I wasn't comparing functions, you were. I was just trying to answer your questions. I had a count function that worked fine for an enum type, but thought why not go polymorphic with it. So I changed it and got the error I originally inquired about. Thanks. 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, 1:18 PM 2009/4/18 michael rice :

...

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? As for comparing functions, I would not do it at all, because comparing functions makes no sense: one way of comparison (extensional equality) is not implementable, whereas the other (object identity) breaks referential transparency and is impossible to reason about and generally doesn't have a single reason to work correctly at all. Object identity has no reason to exist in a language without mutable objects, like Haskell. That's probably the key. As for implementing the count function, I would implement it in the way that people already suggested to you: by putting a constraint onto its type, namely a constraint that makes it polymorphic *over types whose values can be compared for equality*, t.i. over members of the Eq class. This is perfectly correct and does not cause any mathematical unsoundness.

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 :

...

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/  :? 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 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, 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 :

...

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 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, 11:03 AM

What should

count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined))

return?

2009/4/18 michael rice :

...

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/  :? 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

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