Hi Richard, I'm not sure if I'm missing something, but my adolescent naivety in frontend matters would try to reach for https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0155-ty... and write MkRec { field = \@a -> ... } and I hope that will do the right thing. Indeed, I interpret your proposed `field @a = ...` as much the same. Sebastian ------ Originalnachricht ------ Von: "Richard Eisenberg" An: "Erdi, Gergo via ghc-devs" 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

Hi, Am Montag, dem 16.05.2022 um 19:09 +0000 schrieb Richard Eisenberg:

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.)

I’m probably revealing ignorance of #448 this way, but why would MkRec { field = \@a -> ...} not work with -XTypeAbstractions Cheers, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/