warnings for pattern matches generated by the compiler

[Simon Peyton-Jones]

GHC's pattern-match overlap checker is a regular source of (justified)
complaints.  Somehow it never gets to the top of our agenda.

It's a well-identified piece of the compiler if anyone feels motivated
to lend a hand.  Otherwise I can't promise when we'll get to it.

But keep prodding us at intervals if it is important to you.  Maybe
one of us will have a rush of blood to the head.

Simon

| -----Original Message-----
| From: Feliks Kluzniak [mailto:feliks at crt.se] 
| Sent: 09 March 2002 12:33
| To: glasgow-haskell-bugs at haskell.org
| Subject: warnings for pattern matches generated by the compiler
| 
| 
| 
| Hello!
| 
| I like to have the warnings about non-exhaustive pattern 
| matches etc. turned on: I find them very useful.  
| Unfortunately, this prevents me from taking full advantage of 
| field labels.
| 
| Here is an (artificial) example:
| 
|     module See ( D, foo ) where
|     
|     data  D  =  U | D{ value :: Int }
|     
|     foo ::  D -> D
|     
|     foo d  =  case  d
|               of
|                  D{}  ->  d{ value = value d + 1 }
|                  U    ->  D{ value = 0 }
|     
| GHC complains as follows:
|     
|     ghc --make -odir../ghc -hidir../ghc -Wall 
| -fno-warn-missing-signatures See.hs
|     ghc-5.02.2: chasing modules from: See.hs
|     Compiling See              ( See.hs, ../ghc/See.o )
|     
|     See.hs:9: Warning: Pattern match(es) are non-exhaustive
|         In a record-update construct (D ds_dpd): Patterns not 
| matched: U
|     
| This is not quite as it should be, is it?  
| 
| Is there any work-around other than abandoning the use of 
| field labels or turning off the warnings?  (In my real 
| application there are quite a few more cases in the union, 
| and more fields.)
| 
| Regards,
| -- Feliks
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