Maybe this is not what you want, but you can also put the 'convl' function in the 'ConvertToInt' class. class ConvertToInt a where conv :: a -> Int convl :: [a] -> [Int] With this approach you don't need any language extension. 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