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{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
-- !!! One method class from Sergey Mechveliani
-- showed up problematic newtype dict rep.
module Main where
import Data.Ratio
class MBConvertible a b where cm :: a -> b -> Maybe b
c :: MBConvertible a b => a -> b -> b
c a b = case cm a b
of
Just b' -> b'
_ -> error "c a b failed"
instance MBConvertible Int Int where cm a _ = Just a
instance (MBConvertible a b,Integral b) => MBConvertible a (Ratio b)
where
cm a f = case cm a (numerator f) of Just a' -> Just (a'%1)
_ -> Nothing
main = let f = 1%1 :: Ratio Int
n2 = 2::Int
g = (c n2 f) + f
in
putStr (shows g "\n")
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