question

gistfile1.hs

-- generate an infinite sequence of sign alternating numbers
-- how to make it really infinite so foldl doesn't take away the laziness of the evaluation?

alternating count = foldl (\x (a,b) -> x ++ [a,b]) [] $ take count [(x,y) | x <- [1..], let y = negate x]

-- out:
[1,-1,2,-2,3,-3,4,-4,5,-5,6,-6,7,-7,8,-8,9,-9,10,-10]

-- variants:

-- using foldr
alternating = foldr (\(a,b) x -> [a,b] ++ x) [] [(x,y) | x <- [1..], let y = negate x]

-- concatenating list of lists
alternating = concat [[x,y] | x <- [1..], let y = negate x]

-- I like this one more :)

alternating count = concat $ take count [[x,y] | x <- [1..], let y = negate x]

Thanks for the hint!

hs> let alternating = concat [[x,y] | x <- [1..], let y = negate x]
hs> take 10 alternating
[1,-1,2,-2,3,-3,4,-4,5,-5]

^^ i wanted this :)

OK, you're welcome :)