You have two instance declarations: instance SubType a (Either a x) where instance (SubType a b) => 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 | | | I have encountered an unexpected type error of the form: | | >> Could not unambiguously deduce <type> from the context <context> | | I was able to generate a quite simple test case to demonstrate | the problem, that I include at the end of this message. | | I am using Version 5.04.2 of the Glasgow Compiler, with options: | | -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances | | Here is the output generated when I attempt to load the file, test.hs, | (test.hs appears at the end of this message): | | >> 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> | | I do not understand why the compiler cannot deduce | (SubType x Value) from (SubType x BaseType) | | since: | | type Value = (Either Double BaseType) | | and | instance (SubType a b) => SubType a (Either x b) where | | The message actually says it cannot "unambiguously" make the | deduction. | | Can someone explain to me why this is, and more importantly | how I can resolve the issue? | | The suggestion, | >> Add (SubType x Value) | >> to the existential context of a data constructor | | 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. | | Thanks in advance for your help ! | | Ed Komp | komp@ittc.ku.edu | | -- ----------------------------------------------------------- | -- test.hs | -- ----------------------------------------------------------- | module Test where | | import Control.Monad.Error | | class SubType a b where | inj :: a -> b | prj :: b -> Maybe a | | instance SubType a (Either a x) where | inj = Left | prj = either Just (const Nothing) | | instance (SubType a b) => SubType a (Either x b) where | inj = Right . inj | prj = either (const Nothing) prj | | -- ----------------------------------------------------------- | | type BaseType = Either Integer ( Either Bool () ) | | type Value = (Either Double BaseType) | | data Foo = forall x. (SubType x BaseType) => MkFoo x | | test :: Foo -> Value | test (MkFoo x) = inj x | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users