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 [()]
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