"Could not unambiguously deduce"..

[Simon Peyton-Jones]

| What I don't understand, however, is why adding that extra
| constraint helps in any way? Shouldn't the addition of new
| things in the context only make more options available? Why
| does it make some of the existing, amgibuous options go away?

With the extra constraint, the instance decl can build a C (T a)
dictionary from
a (C a, C [a]) dictionary, by choosing the expl method from the C [a]
dictionary it is passed (ignoring the C a one).  That defers the choice
of which of the overlapping instance decls we are going to use.  Perhaps
it'll be postponed to a point at which 'a' is known, in which case the
choice is easy.  The point is that we don't want to have to choose
between the C String and C [a] instance decls until we know enough about
'a' to choose the right one.

Simon


| data T a =3D D [a]
|=20
| class C t where
|    expl :: t -> String
|    expl x =3D "default"
|=20
| instance        C String where expl s =3D "String"
| instance C a =3D> C [a]    where expl l =3D "[a]"
|=20
| instance (C a {- ,C [a] -} ) =3D> C (T a) where
|     expl (D xs) =3D expl xs
|=20
| main =3D putStrLn $ expl "hi"
|=20
| ------------
|=20
| 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:
|=20
|   $ ghc --make Tst.hs
|   c:\ghc\ghc-5.04\bin\ghc.exe: chasing modules from: Tst.hs
|   Compiling Main             ( Tst.hs, ./Tst.o )
|=20
|   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
|=20
|=20
| Confused,
| Claus
|=20
| PS. Perhaps related, but why does Hugs seem to ignore the
|     C a constraint in the context of the original version?
|=20
|     $ 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
|     _________________________________________
|=20
|     Hugs mode: Restart with command line option +98 for Haskell 98
mode
|=20
|     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) =3D> T a -> String
|     *** Given context : C (T a)
|     *** Constraints   : C [a]
|=20
|     Prelude>
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