GHC derives and accepts illegal Haskell98 contexts

[Simon Peyton-Jones]

Thanks for pointing this out.  You're the second person who has found
that GHC is a bit too liberal in accepting non-H98 programs.  The reason
for this was that checking of types themselves was a bit contorted.
I've 
rejigged it now, and (once I've committed it) you should find that GHC
only
accepts H98 types.

Simon

| -----Original Message-----
| From: kahl at heraklit.informatik.unibw-muenchen.de 
| [mailto:kahl at heraklit.informatik.unibw-muenchen.de] 
| Sent: 24 July 2001 12:38
| To: glasgow-haskell-bugs at haskell.org
| Subject: GHC derives and accepts illegal Haskell98 contexts
| 
| 
| 
| I have experimented with the definition of class assertions 
| in section 4.1.3 in the Haskell 98 report, and found that GHC 
| 4.08 and GHC 5.00.1 (invoked via ``ghc -c Test.hs'') both 
| derive and accept type arguments (in class assertions) that 
| do not have a variable as head.
| 
| I cannot really imagine where this might hurt, though.
| 
| 
| By the way, I find that the .hi format of 5.00 is, from a 
| human-readability point of view, in my opinion not optimal in 
| that there is no separator after the __forall-bound variables:
| 
| r2 :: __forall b (E (PrelBase.String, b)) => b -> PrelBase.Bool;
|                 ^
| 
| There is no comparison with NHC since NHC still has the 
| ``simple context restriction''.
| 
| 
| Best regards,
| 
| Wolfram
| 
| ==============================================================
| ===========
| 
| module Test where
| 
| class T a where
|   t :: a b c -> Bool
| 
| class E a where
|   e :: a -> a -> Bool
| 
| instance T (,) where
|   t (a,b) = True
| 
| q1 :: (T a,Eq (a String b)) => a String b -> Bool
| q1 x = if x == x then t x else True
| 
| --r1 :: Eq (String, b) => b -> Bool -- derived and accepted by GHC
| --r1 :: Eq a => a -> Bool           -- derived by Hugs98,
|                                     -- accepted by both 
| Hugs98 and GHC r1 x = q1 ("asd",x)
| 
| 
| q2 :: (T a,E (a String b)) => a String b -> Bool
| q2 x = if e x x then t x else True
| 
| --r2 :: E (String, b) => b -> Bool  -- derived and accepted by GHC
| r2 x = q2 ("asd",x)                 -- not accepted by hugs +98
| 
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