Well, this definition, although correct, contradicts the OP's claim that \x->x /= \y->if 1==1 then y else undefined :)
I'm leading the OP to the thought that necessity Eq constraint and lack of an Eq instance for functions (and thus non-existence of a universally polymorphic 'count' function) is something dictated by the nature of the world, not by the nature of Haskell. 2009/4/18 John A. De Goes <j...@n-brain.net>: > > Two functions are equal iff they have the same domain and range and the same > outputs for the same inputs. Simple to state, but extremely difficult to > implement in a useful way, and impossible to implement in a perfect way. > > If you had a compiler or algorithm capable of determining function equality, > you could use it to prove or disprove arbitrary theorems in mathematics. > > Regards, > > John A. De Goes > N-BRAIN, Inc. > The Evolution of Collaboration > > http://www.n-brain.net | 877-376-2724 x 101 > > On Apr 18, 2009, at 9:39 AM, Eugene Kirpichov wrote: > >> Could you then provide an example of two functions that *are* equal, >> or, even better, a definition of equality for arbitrary functions? >> Since Haskell may be compiled into C, this must be a definition that >> is implementable in C. >> >> 2009/4/18 michael rice <nowg...@yahoo.com>: >>> >>> Though I haven't tried it out, it's trying to use my function to count >>> functions. >>> >>> The first argument is the identity function. >>> >>> The second argument is a list of a different form of the identity >>> function. >>> >>> Though the two identity functions, given the same input, would produce >>> the >>> same output, I doubt they would be equal. >>> >>> So my guess at an answer would be zero. >>> >>> Michael >>> >>> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpic...@gmail.com> wrote: >>> >>> From: Eugene Kirpichov <ekirpic...@gmail.com> >>> Subject: Re: [Haskell-cafe] General function to count list elements? >>> To: "michael rice" <nowg...@yahoo.com> >>> Cc: haskell-cafe@haskell.org >>> Date: Saturday, April 18, 2009, 11:03 AM >>> >>> What should >>> >>> count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) >>> >>> return? >>> >>> 2009/4/18 michael rice <nowg...@yahoo.com>: >>>> >>>> Is there a general function to count list elements. I'm trying this >>>> >>>> count :: a -> [a] -> Int >>>> count x ys = length (filter (== x) ys) >>>> >>>> with this error upon loading >>>> >>>> ============= >>>> >>>> [mich...@localhost ~]$ ghci count >>>> GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help >>>> Loading package ghc-prim ... linking ... done. >>>> Loading package integer ... linking ... done. >>>> Loading package base ... linking ... done. >>>> [1 of 1] Compiling Main ( count.hs, interpreted ) >>>> >>>> count.hs:2:29: >>>> Could not deduce (Eq a) from the context () >>>> arising from a use of `==' at count.hs:2:29-32 >>>> Possible fix: >>>> add (Eq a) to the context of the type signature for `count' >>>> In the first argument of `filter', namely `(== x)' >>>> In the first argument of `length', namely `(filter (== x) ys)' >>>> In the expression: length (filter (== x) ys) >>>> Failed, modules loaded: none. >>>> Prelude> >>>> >>>> ============= >>>> >>>> Not sure what it's trying to tell me other than I need an (Eq a) >>>> somewhere. >>>> >>>> Michael >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe@haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >>> >>> >>> -- >>> Eugene Kirpichov >>> Web IR developer, market.yandex.ru >>> >>> >> >> >> >> -- >> Eugene Kirpichov >> Web IR developer, market.yandex.ru >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Eugene Kirpichov Web IR developer, market.yandex.ru _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
