Evan, Lennart Thanks for the provocation. I've committed a patch that fixes all these issues. Try now! Simon Thu Aug 20 13:34:43 BST 2009 simonpj@microsoft.com * Improvements to record puns, wildcards * Make C { A.a } work with punning, expanding to C { A.a = a } * Make it so that, with -fwarn-unused-matches, f (C {..}) = x does not complain about the bindings introduced by the "..". * Make -XRecordWildCards implies -XDisambiguateRecordFields. * Overall refactoring of RnPat, which had become very crufty. In particular, there is now a monad, CpsRn, private to RnPat, which deals with the cps-style plumbing. This is why so many lines of RnPat have changed. * Refactor the treatment of renaming of record fields into two passes - rnHsRecFields1, used both for patterns and expressions, which expands puns, wild-cards - a local renamer in RnPat for fields in patterns - a local renamer in RnExpr for fields in construction and update This make it all MUCH easier to understand * Improve documentation of record puns, wildcards, and disambiguation | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Evan Laforge | Sent: 12 August 2009 23:59 | To: Simon Peyton-Jones | Cc: Augustsson, Lennart; haskell; GHC users | Subject: Re: [Haskell-cafe] generalize RecordPuns and RecordWildCards to work with | qualified names? | | > | Even is suggesting that instead of reporting an error, in the second | > | case we could use the translation: | > | | > |   f (A.A { A.a }) = a   -->   f (A.A { A.a = a }) | > | | > | (i.e., when punning occurs with a qualified name, use just the | > | unqualified part of the name in the pattern) | > | > Yes, that'd be possible.   But it seems debatable -- it doesn't *look* as if the | pattern (A.A { A.a }) binds 'a' -- and it seems even less desirable in record | construction and update.  To be concrete, would you expect these to work too? | > | >  g a = A.A { A.a }     -->    g a = A.A { A.a = a } | >  h x a = x { A.a }     -->    h x a = a { A.a = a } | | Oh, I didn't realize that record punning included construction as | well. Yeah, that's a little funky looking. I don't mind seeing the | binding form and I think a new reader could figure it out without too | much trouble but I would be a little confused by the construction form | and think a new reader would also be confused. | | > With -XDisambiguateRecordFields you could say | > | >  g a = A.A { a } | > | > which seems better.  (But there's no help for record update, since we don't know | which data constructor is involved.) | | I didn't know about DisambiguateRecordFields! Looks like that also | makes the wildcard work like I want it to. | | The ghc docs for DisambiguateRecordFields don't make this very clear | to me... it talks about disambiguating names in scope, but if I say | "R.R { a = val}" I wouldn't expect it to "disambiguate" 'a', which is | not in scope at all, to 'R.a' which looks like a completely different | name. Rereading the paragraph at 7.3.11 I'm still surprised this | works. Maybe add something like: | | ... preceeding docs ... | | This also means that if you use qualified imports you can still use | unqualified field names. E.g. in the pattern @(R.R { a = a_val })@, | @a@ will be disambiguated to @R.a@, even if @R@ is imported qualified. | | I gather we're not supposed to call them "records" anymore, they're | supposed to be something I forget now, but the rest of the ghc docs | says records, so... | | > So my current conclusion is: improve the error message, perhaps suggesting the | flag -XDismabiguateRecordFields, but don't add the change you suggest. | > | > Comments? | | Sounds good to me. I'll try adding DisambiguateRecordFields and try | out the new punning, thanks! | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe