Thanks Roman. Tried it and implemented, but had troubles until I realized that
for String, 10 test take quite long. :-)
However, I decided to solve this problem in a more "natural" way
-------- Original Message --------
Subject: Re: [Haskell-cafe] Comparing functions
From: Roman Cheplyaka <r...@ro-che.info>
To: Vlatko Basic <vlatko.ba...@gmail.com>
Cc: Haskell-Cafe <haskell-cafe@haskell.org>
Date: 11.07.2013 20:10
* 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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