Request: Infer generics

[babakness]

The request is to let infer support generics.

Background:

I recently published a recursive Pipe and Compose. It offers some key advantages, including the preservation of variable names.

https://github.com/babakness/pipe-and-compose-types

Its shortcoming is in the case of generics. This is because infer does not work well with generics. Example

/**
 * Extracts function arguments
 */
export type ExtractFunctionArguments < Fn > = Fn extends  ( ...args: infer P ) => any  ? P : never

/**
 * Extracts function return values
 */
export type ExtractFunctionReturnValue<Fn> = Fn extends  ( ...args: any[] ) => infer P  ? P : never

type Foo = ExtractFunctionArguments< <A>(a:A, b:number) => A >
// Foo has type [ {} , number ]

TypeScript 3.4 improved how pipe and compose functions work with generics with higher order type inference from generic functions.

Common implementations of pipe and compose rely on parameter overloading. However, even here, there is an issue with variadic functions. See example in this issue:

#30727

The broader issue with the parameter overloading version of pipe and compose is that parameter names are lost, decreasing code clarity. The recursive version, relying on infer does maintain parameter names.

Providing support generics with infer will present a major productivity bump and will nearly complete the community's need for the common place pipe and compose in FP patterns without compromise.

The only outlier is a small situation with parameter names on variadic functions of a minimum arity of zero as opposed to at least one. Can provide examples if needed since it is a separate issue.

[tani]

I also faced this issue when I proposed new Curring interface to fp-ts (gcanti/fp-ts#843) . I write the following function alias.

export type CurriedN<A extends Array<unknown>, B> = A extends []
  ? () => B
  : A extends [unknown]
  ? ((...args: A) => B)
  : ((...args: A) => unknown) extends ((head: infer H, ...tail: infer T) => unknown)
  ? ((...args: [H]) => CurriedN<T, B>)
  : never

However, this alias drops generics parameter.

According to @gcanti ,

import { FunctionN } from 'fp-ts/lib/function'

export type CurriedN<A extends Array<unknown>, B> = A extends []
  ? () => B
  : A extends [unknown]
  ? ((...args: A) => B)
  : ((...args: A) => unknown) extends ((head: infer H, ...tail: infer T) => unknown)
  ? ((...args: [H]) => CurriedN<T, B>)
  : never

declare function curry2<A extends Array<unknown>, B>(f: FunctionN<A, B>): CurriedN<A, B>

declare function map<A, B>(as: Array<A>, f: (a: A) => B): Array<B>

const mapCurried2 = curry2(map)
// inferred type: mapCurried2: (args_0: {}[]) => (f: (a: {}) => {}) => {}[]

I also request this feature. Thank you.

[tani]

@RyanCavanaugh @babakness I confirm to work my example in v4.1.0-beta! We can close this issue. Cheers 🍻

[babakness]

@RyanCavanaugh @tani Hmm... this isn't working for me using playground for v4.1.0-beta. @tani Can you send me a link or provide instructions on how this is working for you? Otherwise, could we open this again?

https://www.typescriptlang.org/play?ts=4.1.0-beta#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgMQFclsYEIkA5AHgEE5RUkATAZzgCVhNmKAbAJ60oUTAOrEA1kggB3JAD4ANHABCCuAG8AUHDgAKAHTHMUAOasAXHFoBKa6u0BfbdtCRYcGALB4AwoQiCMDMNPSMwCzswqLiUjLyymoaALw2DCBMbHAA2gC6unAA-Aa2cCkajnrW4ZmR2TnxckgFeiX6RibmVjZlFWq2hdYdxoamFjV9Gk3yZRFRBvoAFtzM1sjoaHAAEiqjMJgIfOtIm1BwACpTcDNIg22Lo+M9Odt51wFBITQXKur3cGsSGAADc0K5mMBsHxTHh0MRSOQkHBsIEoAIAEx0DJZaIiMQSJDSZpJdT6dDWIgkMgUMJ-BT2OCfKDBUJ0ekQqEwqBwhE05EAW0wYHZyX0mB6MQJtCSFIMmEm5UqjKl4nUrmwFFY8CFYGZrIx5RRaMx+l1g20mqQ2rg6HQRt1+pCGP0OQAjEoMUoAMzvbR2-QAFjKAHoQzZzIQBZF4BB7d5fHAAORIKMAIzQScQ7Bk8AlrAQZiQmDTfDwMAgcDApkw0dQ5zjXh8eCT4ust2utyTQA

[tani]

@RyanCavanaugh @tani Hmm... this isn't working for me using playground for v4.1.0-beta. @tani Can you send me a link or provide instructions on how this is working for you? Otherwise, could we open this again?

https://www.typescriptlang.org/play?ts=4.1.0-beta#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgMQFclsYEIkA5AHgEE5RUkATAZzgCVhNmKAbAJ60oUTAOrEA1kggB3JAD4ANHABCCuAG8AUHDgAKAHTHMUAOasAXHFoBKa6u0BfbdtCRYcGALB4AwoQiCMDMNPSMwCzswqLiUjLyymoaALw2DCBMbHAA2gC6unAA-Aa2cCkajnrW4ZmR2TnxckgFeiX6RibmVjZlFWq2hdYdxoamFjV9Gk3yZRFRBvoAFtzM1sjoaHAAEiqjMJgIfOtIm1BwACpTcDNIg22Lo+M9Odt51wFBITQXKur3cGsSGAADc0K5mMBsHxTHh0MRSOQkHBsIEoAIAEx0DJZaIiMQSJDSZpJdT6dDWIgkMgUMJ-BT2OCfKDBUJ0ekQqEwqBwhE05EAW0wYHZyX0mB6MQJtCSFIMmEm5UqjKl4nUrmwFFY8CFYGZrIx5RRaMx+l1g20mqQ2rg6HQRt1+pCGP0OQAjEoMUoAMzvbR2-QAFjKAHoQzZzIQBZF4BB7d5fHAAORIKMAIzQScQ7Bk8AlrAQZiQmDTfDwMAgcDApkw0dQ5zjXh8eCT4ust2utyTQA

Thank you, I agree with you. I forget what I do that time. Next time, I keep my reproducible code.