type error

[Simon Peyton-Jones]

You have two instance declarations:
	instance SubType a (Either a x) where
	instance (SubType a b) =3D> SubType a (Either x b) where

Which one should GHC use to resolve the constraint?
	(SubType x Value)

It can't make an unambiguous choice, hence the error message.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-admin@haskell.org
[mailto:glasgow-haskell-users-admin@haskell.org]
| On Behalf Of Ed Komp
| Sent: 02 June 2003 15:43
| To: glasgow-haskell-users@haskell.org
| Subject: type error
|=20
|=20
| I have encountered an unexpected type error of the form:
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|  >>     Could not unambiguously deduce <type> from the context
<context>
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| I was able to generate a quite simple test case to demonstrate
| the problem, that I include at the end of this message.
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| I am using Version 5.04.2 of the Glasgow Compiler, with options:
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| -fglasgow-exts -fallow-overlapping-instances
-fallow-undecidable-instances
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| Here is the output generated when I attempt to load the file, test.hs,
| (test.hs appears at the end of this message):
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|  >> fleck [61] % ghci --version
|  >> The Glorious Glasgow Haskell Compilation System, version 5.04.2
|  >> fleck [62] % ghci -fglasgow-exts -fallow-overlapping-instances
| -fallow-undecidable-instances test.hs
|  >>    ___         ___ _
|  >>   / _ \ /\  /\/ __(_)
|  >>  / /_\// /_/ / /  | |      GHC Interactive, version 5.04.2, for
Haskell 98.
|  >> / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
|  >> \____/\/ /_/\____/|_|      Type :? for help.
|  >>
|  >> Loading package base ... linking ... done.
|  >> Loading package haskell98 ... linking ... done.
|  >> Compiling Test             ( test.hs, interpreted )
|  >>
|  >> test.hs:26:
|  >>     Could not unambiguously deduce (SubType x Value)
|  >> 	from the context (SubType x BaseType)
|  >>     The choice of (overlapping) instance declaration
|  >> 	depends on the instantiation of `x'
|  >>     Probable fix:
|  >> 	Add (SubType x Value)
|  >> 	to the existential context of a data constructor
|  >> 	Or add an instance declaration for (SubType x Value)
|  >>     arising from use of `inj' at test.hs:26
|  >>     In the definition of `test': inj x
|  >> Failed, modules loaded: none.
|  >> Prelude>
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| I do not understand why the compiler cannot deduce
| (SubType x Value) from (SubType x BaseType)
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| since:
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| type Value =3D (Either Double BaseType)
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| and
| instance (SubType a b) =3D> SubType a (Either x b) where
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| The message actually says it cannot "unambiguously" make the
| deduction.
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| Can someone explain to me why this is, and more importantly
| how I can resolve the issue?
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| The suggestion,
|  >> 	Add (SubType x Value)
|  >> 	to the existential context of a data constructor
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| will work in this simple example (I tested this);
| but in the code where I originally encountered the problem
| this is not a viable alternative since it would generate
| circular dependencies.
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| Thanks in advance for your help !
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| Ed Komp
| komp@ittc.ku.edu
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| -- -----------------------------------------------------------
| -- test.hs
| -- -----------------------------------------------------------
| module Test where
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| import Control.Monad.Error
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| class SubType a b where
|   inj :: a -> b
|   prj :: b -> Maybe a
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| instance SubType a (Either a x) where
|   inj       =3D Left
|   prj       =3D either Just (const Nothing)
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| instance (SubType a b) =3D> SubType a (Either x b) where
|     inj       =3D Right . inj
|     prj       =3D either (const Nothing) prj
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| -- -----------------------------------------------------------
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| type BaseType =3D Either Integer ( Either Bool () )
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| type Value =3D (Either Double BaseType)
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| data Foo =3D  forall x. (SubType x BaseType)  =3D> MkFoo x
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| test :: Foo -> Value
| test (MkFoo x) =3D inj x
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