[Haskell-cafe] Comparing functions

[Timon Gehr]

On 07/11/2013 07:33 PM, Vlatko Basic wrote:
> 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')
>

Yes, (==) is a variable name in a pattern. Hence you are creating a new 
definition for (==) bound to the constructor argument to CF, which hides 
the (==) defined within the Eq type class.

> Is it possible at all to compare two functions

Function types are opaque and values do not have an identity.

> or how to solve this problem, to show some string for a specific function?
>
>
> br,
> vlatko

You could store the string alongside the function inside the data type 
in some way.