RE: [Haskell-cafe] generalize RecordPuns and RecordWildCards to work with qualified names?
Oh, now I get it, thanks. This message concerns design choices for record-syntax-related GHC extensions. Lennart, pls tune in. You don’t need to have read the thread to understand this message. | I think that Even refers to an example like this: | | module A where | data A = A { a :: Int } | | The following works: | | {-# LANGUAGE NamedFieldPuns #-} | module B where | import A | f (A { a }) = a | | However, if we import "A" qualified, then punning does not seem to work: | | {-# LANGUAGE NamedFieldPuns #-} | module B where | import qualified A | f (A.A { a }) = a | | This results in: Not in scope: `a' Right. What is happening is that GHC looks up the first 'a' (the one on the LHS) and finds it not in scope. If you add -XDisambiguateRecordFields, it works fine. But admittedly, the error message is unhelpful. I could improve that. Now on to the suggested change: | {-# LANGUAGE NamedFieldPuns #-} | module B where | import qualified A | | f (A.A { A.a }) = a | | This results in: Qualified variable in pattern: A.a | | 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 } In these cases, I think the abbreviated code looks too confusing. 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.) So my current conclusion is: improve the error message, perhaps suggesting the flag -XDismabiguateRecordFields, but don't add the change you suggest. Comments? Simon
At a minimum I think the error message should be better.
I also think it would be natural to use the DisambiguateRecordFields
for the places where RecordWildcards are used.
I mean, if I change from unqualified import to a qualified one, and
then change all visible names to be qualified I would expect things to
still work.
For RecordPuns I don't have an opinion on what to do.
-- Lennart
On Sun, Aug 9, 2009 at 9:42 PM, Simon Peyton-Jones
Oh, now I get it, thanks. This message concerns design choices for record-syntax-related GHC extensions. Lennart, pls tune in. You don’t need to have read the thread to understand this message.
| I think that Even refers to an example like this: | | module A where | data A = A { a :: Int } | | The following works: | | {-# LANGUAGE NamedFieldPuns #-} | module B where | import A | f (A { a }) = a | | However, if we import "A" qualified, then punning does not seem to work: | | {-# LANGUAGE NamedFieldPuns #-} | module B where | import qualified A | f (A.A { a }) = a | | This results in: Not in scope: `a'
Right. What is happening is that GHC looks up the first 'a' (the one on the LHS) and finds it not in scope. If you add -XDisambiguateRecordFields, it works fine. But admittedly, the error message is unhelpful. I could improve that.
Now on to the suggested change:
| {-# LANGUAGE NamedFieldPuns #-} | module B where | import qualified A | | f (A.A { A.a }) = a | | This results in: Qualified variable in pattern: A.a | | 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 }
In these cases, I think the abbreviated code looks too confusing.
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.)
So my current conclusion is: improve the error message, perhaps suggesting the flag -XDismabiguateRecordFields, but don't add the change you suggest.
Comments?
Simon
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participants (2)
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Lennart Augustsson -
Simon Peyton-Jones