hkt-toolbelt is a collection of type-level utilities that can be mapped and combined in functional ways using higher-kinded types.

Our composable and compile time-efficient types enable users to write expressive and readable type-level code without getting mired in complexity.

Getting Started

Install

> npm install hkt-toolbelt
> yarn add hkt-toolbelt

Import

import { 
  $, $$, $N, 
  Boolean, Conditional, Function,
  List, Object, String, Union,
  Number, Integer, NaturalNumber,
  Kind, Type, Combinator, Parser,
} from "hkt-toolbelt";

Usage

1) $: Apply a Function to an Argument

$<KIND, ARG>

import { $, List, Conditional } from "hkt-toolbelt";

type HelloWorld = $<$<List.Push, "world">, ["hello"]>;  // ["hello", "world"]

type OneTwoThree = $<
  $<List.Filter, $<Conditional.Extends, number>>,
  [1, "foo", 2, 3, "bar"]
>; // [1, 2, 3]

2) $N: Pass Multiple Arguments into an uncurried Function

$N<KIND, [ARG1, ARG2, ...]>

import { $, $N, List, NaturalNumber } from "hkt-toolbelt";

// Example 1: full application (invoked with all three arguments)
type ReduceSum1to5 = $N<List.Reduce, [
  NaturalNumber.Add,  // callback
  0,                  // initial value
  [1, 2, 3, 4, 5]     // target array
]>;  // 15

// Example 2: partial application (invoked with only the first two arguments)
type ReduceSum = $N<List.Reduce, [NaturalNumber.Add, 0]>;
type ReduceSum1to5 = $<ReduceSum, [1, 2, 3, 4, 5]>;  // 15
type ReduceSum1to10 = $<ReduceSum, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]>;  // 55

3) $$: Pipe an Argument through Multiple Functions

$$<[KIND1, KIND2, ...], ARG>

import { $, $$, List, String } from "hkt-toolbelt";

type UnshiftPushJoin = $$<[
    $<List.Unshift, "first">,  // ["first", "second"]
    $<List.Push, "third">,     // ["first", "second", "third"]
    $<String.Join, ", ">,      // "first, second, third"
  ], ["second"]>;