[Haskell] Newbie help with type-classes

[Bas van Dijk]

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#instance-overlap

regards,

Bas van Dijk

On 5/11/07, Ryan Ingram <ryani.spam at 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 [()]
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