Here's a puzzle to fry your brains ....

[John Launchbury]

Folks,

My student Levent Erkok and I have been playing about with the idea of mutual 
recursive bindings in the do notation. Mostly it's clear how they should behave, but 
we struggle with the following example. I would love to hear some (considered) 
opinions about this.

John.

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test :: [Int]
test = do (x,z) <- [(y,1),(2*y,2), (3*y,3)]
          Just y <- map isEven [z .. 2*z]
          return (x+y)

isEven x = if even x then Just x else Nothing