strong type derivation in ghc 6.12 compared to 6.12

[Simon Peyton-Jones]

See my comment http://hackage.haskell.org/trac/ghc/ticket/4028#comment:4

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Seyed Hosein Attarzadeh Niaki
| Sent: 27 April 2010 20:41
| To: glasgow-haskell-users at haskell.org
| Subject: strong type derivation in ghc 6.12 compared to 6.12
| 
| I have problems with upgrading some already working code for GHC 6.12.
| The following code:
| 
| -----------------------------------------------------------
| {-# LANGUAGE DeriveDataTypeable,
|              GeneralizedNewtypeDeriving, TemplateHaskell  #-}
| 
|  import Data.Data
|  import Language.Haskell.TH.Syntax
| 
|  class Nat a
|  class (Data a, Lift a) => ProcType a
| 
| 
|  instance (Lift a, Nat s) => Lift (FSVec s a) where
|   lift (FSVec xs) = undefined
| 
| 
|  newtype Nat s => FSVec s a = FSVec {unFSVec :: [a]}
|   deriving (Eq, Typeable, Data)
| 
|  -- The following line does not cause a problem in GHC 6.10
|  -- instance (Typeable s, Nat s, ProcType a) => ProcType (FSVec s a)
| -----------------------------------------------------------
| 
| works fine in ghc 6.10.4 and I get (extra stuff removed):
| //
| *Main> :i FSVec
| newtype (Nat s) => ...
| instance (Data a, Typeable s, Nat s) => Data (FSVec s a)
|   -- Defined at ghctest.hs:16:26-29
| ...
| //
| 
| but in 6.12.2 what I get is:
| //
| *Main> :i FSVec
| newtype (Nat s) => ...
| instance (Data s, Data a, Nat s) => Data (FSVec s a)
| ...
| //
| 
| So, although the last line in the above code compiles correctly in ghc
| 6.10.4, in 6.12.2 GHC complains with:
| //
| ghctest.hs:19:10:
|     Could not deduce (Data s)
|       from the context (Typeable s, Nat s, ProcType a)
|       arising from the superclasses of an instance declaration
|                    at ghctest.hs:19:10-64
|     Possible fix:
|       add (Data s) to the context of the instance declaration
|     In the instance declaration for `ProcType (FSVec s a)'
| Failed, modules loaded: none.
| //
| 
| Why are they different? Why 6.12 derives a stronger constraint? Is it a
| bug or it was not working correctly before?
| 
| --
| Hosein Attarzadeh
| 
| 
| _______________________________________________
| Glasgow-haskell-users mailing list
| Glasgow-haskell-users at haskell.org
| http://www.haskell.org/mailman/listinfo/glasgow-haskell-users