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 <nowg...@yahoo.com>: > 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 <hask...@colquitt.org> wrote: > > From: John Dorsey <hask...@colquitt.org> > 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, 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 > > > > _______________________________________________ > 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
