[ ghc-Bugs-759910 ] Non-exhaultive patterns from derived Read

Bugs item #759910, was opened at 2003-06-24 16:14
Message generated for change (Comment added) made by simonpj
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=759910&group_id=8032

Category: Compiler
Group: None
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Non-exhaultive patterns from derived Read

Initial Comment:
When switching from GHC 5.04.1 to 6.0 I started getting 
a load of warnings like these:




 source/SDL/Types.hs:33:


     Warning: Pattern match(es) are non-exhaustive


          In a pattern binding in


             a 'do' expression:


              Patterns not matched:


                  Text.Read.Lex.EOF


                  Text.Read.Lex.Rat _


                  Text.Read.Lex.Int _


                  Text.Read.Lex.Symbol _


                  ...




The source in question looks like this




data Expected


        = ExpectedType String


        | ExpectedValue String


        deriving (Read, Show)




It looks as though everything deriving Read causes this 
warning (there are several such constructs in the file).  
Deriving only Show does not.  Found no mention of 
change to behavior of derived reads in release notes or in 
google.




Cheers,


Jason Feingold


jfeingol@xxxxxxxxxxxxx


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>Comment By: Simon Peyton Jones (simonpj)
Date: 2003-06-25 08:16

Message:
Logged In: YES 
user_id=50165

You must be using -fwarn-simple-patterns too, right?

You're right that the error message is misleading, but it's not 
particularly easy to fix, so I've settled for improving the 
documentation of that flag.

----------------------------------------------------------------------

Comment By: Nobody/Anonymous (nobody)
Date: 2003-06-24 16:16

Message:
Logged In: NO 

BTW, I'm running on Windows 2000.


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