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

[Luke Palmer]

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