I'm not sure if I'm missing something, but my adolescent naivety in frontend matters would try to reach for

MkRec { field = \@a -> ... }

and I hope that will do the right thing. Indeed, I interpret your proposed `field @a = ...` as much the same.

------ Originalnachricht ------

Von: "Richard Eisenberg" <lists@richarde.dev>

An: "Erdi, Gergo via ghc-devs" <ghc-devs@haskell.org>

Gesendet: 16.05.2022 21:09:33

Betreff: ambiguous record field (but not *that* kind of ambiguous record field)

Hi all,

On a project I'm working on, I wish to declare something like

data Rec = MkRec { field :: forall a. SomeConstraint a => ... }

where the ... contains no mention of `a`.

Even with https://github.com/ghc-proposals/ghc-proposals/pull/448, I think there is no way to avoid the ambiguity when setting `field`. Is that correct? If so, what shall we do about it? The natural answer is somehow to write ... MkRec { field @a = ... } ... but that would break significant new syntactic ground. (Maybe it's good new syntactic ground, but it would still be very new.)

Thanks,
Richard