Add: -fallow-overlapping-instances to your OPTIONS pragma and read about overlapping instances in the GHC User Guide: http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html... regards, Bas van Dijk On 5/11/07, Ryan Ingram <ryani.spam@gmail.com> wrote:

Here's a test case for the problem I'm having; I'm using runhaskell from ghc v6.6.

Problem #1) Without -fallow-undecidable-instances, I get the following error: Constraint is no smaller than the instance head in the constraint: ConvertToInt a (Use -fallow-undecidable-instances to permit this) In the instance declaration for `ConvertToIntList a'

Problem #2) With -fallow-undecidable-instances, I get this error instead: Overlapping instances for ConvertToIntList () arising from use of `convl' at testcase.hs:28:6-15 Matching instances: instance (ConvertToInt a) => ConvertToIntList a -- Defined at testcase.hs:15:0 instance ConvertToIntList () -- Defined at testcase.hs:18:0 In the expression: convl [()] In the definition of `xl2': xl2 = convl [()]

I don't understand why there is an overlapping instances error; () is not an instance of ConvertToInt so how could that instance ever apply?

Is there something basic about type-classes that I'm not understanding here? My actual problem is more complicated than this, but this test-case covers the basic issue; something being an instance of class A means that I can derive an instance of class B for it, but I want to implement other instances of class B as well.

Code below:

{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}

module TestCase where class ConvertToInt a where conv :: a -> Int

class ConvertToIntList a where convl :: [a] -> [Int]

instance ConvertToInt Int where conv = id

instance ConvertToInt a => ConvertToIntList a where convl = map conv

instance ConvertToIntList () where convl x = []

x :: Int x = 5

xl :: [Int] xl = convl [x]

xl2 :: [Int] xl2 = convl [()] _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell

Ryan Ingram wrote: haskell-cafe@haskell.org is better for this type of question. Follow-up is set to it.

Here's a test case for the problem I'm having; I'm using runhaskell from ghc v6.6.

Problem #1) Without -fallow-undecidable-instances, I get the following error: Constraint is no smaller than the instance head This is bad.

Problem #2) With -fallow-undecidable-instances, I get this error instead: Overlapping instances for ConvertToIntList ()

I don't understand why there is an overlapping instances error; () is not an instance of ConvertToInt so how could that instance ever apply?

I write anywhere, instance ConvertToInt () where ... Now it is overlapping.

Is there something basic about type-classes that I'm not understanding here? They are open (Open World Assumption). I can add a new instance anywhere at any time.

Code below: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}

module TestCase where

class ConvertToInt a where conv :: a -> Int

class ConvertToIntList a where convl :: [a] -> [Int]

instance ConvertToInt Int where conv = id

instance ConvertToInt a => ConvertToIntList a where This is what it's complaining about in the first error; this doesn't work.

convl = map conv

instance ConvertToIntList () where convl x = []