Re: [Haskell-cafe] Comparing functions

[Vlatko Basic]

-------- Original Message -------- Subject: Re: [Haskell-cafe] Comparing functions From: Brandon Allbery <allber...@gmail.com> To: Vlatko Bašić <vlatko.ba...@gmail.com> Cc: Haskell-Cafe <haskell-cafe@haskell.org> Date: 11.07.2013 19:50 On Thu, Jul 11, 2013 at 1:33 PM, Vlatko Basic <vlatko.ba...@gmail.com> 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 Hm, I thought it is a pattern match with constant, as in f ('a':xs) == 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 allber...@gmail.com                                  ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

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