type-ts

Typescript Type-level playground

This project is a playground of type-level operations for TypeScript.

Installation

npm i type-ts

TypeScript compatibility

The stable version is tested against TypeScript 3.1.6+.

API

Logic

Extends<A, B>

Returns true if A extends from B, false otherwise.

Extends<'foo', string> // true
Extends<string, number> // false

Equals<A, B>

Returns true if A and B are equal types, false otherwise.

Equals<string, string> // true
Equals<string, number> // false

And<A, B>

Returns true if A and B are equal types to true, false otherwise.

And<true, true> // true
And<true, false> // false

Or<A, B>

Returns true if either A or B are true, false otherwise.

Or<false, true> // true
Or<false, false> // false

Xor<A, B>

Returns true if A and B differ, one is true and the other is false. false otherwise.

Xor<false, true> // true
Xor<true, true> // false

Not<A>

Returns true if A is false. false otherwise.

Not<false> // true
Not<true> // false

If<C, T, E>

Returns T if C is true. E otherwise.

If<true, number, string> // number
If<false, number, string> // string

Newtype

Newtype<T, NAME>

Same representation, different type for the typesystem.

type USD = Newtype<number, "USD">;
type EUR = Newtype<number, "EUR">;

const usd = 10 as USD;
const eur = 10 as EUR;

function gross(net: USD, tax: USD): USD {
  return (net + tax) as USD; // We can treat them like the underlying type
}

gross(usd, usd); // $ExpectType Newtype<number, "USD">
// $ExpectError
gross(eur, usd); // Type '"EUR"' is not assignable to type '"USD"'.
// $ExpectError
gross(1, 1.2); // Type 'number' is not assignable to type '"USD"'.

Type

Omit<A, K>

Extracts a super-type of A identified by its keys K.

Omit<{ a: string; b: number }, 'a'> // { b: number }

Overwrite<A, B>

Overwrites A with the properties in B.

Overwrite<{ a: string; b: number }, { b: boolean }> // { a: string; b: boolean }

Diff<A, K>

Make the specified properties K partial in type A.

Diff<{ a: string; b: number }, 'b'> // { a: string; b?: number }

Lacks<A, K>

Encodes the constraint that a given object A does not contain specific keys K

declare function f(x: Lacks<{ a: string; b: number }, "a">): void;
// $ExpectError
f({ a: "foo", b: 1 });

Exact<A>

Encodes that A cannot have extra propeties.

type E = Exact<{ a: string; b: number }>;
declare function f(a: E): void;
declare const x: { a: string };
declare const y: { a: string; b: number };
declare const z: { a: string; b: number; c: boolean };
f(x); // $ExpectError
f(y); // Ok
f(z); // $ExpectError

KeysOfType<A, B>

Picks from A only the keys of a certain type B.

KeysOfType<{a: string, b: string | boolean, c: boolean, d: string}, string> // "a" | "d"

DeepReadonly<A>, DeepMutable<A>, DeepRequired<A>, DeepPartial<A>

Recursively sets modifiers readonly and ?.

interface A {
  b: {
    c: string;
    d: Array<{ e: number }>;
  };
}
type ReadonlyA = DeepReadonly<A>;
declare const a: ReadonlyA;
a.b.d[1].e = 1; // $ExpectError

Intersection<A, B>

Extracts from A only the properties found on B.

Intersection<{ a: number; b: number }, { b: number; c: number }> // { b: number }

ValuesOf<A>

Returns the union of all value types from A.

ValuesOf<{ a: number; b: boolean; c: string; d: number }> // number | boolean | string

SetIntersection<A, B>

Intersection of the literal union sets A and B.

SetIntersection<"a" | "b", "c" | "d"> // never
SetIntersection<"a" | "b", "b" | "c"> // "b"
SetIntersection<"a" | "b" | "c" , "b" | "c"> // "b" | "c"

SetDifference<A, B>

Difference of the literal union sets A and B.

SetDifference<"a" | "b", "c"> // "a" | "b"
SetDifference<"a" | "b", "b"> // "a"

SetUnion<A, B>

Union of the literal union sets A and B.

SetUnion<"a" | "b", "b" | "c"> // "a" | "b" | "c"

Pretty<A>

Display purpouse only. Useful when dealing with complex types and want to see something simpler in the IDE.

Pretty<{ a: 1 } & { b: 2 }> // { a: 1, b: 2 }

NoInfer<A>

Use to prevent a usage of type T from being inferred in other generics. See

JSONValue

Represents JSON representable types.

Awaited<A>

Unwraps nested Promises. Typescript does not collapse nested promises. See

type x = Awaited<number>; // Promise<number>
type y = Awaited<Promise<Promise<Promise<string>>>>; // Promise<string>
type z = Awaited<Awaited<Awaited<boolean>>>; // Promise<boolean>

Number

Peano numbers implementation.

Zero

Zero

Succ<N>

Succ<Zero> // _1

Digit = _0 | _1 | _2 | _3 | _4 | _5 | _6 | _7 | _8 | _9

Predefined aliases.

