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List:       hugs-bugs
Subject:    RE: "Could not unambiguously deduce"..
From:       "Simon Peyton-Jones" <simonpj () microsoft ! com>
Date:       2002-11-06 17:40:54
Message-ID: DE924A789E3C124F8C8A319F273357E7CC14BE () lon-msg-01 ! europe ! corp ! microsoft ! com
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> Yes, but isn't that an implementation problem surfacing at the
> language level? All the dictionaries needed to delay the decision to
> the point of use could also be made available when compiling the
> original program, no? After all, that's the reason why there's an
> ambiguity in the first place.

No, it's not.   It's tricky to understand and tricky to explain.  I'll have one more \
go.

Consider just this:

> > > class C t where
> > > expl :: t -> String
> > > expl x "default"
> > > 
> > > instance           C String where expl s = "String"
> > > instance C a => C [a]    where expl l = "[a]"

Now try this:

	f :: C a => [a] -> String
	f xs = expl xs ++ "\n"

	foo1 = f "wig"
	foo2 = expl "wig" ++ "\n"

	baz1 = f [True]
	baz2 = expl [True] ++ "\n"

I think we'll agree that foo2 should return "String\n" and baz2 should return \
"[a]\n".] But what about foo1 and baz1?  Both execute the code for 'f'.  What does f \
do?  It needs a dictionary for C[a] to deal with the call (expl xs).  BUT it does not \
know what 'a' is going to be.  It can't choose the String instance.  So perhaps it \
should choose the C [a] instance?  Well, it could, but then (f xs) would always \
return "[a]\n".  And 'f' must choose one of the two instances right now, because all \
it's passed is a dictionary for (C a).

If, instead, we say

	f :: C [a] => [a] -> String

then f doesn't have to choose which instance... it can just use the C[a] dictionary \
that is passed.  Then in foo1, the compiler knows that it should pass a (C String) \
dictionary, and in baz1 it should pass a (C [Boo]) dictionary and all is well.


In your example things are slightly more complicated because it's the instance decl \
that gives rise to the problem, but it's just the same issue.

Simon






> 
> Not to mention that in the case for which there is an overlap, the
> String instance will always be chosen as the more specific one..
> 
> Claus
> 
> > > data T a ý [a]
> > > 
> > > class C t where
> > > expl :: t -> String
> > > expl x ÿdefault"
> > > 
> > > instance        C String where expl s "String"
> > > instance C a ÿC [a]    where expl l "[a]"
> > > 
> > > instance (C a {- ,C [a] -} ) ÿC (T a) where
> > > expl (D xs) þxpl xs
> > > 
> > > main ÿutStrLn $ expl "hi"
> > > 
> > > ------------
> > > 
> > > As is, both ghc and hugs reject the program, whereas
> > > both accept it with the extra constraint in the C (T a)
> > > instance.. Now, I think I can see how the right-hand-side
> > > expl could come either from the C String or from the C [a]
> > > instance - hence ghc's message:
> > > 
> > > $ ghc --make Tst.hs
> > > c:\ghc\ghc-5.04\bin\ghc.exe: chasing modules from: Tst.hs
> > > Compiling Main             ( Tst.hs, ./Tst.o )
> > > 
> > > Tst.hs:15:
> > > Could not unambiguously deduce (C [a])
> > > from the context (C (T a), C a)
> > > The choice of (overlapping) instance declaration
> > > depends on the instantiation of `a'
> > > Probable fix:
> > > Add (C [a]) to the class or instance method `expl'
> > > Or add an instance declaration for (C [a])
> > > arising from use of `expl' at Tst.hs:15
> > > In the definition of `expl': expl xs
> > > 
> > > 
> > > Confused,
> > > Claus
> > > 
> > > PS. Perhaps related, but why does Hugs seem to ignore the
> > > C a constraint in the context of the original version?
> > > 
> > > $ hugs -98 Tst.hs
> > > __   __ __  __  ____   ___
> > > _________________________________________
> > > > > > > > > > > > > > > > > __      Hugs 98: Based on the Haskell 98
> > > standard
> > > > > ___|| ||__|| ||__||  __||     Copyright (c) 1994-2001
> > > > > ---||         ___||           World Wide Web:
> > > http://haskell.org/hugs
> > > > > > > Report bugs to:
> > > hugs-bugs@haskell.org
> > > > > > > Version: December 2001
> > > _________________________________________
> > > 
> > > Hugs mode: Restart with command line option +98 for Haskell 98
> > mode
> > > 
> > > Reading file "c:\Program Files\Hugs98\\lib\Prelude.hs":
> > > Reading file "Tst.hs":
> > > Type checking
> > > ERROR "Tst.hs":15 - Cannot justify constraints in instance member
> > > binding
> > > *** Expression    : expl
> > > *** Type          : C (T a) ÿT a -> String
> > > *** Given context : C (T a)
> > > *** Constraints   : C [a]
> > > 
> > > Prelude>
> > > _______________________________________________
> > > Glasgow-haskell-bugs mailing list
> > > Glasgow-haskell-bugs@haskell.org
> > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
> 


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