[Haskell-beginners] Perfect numbers

[Matthew Williams]

Hi Guys,
 
I'm new to Haskell and I was wondering if you can help me:
 
One of the first program's I tend to write when I'm looking at a new
language is a program to generate a list of perfect numbers:
 
--My First Perfect Number Generator
factors :: Integer -> [Integer]
factors x = [z | z <- [1..x-1], x `mod` z == 0]
 
is_perfect :: Integer -> Bool
is_perfect x = if sum(factors x) == x then True else False
 
do_perfect :: [Integer] -> [Integer]
do_perfect x = [z |z <- x, is_perfect z ]
 
Then to run it:
> do_perfect [1..9000]
 
I'm using GHC to run it. My problem / question is this: It's running
quite a lot slower than equivalent programs in erlang and python. I
suspect it's down to the way I've written it. Any thoughts (or comments
in general)
 
Many thanks
 
Matt

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