Hello,
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'
{-# 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)
Hope that this helps,
-Iavor
On Thu, Jul 23, 2009 at 12:51 PM, Simon
Peyton-Jones
Can you give a concrete program to illustrate your point, please? I'm not getting it.
Simon
| -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Evan Laforge | Sent: 17 July 2009 23:57 | To: haskell | Subject: [Haskell-cafe] generalize RecordPuns and RecordWildCards to work with | qualified names? | | Record punning is not all that useful with qualified module names. If | I write '(M.Record { M.rec_x })' it says " Qualified variable in | pattern" and if I write '(M.Record { rec_x })' it says 'Not in scope: | `rec_x''. Could it be this extension be further extended slightly so | that 'f (M.Record { M.rec_x })' will desugar to 'f (M.Record { M.rec_x | = rec_x })'? | | Similarly, RecordWildCards could support this too. | | It seems simple and useful to me... am I missing anything fatally | problematic about this? Would anyone else use it? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe
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