On Thu, Jul 11, 2013 at 1:33 PM, Vlatko Basic <vlatko.basic@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

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

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