Hi All, I was looking at the definition of intersperse on http://hackage.haskell.org/package/base-4.7.0.1/docs/src/Data-List.html#inte... And found this -- | The 'intersperse' function takes an element and a list and -- \`intersperses\' that element between the elements of the list. -- For example, -- -- > intersperse ',' "abcde" == "a,b,c,d,e" intersperse :: a -> [a] -> [a] intersperse _ [] = [] intersperse sep (x:xs) = x : prependToAll sep xs -- Not exported: -- We want to make every element in the 'intersperse'd list available -- as soon as possible to avoid space leaks. Experiments suggested that -- a separate top-level helper is more efficient than a local worker. prependToAll :: a -> [a] -> [a] prependToAll _ [] = [] prependToAll sep (x:xs) = sep : x : prependToAll sep xs I don't understand why we need to "make every element in the 'intersperse'd list available as soon as possible to avoid space leaks." Could somebody shed some light on what could cause a space leak here? In particular, would this simpler version cause problems? intersperse :: a -> [a] -> [a] intersperse _ [] = [] intersperse _ [x] = [x] intersperse a (x: xs@(y:ys)) = x : a : intersperse a xs Thanks, Dimitri