Support for SML/NJ curried functor syntax

[hyt658]

Problem

SML/NJ accepts curried functor parameter syntax, for example:

functor F(M: ORD_MAP) (S: ORD_SET) :>
sig
    val func: string list -> S.set M.map
end =
    (* ... *)
end

However, this triggers a parse error: expected '=' at the second parameter group (S : ...). It looks like Millet parses functor F (M : ...) and then expects = / : ... / :> ..., so the extra (S : ...) is rejected and causes cascading errors.

Solution

The curried functor parameter is the extention syntax in SML/NJ. It would be very helpful if Millet could support (or provide a mode for) this.

[azdavis]

we'd want to put this behind an extension configurable in the millet.toml, as we've done for other extensions like successor ml.

the semantics of this would probably be make an anonymous signature that takes both things and opens them in the functor body. we'd also have to handle curried applications which might be a bit harder since you can partially apply the outer functor.

i might take a look at some point