Add<A, B>

Add<_4, _3> // _7
Add<_5, _4> // _9

Substract<A, B>

Substract<_0, _1> // never
Substract<_4, _3> // _1
Substract<_5, _2> // _3

Multiply<A, B>

Multiply<_2, _3> // _6
Multiply<_3, _3> // _9

Quotient<A, B>

Quotient<_4, _0> // never
Quotient<_3, _3> // _1
Quotient<_0, _6> // _0
Quotient<_4, _2> // _2
Quotient<_6, _5> // _1

Quotient<A, B>

Quotient<_3, _3> // _0
Quotient<_5, _3> // _2
Quotient<_4, _2> // _0
Quotient<_6, _5> // _1

GreaterThan<A, B>

GreaterThan<_1, _0> // true
GreaterThan<_5, _2> // true
GreaterThan<_0, _0> // false
GreaterThan<_0, _1> // false

LowerThan<A, B>

LowerThan<_1, _0> // false
LowerThan<_5, _2> // false
LowerThan<_0, _0> // false
LowerThan<_0, _1> // true

EqualsTo<A, B>

EqualsTo<_0, _1> // false
EqualsTo<_2, _2> // true

AsNat<N>

Convert a number to a Nat. Do not use with negative numbers.

AsNat<2> // _2 or Succ<Succ<_0>>

AsNumbe<N>

Convert a Nat to a number.

AsNumber<_2> // 2

List

Tuple<A>

Non empty tuple.

Nil

Empty tuple.

StaticArray<A>

Compile time arrays (aka array literals).

AnyStaticArray

Alias for StaticArray<any>.

Head<A>

Returns the first type of the static array.

Head<[]> // never
Head<[1, 2, 3]> // 1
Head<[number, 2, 3]> // number

Tail<A>

Tail of the static array.

Tail<[]> // never
Tail<[1, 2]> // [2]
Tail<[1, 2, 3]> // [2, 3]

Last<A>

Returns the last type of the static array.

Last<[]> // never
Last<[1, 2]> // 2
Last<[1, 2, 3]> // 3

Cons<H, L>

Adds a type to the front of the static array.

Cons<1, []> // [1]
Cons<1, Cons<2, []>> // [1, 2]
Cons<1, [2, 3]> // [1, 2, 3]

Drop<A, N>

Drops N elements from the front of the static array.

Drop<[], _2> // []
Drop<[1, 2], _1> // [2]
Drop<[1, 2, 3], _2> // [3]

Reverse<A>

Reverses a static array.

Reverse<[]> // []
Reverse<[1, 2]> // [2, 1]
Reverse<[1, 2, 3]> // [3, 2, 1]

Take<A, N>

Return the first N types of the static array.

Take<[], _2> // []
Take<[1, 2], _1> // [1]
Take<[1, 2, 3], _2> // [1, 2]

Concat<A, B>

Concatenates two static arrays.

Concat<[], []> // []
Concat<[], [1]> // [1]
Concat<[2], [1]> // [2, 1]
Concat<[1, 2], [3]> // [1, 2, 3]
Concat<[1], [2, 3]> // [1, 2, 3]

Zip<A, B>

Zips two static arrays into one with length equals to the shortest length where at each position there is a tuple with the types of each one of them.

Zip<[], []> // []
Zip<[], [1]> // []
Zip<[2], [1]> // [[2, 1]]
Zip<[1, 2], [3]> // [[1, 3]]
Zip<[1], [2, 3]> // [[1, 2]]
Zip<[1, 2], [3, 4]> // [[1, 3], [2, 4]]

Repeat<A, N>

Returns a static array of length N with A in each position.

Repeat<0, _0> // []
Repeat<0, _1> // [0]
Repeat<0, _2> // [0, 0]
Repeat<0, _3> // [0, 0, 0]

Length<A>

Returns the length of a static array.

Length<[]> // 0
Length<[1]> // 1>
Length<[1, 2]> // 2
Length<[1, 2, 3]> // 3

type _10 = Add<_9, _1>;
type _20 = Multiply<_10, _2>;
Length<Repeat<0, _20>> // 20

LengthN<A>

Same as Length but returns a Nat instead.

LengthN<[]> // _0
LengthN<[1]> // _1
LengthN<[1, 2]> // _2
LengthN<[1, 2, 3]> // _3
LengthN<Repeat<0, _20>> // _20

Push<A, B>

Adds a type to the end of a static array.

Push<1, []> // [1]
Push<1, Cons<2, []>> // [2, 1]
Push<1, [3, 2]> // [3, 2, 1]

Map<A, B> and Reduce<A, B>

Type-level version of this operations.

declare module "type-ts/list" {
  interface Map1<A = unknown> {
    AsNat: A extends number ? AsNat<A> : never;
  }

  interface Reduce2<A = unknown, B = unknown> {
    Add: A extends Nat ? (B extends Nat ? Add<A, B> : never) : never;
  }
}

Map<[0, 1, 2], "AsNat"> // [_0, _1, _2]
Reduce<[_1, _2, _3] "Add", _0> // _6

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