basicTypes/Var.lhs:194: Non-exhaustive patterns in function r eadMutTyVar

[Simon Peyton-Jones]

I havn't seen this one myself, but I defer to Jeff.

Jeff: any chance you can look at this?  We want to put out 4.08.2 shortly,
so it would be nice to fix this.

Thomas, in parallel, I'd like to get this bug into our regression test
suite.  How hard would it be to boil down your program to something
of modest size that still shows the bug?

Simon

| -----Original Message-----
| From: Thomas Hallgren [mailto:hallgren at cs.chalmers.se]
| Sent: 27 November 2000 10:59
| To: glasgow-haskell-bugs at haskell.org
| Subject: basicTypes/Var.lhs:194: Non-exhaustive patterns in function
| readMutTyVar
| 
| 
| Hi,
| 
| In some Haskell code I am writing at the moment I use a number of 
| multi-parameter classes, with various interrelated instances. 
| One class 
| looks like
| 
|    class Convert from to where conv :: from -> to
| 
| Everything works fine, but when I add functional 
| dependencies, to reduce 
| the need for type signatures,
| 
|    class Convert from to | from -> to where conv :: from -> to
| 
| GHC 4.08.1 fails with the message
| 
|    basicTypes/Var.lhs:194: Non-exhaustive patterns in 
| function readMutTyVar
| 
| (I also get messages like
| 
|    zonkIdOcc:  FunDep_a1sp
|    zonkIdOcc:  FunDep_a1sj
|    zonkIdOcc:  FunDep_a1sg
|    zonkIdOcc:  FunDep_a1s8
|    zonkIdOcc:  FunDep_a1sa
| 
| but these seem to be harmless.)
| 
| Is there a fix for this already, or should I prepare a more 
| detailed bug 
| report?
| 
| Regards,
| Thomas Hallgren
| 
| PS My code is accepted by Hugs...
| 
| 
| 
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