On Thu, Jul 11, 2013 at 1:33 PM, Vlatko Basic wrote:

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')

The problem here isn't quite what you think it is; (==) is not a constructor, therefore it is a *variable*. It's exactly the same problem as a = 5 -- ... foo a = 3 -- this does NOT compare with the previous value of "a"; it's identical to the next line! foo x = x Just as with the above, the normal way to do it would be to use a guard... but functions don't have an Eq instance, and *can't* have one. How do you meaningfully compare them? And for a typeclass function like (==), do you want (==) instantiated for Int to compare equal to (==) instantiated for Integer? Does a native-compiled function compare equal to an interpreted function? Remember referential transparency; the concept of comparing pointers used in some languages is not applicable to Haskell. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

* Vlatko Basic [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