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

18 Apr 2009

      On Sat, Apr 18, 2009 at 11:11 AM, michael rice  wrote:
...
...
To compare two functions in C, I would compare their machine addresses.
Why would you need that at all?
How would *you* do it?
Do what?  What is the problem?

No, I don't mean "comparing two functions".  I rather mean, why are you
comparing these functions?  What question is the comparison answering, and
how are you using that answer?  Is address comparison really what your code
wants, or do you want some stronger property?

Luke
...
Michael
================
--- On *Sat, 4/18/09, Eugene Kirpichov * wrote:
From: Eugene Kirpichov 
Subject: Re: [Haskell-cafe] General function to count list elements?
To: "michael rice" 
Cc: haskell-cafe@haskell.org
Date: Saturday, April 18, 2009, 12:39 PM
2009/4/18 michael rice http://mc/compose?to=nowgate@yahoo.com
...
:
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
That's not the functions being compared, but the memory addresses of
the code implementing them. If your goal is comparing functions to
answer a question "Are these two values indistinguishable?", equal?
doesn't help you, because it may answer 'false' even if these two
values are indistinguishable from a mathematical point of view.
...
but apparently not in Haskell
[michael@localhost ~]$ ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/ http://www.haskell.org/ghc/%C2%A0:? 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.
Again, this is first-class support for memory addresses of code
representing functions.
...
To compare two functions in C, I would compare their machine addresses.
Why would you need that at all?
...
Michael
--- On Sat, 4/18/09, Eugene Kirpichov http://mc/compose?to=ekirpichov@gmail.com>
wrote:
From: Eugene Kirpichov http://mc/compose?to=ekirpichov@gmail.com
Subject: Re: [Haskell-cafe] General function to count list elements?
To: "michael rice" http://mc/compose?to=nowgate@yahoo.com
Cc: haskell-cafe@haskell.orghttp://mc/compose?to=haskell-cafe@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.
...
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
2009/4/18 michael rice http://mc/compose?to=nowgate@yahoo.com
:
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 http://mc/compose?to=ekirpichov@gmail.com>
wrote:
From: Eugene Kirpichov http://mc/compose?to=ekirpichov@gmail.com
...
Subject: Re: [Haskell-cafe] General function to count list elements?
To: "michael rice" http://mc/compose?to=nowgate@yahoo.com
...
Cc: haskell-cafe@haskell.orghttp://mc/compose?to=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 http://mc/compose?to=nowgate@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@localhost ~]$ ghci count
GHCi, version 6.10.1: http://www.haskell.org/ghc/ http://www.haskell.org/ghc/%C2%A0:? 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.orghttp://mc/compose?to=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
--
Eugene Kirpichov
Web IR developer, market.yandex.ru
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

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

Luke Palmer