| 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:
| > | [EMAIL PROTECTED]
| > | || || 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
| > | [EMAIL PROTECTED]
| > | http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
|
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