Philip Thanks for your msg (which appears in full below for reference.) | data RecContTest a where | C { showable :: Show a => a } :: RecContTest a This definitely isn't going to work: it declares the 'showable' field to be a function that *requires* a Show context, not one that *provides* a Show context. | type a in function c. My definition of RecContTest is the only | syntactical form I could come up with that GHC accepts. Is there a | syntax for what I want here? Not at the moment, I'm afraid. The problem is mainly to find a decent syntax -- and that problem made me simply omit it altogether. Probably the most plausible possibilities are (A) data RecContTest a where Show a => C { showable :: Show a => a } :: RecContTest a (B) data RecContTest a where C :: Show a => { showable :: Show a => a } -> RecContTest a The latter (B) looks best to me. I dislike (A) because part of the type (the "Show a =>") occurs before the constructor name C, and part appears after. On the other hand, (B) has something that looks vaguely like a type { x::ty, y::ty } -> ty but that's not really valid type syntax. (Mind you, the ! marks in a constructor signature aren't part of valid types either, so maybe it's not so bad to have a special form in constructor declarations.) But if we were going to adopt (B), then even when there is no class context we should really say (B) data RecTest a where B :: { arg :: a } -> RecTest a rather than the current syntax which is (A) data RecTest a where B { arg :: a } :: RecTest a [Note the different placement of the double colon and arrow in (B).] My take on this (B) looks nicer, but it would represent a breaking change But perhaps not many people use record-style syntax + GADT-style syntax And better to make breaking changes sooner than later Question for everyone: * are (A) and (B) the only choices? * do you agree (B) is best Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Philip K.F. Hölzenspies | Sent: 15 June 2009 11:09 | To: glasgow-haskell-users@haskell.org | Subject: GADT record syntax and contexts | | Dear GHCers, | | Maybe this question is more for Haskell-cafe, but since it involves | language extensions as provided by GHC, it seems reasonable to post it | here. | | The GHC user guide lists under 7.4 [1] the extensions to data types and | type synonyms. My question involves 7.4.5 (although the overlap with | 7.4.6 is considerable; maybe the two should be merged). A property of | GADTs that I really appreciate is the fact that constructor contexts are | made available by pattern matching (I don't quite understand why normal | ADTs could do this, btw, but that's another question). The problem I | have now, though, is that there doesn't seem to be a syntax to combine | contexts and records in GADTs. Consider this program: | | data ContTest a where | A :: Show a => a -> ContTest a | | data RecTest a where | B { arg :: a } :: RecTest a | | data RecContTest a where | C { showable :: Show a => a } :: RecContTest a | | a :: ContTest a -> String | a (A a) = show a | | b :: Show a => RecTest a -> String | b = show . arg | | c :: RecContTest a -> String | c (C { showable = x }) = show x | | Both ContTest and RecTest work swimmingly (with functions a and b, | respectively). However, GHC complains that it can not deduce Show for | type a in function c. My definition of RecContTest is the only | syntactical form I could come up with that GHC accepts. Is there a | syntax for what I want here? | | Regards, | Philip | | | | PS. As a side-note. In section 7.4.5, the manual states that: | | "At the moment, record updates are not yet possible with GADT-style | declarations, so support is limited to record construction, selection | and pattern matching." | | However, if I add: | | update org@(B {}) x = org { arg = x } | | and evaluate "b $ update (B True) False" the response is "False" as one | would expect. Is this line in the user guide outdated, or is support for | updates unstable or partial? | | | [1]http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions...