[Haskell-cafe] \_ -> not equivalent to const $

In attempting to devise a variant of cycle which did not keep its argument alive (for the purpose of cycle [1::Int..]), I came across this peculiar behavior: import Debug.Trace cycle' :: (a -> [b]) -> [b] cycle' xs = xs undefined ++ cycle' xs

take 20 $ cycle' (const $ 1:2:3:4:trace "x" 5:[]) [1,2,3,4,x 5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5]

Nuts. Oh, but wait:

take 20 $ cycle' (\_ -> 1:2:3:4:trace "x" 5:[]) [1,2,3,4,x 5,1,2,3,4,x 5,1,2,3,4,x 5,1,2,3,4,x 5]

Hey, it worked! Can someone explain what the heck is going on here? Luke