compiler change

[Simon Peyton-Jones]

Aha!  You are the second real customer to discover a change I made to
GHC 6.6, namely that pattern bindings are not generalised.  It's a
documented change (see "Monomorphic pattern bindings" in the 6.6 user
manual), but it means that 6.6 is not, by default, exactly Haskell 98.
To recover the previous behaviour use -fno-mono-pat-binds.

There is a reason for the change. Generalising pattern bindings is a
rather significant complication in the language design.  My theory is
that it's a feature that is almost never used -- and on the rare
occasion that it is used, you can get around it.   

You can solve your problem in three ways
* -fno-mono-pat-binds
* use the fst/snd thing as you suggest
* put the polymorphic function in the tuple as Bulat suggested (this
makes use of GHC's new ability to impredicative polymorphism.

Simon



| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Christian Maeder
| Sent: 21 September 2006 09:56
| To: GHC Users Mailing List
| Subject: compiler change
| 
| I have to port the following piece of code to ghc-6.6:
| 
| [...]
| addSimpleFallOut :: IO a -> IO (Either String a)
| simpleFallOutId :: ObjectID
| (simpleFallOutId, addSimpleFallOut) = mkSimpleFallOut
| 
| mkSimpleFallOut :: (ObjectID,IO a -> IO (Either String a))
| mkSimpleFallOut = unsafePerformIO newFallOut
| {-# NOINLINE mkSimpleFallOut #-}
| 
| newFallOut :: IO (ObjectID, IO a -> IO (Either String a))
| [...]
| 
| ghc-6.5.20060919 reports:
|     Couldn't match expected type `forall a.
|                                   IO a -> IO (Either String a)'
|            against inferred type `IO a -> IO (Either String a)'
|       Expected type: (ObjectID,
|                       forall a1. IO a1 -> IO (Either String a1))
|       Inferred type: (ObjectID, IO a -> IO (Either String a))
|     In the expression: mkSimpleFallOut
|     In a pattern binding:
|         (simpleFallOutId, addSimpleFallOut) = mkSimpleFallOut
| 
| I can make the code compile by rewriting the pair as:
| 
| simpleFallOutId = fst mkSimpleFallOut
| addSimpleFallOut = snd mkSimpleFallOut
| 
| However, in this case I have inlined mkSimpleFallOut manually! Does
this
| matter? If so, how could I rewrite the above code?
| 
| Cheers Christian
| 
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