Functor supertype has conflicting instances

Question

I defined the Fine-Grained Functor ( FgFunctor) class to apply a constraint to the type of functions that can be displayed by my ordered triple data type ( OrdTriple), which requires the contained type to be ordered.

import Data.List (sort)

-- 'fine-grained' functor
class FgFunctor f a b where
    fgmap :: (a -> b) -> f a -> f b

data OrdTriple a = OrdTriple a a a deriving Show

instance (Ord a, Ord b) => FgFunctor OrdTriple a b where
    fgmap f (OrdTriple n d x) = OrdTriple n' d' x'
        where [n', d', x'] = sort [f n, f d, f x]

main :: IO ()
main = do
    let test = fgmap (* 10^4) $ OrdTriple 1 6 11 
    print test

The code works fine until I define all the others Functoralso be FgFunctors, for example:

-- all regular functors are also fine-grained ones
instance Functor f => FgFunctor f a b where
    fgmap = fmap

With this instance declaration, as soon as I try to use fgmapin my type OrdTriple, the compiler complains about overlapping instance declarations

Overlapping instances for FgFunctor OrdTriple b0 b0
  arising from a use of ‘fgmap’
Matching instances:
  instance Functor f => FgFunctor f a b
    -- Defined at OrdTriple.hs:15:10
  instance (Ord a, Ord b) => FgFunctor OrdTriple a b
    -- Defined at OrdTriple.hs:18:10
In the expression: fgmap (* 10 ^ 4)
In the expression: fgmap (* 10 ^ 4) $ OrdTriple 1 6 11
In an equation for ‘test’:
    test = fgmap (* 10 ^ 4) $ OrdTriple 1 6 11

" ", , OrdTriple, OrdTriple Functor, , .

Answer 1

(GHC) , . FgFunctor OrdTriple, OrdTriple Functor.

GHC/AdvancedOverlap , .