decode } from "fp-ts-extras/lib/TaskEither";
import * as t from "io-ts";
import { pipe } from "fp-ts/lib/Function";

pipe(
  decode(t.string, null)
  // compose with other `TaskEither`s
);

Tuple

Various useful functions on tuples, overloaded on tuple size.

UUID

UUID generator that returns type UUID

Related projects

• https://github.com/gcanti/fp-ts-contrib
• https://github.com/samhh/fp-ts-std