[explicit-function-return-type] request option for curried arrow functions

[skylize]

Proposal

Additional option for explicit-function-return-type ignores all but the final returned function when multiple fat-arrows appear in a function expression before a shared body.

{
  "rules": {
    "@typescript-eslint/explicit-function-return-type": [
      "error",
      {
        "allowCurrying": true
      }
    ]
  }
}

Purpose

In functional programming, currying is a common pattern. With this pattern a function taking multiple parameters is instead modeled as a unary (1-parameter) function that returns a unary function that returns a unary function, .etc. With arrow functions, you can express this quite elegantly, but the intermediate return types are (usually) not of meaningful importance.

Example

Good: Explicit return given for last function returned by expression.

const foo = ( bar: string ) => ( baz: string ) => ( buz: string ) => ( biz: string ) : string =>
    `${ bar } ${ baz } ${ buz } ${ biz }`

const fiz = foo( 'bar' )( 'baz' )( 'buz' )( 'biz' )

Bad: Applied all parameters, but still failed to express a return type.

const badFoo = ( bar: string ) => ( baz: string ) => ( buz: string ) => ( biz: string ) =>
    `${ bar } ${ baz } ${ buz } ${ biz }`

Note that in the example, there are technically 4 different functions in the foo expression. But from a conceptual level, you can think of it as 1 function that must be called 4 times to apply all the parameters.

The type of foo( 'bar' ) is "foo with the first parameter fixed as 'bar'". Trying to express that explicitly through the type system is a painful exercise in meaningless verbosity.

However, once all the parameters are given, it still makes sense to enforce an explicit return type for the final returned value.

[bradzacher]

good idea and pretty easy to implement!

[lbssousa]

Any news about this issue?

[bradzacher]

Accepting PRs from the community (we love new contributors!!)
Otherwise will get done eventually - we're only a few volunteers (fixes take priority over features when I'm picking new work).

[Svish]

I think I actually managed to implement it! 😳 #538 🎉