Manual partial application of optional labelled args?

1

I’m not sure why this doesn’t work, could it be a compiler bug?

https://rescript-lang.org/try?code=DYUwLgBAhgJjEF4IAoB+UEH4A0FUCMtdkBKExAPgCgJoBaCgIRGDADoB5ABzAEsB7AHZsA5uADqvMAAsAIiABmUAK6tkABnIBqCPgbNWnHgOFiwkmfKWqwGklSqhIsGAFZEKAkRRlK0OGgYrrgExGRUQA

I could define add5 as add(~a=5) but I’ve heard some talk about this feature being removed in the future so would ideally want to avoid it.

2

You need to use explicitly passed optional.

This works:

let add = (~a=?, ~b=?, ()) =>
  a->Belt.Option.getWithDefault(0) + b->Belt.Option.getWithDefault(0)

let add5 = (~b=?, ()) => add(~a=5, ~b?, ()) // Notice the ~b?

What’s happening is that add's ~b parameter takes an int, and add5's ~b parameter is option<int>, thus the type error. By changing ~b=b (or ~b with punning) to ~b=?b (or ~b? with punning) you tell the compiler to not automatically wrap the value in an option when it passes to add, so you can now just use a plain option value.

1 Like
3

Wow, that’s confusing! Thanks :pray: