allow partially specified type arguments

[zpdDG4gta8XKpMCd]

Currently when it comes to using generics a developer has 2 options:

• to specify all type arguments
• to omit all of them altogether

I am a big fan of the latter but it doesn't always work because of inference failure. Quite often it only takes one explicitly specified type argument to help the inference algorithm to deduce the rest of the types, but there is no such option to specify just one.

function fold<Values, Value, Cursor, Direction, Hint, Result>(
   values: Values,
   result: Result,
   start: (values: Values) => Cursor | void,
   advance: (cursor: Cursor) => Cursor | void,
   valueAt: (cursor: Cursor) => Value & Direction,
   hintAt: (cursor: Cursor) => Hint
   folding: (result: Result, value: Value & Direction, hint: Hint) => Result
) : Result {
return undefined;
}

Example. Almost everything is inferred but the Direction type parameter, which is specified explicitly via hypothetical syntax:

let result = fold<,,, AsTowardsLatest,,>(values, result, start, advance, valueAt, hintAt, folding);

Alternatively named arguments might be considered (hypothetical syntax)

let result = fold<Direction = AsTowardsLatest>(values, result, start, advance, valueAt, hintAt, folding);

[mhegazy]

This looks confusing, with little value. #2175 should be the way to go.