S.Length<S extends String>

[MattRCole]

🍩 Feature Request

Is your feature request related to a problem?

It is extremely difficult to derive the length of a string using typescript typings.

Unlike the relatively simple way one can get the length of an array:

type Length<T extends Array> = T['length']

type Test = Length<['a', 'b', 'c']> // Test = 3

The length of a string is very hard to determine

type Length<S extends String> = S['length']

type Test = Length<'abc'> // Test = number

Describe the solution you'd like

A first stab at a solution:

type EmptyString = ''
type NonEmptyString<S extends string> = S extends EmptyString
  ? never
  : S

type SingleChar<S extends string> = S extends `${NonEmptyString<infer X>}${NonEmptyString<infer Y>}`
  ? false
  : true

type Head<S extends string> =
  S extends `${infer X}${string}`
  ? SingleChar<X> extends true
    ? X
    : never
  : never

type Tail<S extends string> =
  S extends `${infer X}${infer Tail}`
  ? SingleChar<X> extends true
    ? Tail
    : never
  : never

type HasTail<S extends string> =
  Tail<S> extends EmptyString
  ? false
  : true

type __Length<S extends string, I extends any[] = []> = {
  0: __Length<Tail<S>, Next<I>>
  1: L.Length<I>
}[
  Tail<S> extends EmptyString
  ? 1
  : 0
]

type _Length<S extends string> =
  S extends EmptyString
  ? 0
  : __Length<S> extends infer X
    ? Increment<Cast<X, number>> // Increment here would probably be something like N.Plus<`${Cast<X, number>}`, '1', 'n'>
    : never

type Length<S extends string> =
  _Length<S> extends infer X
  ? Cast<X, number>
  : never

type Test0 = Length<'abc'> // Test0 = 3
type Test1 = Length<''>       // Test1 = 0

This type might be too "beefy" though and could probably be cut down. It also requires the use of TS 4.1.

Describe alternatives you've considered

I've considered putting in a pull request, but the proposed solution probably needs a lot work, so I've made this feature suggestion instead.

Teachability, Documentation, Adoption, Migration Strategy

A function description would probably be sufficient:

Returns the length of S as a Number

Edit: Clarity

[millsp]

Hey @MattRCole,

I would be all in for such a utility. You had a good practise there with the type system there, well done! But I think that we can indeed do shorter by shipping a Split type that will split on '' empty strings and then we can retrieve the length from there - just to keep it all more generic.

[stale[bot]]

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