here goes a test of  pointer equality (valid for functions too): samepointer :: a -> a -> IO Bool samepointer f1 f2 = do       p1<- varHash f1       p2<- varHash f2       return $  p1 == p2   where    varHash x=  do       sn <- makeStableName x       return  $ hashStableName sn 2009/4/18  :

Hi John,

Yes, I see now what the hullabaloo was about.

I've only been hacking Haskell for a few weeks, so I'm still seeing the "forest" rather than the "trees". But I'm coming along.

Thanks.

Michael

--- On Sat, 4/18/09, John Dorsey wrote:

From: John Dorsey Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" Cc: haskell-cafe@haskell.org Date: Saturday, April 18, 2009, 2:36 PM

Michael,

...

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.

For variety, I'll go a slightly different direction.

If you generalize count to use any predicate, instead of always equality...

    gcount :: (a -> a -> Bool) -> a -> [a] -> Int     gcount pred x0 xs = length (filter (pred x0) xs)

    count = gcount (==)

This will work with any type that you can write a predicate for with the type (a -> a -> Bool).  I can even use this with functions, if I'm careful.

    ghci> gcount (\f g -> True)       (*2) [id,(const 1),(*3)]     3     ghci> gcount (\f g -> f 1 == g 1) (^2) [id,(const 1),(*3)]     2

By the way, do you see why everyone's bothing you about comparing functions?  The type you gave count, which didn't have an Eq constraint, was an assertion that you could compare two values of *any* type.  If there's a type that's not comparable, then count's type was wrong. Functions are the canonical example of an incomparable type.

When you're bored some time, read a bit about the Curry-Howard correspondence.  It's interesting, even if (like me) you don't grok all of its implications.

Regards, John

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