* Vlatko Basic <vlatko.ba...@gmail.com> [2013-07-11 19:33:38+0200]
> Hello Cafe,
>
> I have
>
> data CmpFunction a = CF (a -> a -> Bool)
>
> that contains comparing functions, like ==, <, > ..., and I'm trying
> to declare the Show instance for it like this
>
> instance Show (CmpFunction a) where
> show (CF (==)) = "== " -- no good
> show f = case f of -- no good also
> CBF (==) -> "=="
> _ -> "Other"
>
> but compiler complains for both with
>
> This binding for `==' shadows the existing binding
> imported from `Prelude' at src/Main.hs:6:8-11
> (and originally defined in `ghc-prim:GHC.Classes')
>
> Is it possible at all to compare two functions or how to solve this
> problem, to show some string for a specific function?
Depending on why you need that...
{-# LANGUAGE FlexibleContexts, UndecidableInstances, FlexibleInstances #-}
import Test.SmallCheck
import Test.SmallCheck.Series
import Test.SmallCheck.Drivers
import Control.Monad.Identity
import Data.Maybe
data CmpFunction a = CF (a -> a -> Bool)
feq :: (Show a, Serial Identity a) => CmpFunction a -> CmpFunction a -> Bool
feq (CF f1) (CF f2) =
isNothing $
runIdentity $
smallCheckM 10 (\x1 x2 -> f1 x1 x2 == f2 x1 x2)
instance Show (CmpFunction Integer) where
show f
| f `feq` CF (==) = "=="
| f `feq` CF (/=) = "/="
| f `feq` CF (<) = "<"
| f `feq` CF (<=) = "<="
| otherwise = "Unknown function"
This uses SmallCheck to figure out, with some degree of certainty,
whether two functions are equal.
Of course, Rice's theorem still holds, and the above instance is easy
to fool, but it still might be useful in some cases.
Roman
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