[Haskell-cafe] General function to count list elements?

[michael rice]

I know functions can be compared in Scheme

Welcome to DrScheme, version 4.1 [3m].
Language: Swindle; memory limit: 128 megabytes.
> (equal? equal? equal?)
#t
> 


but apparently not in Haskell

[michael at localhost ~]$ ghci
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.
Prelude> (==) (==) (==)

<interactive>:1:0:
    No instance for (Eq (a -> a -> Bool))
      arising from a use of `==' at <interactive>:1:0-13
    Possible fix: add an instance declaration for (Eq (a -> a -> Bool))
    In the expression: (==) (==) (==)
    In the definition of `it': it = (==) (==) (==)
Prelude>

though I'm new at Haskell and may not be posing the question properly.

I would think a language with 1st-class support for functions would 
certainly include comparing them.

To compare two functions in C, I would compare their machine addresses.

Michael


--- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov at gmail.com> wrote:

From: Eugene Kirpichov <ekirpichov at gmail.com>
Subject: Re: [Haskell-cafe] General function to count list elements?
To: "michael rice" <nowgate at yahoo.com>
Cc: haskell-cafe at haskell.org
Date: Saturday, April 18, 2009, 11:39 AM

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 <nowgate at 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 <ekirpichov at gmail.com> wrote:
>
> From: Eugene Kirpichov <ekirpichov at gmail.com>
> Subject: Re: [Haskell-cafe] General function to count list elements?
> To: "michael rice" <nowgate at yahoo.com>
> Cc: haskell-cafe at 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 <nowgate at 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
>>
>> =============
>>
>> [michael at 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 at 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



      
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090418/03241f34/attachment.htm