[Haskell-beginners] Lazy evaluation and list comprehensions

Obscaenvs obscaenvs at gmail.com
Wed Aug 8 22:48:16 CEST 2012

Hi fellow Haskellers! Trying literary style here, lets just hope it
plays well with your email clients.

Suppose we define a list - somewhat arbitrarily - as

 > xs = [1..42] :: [Int]

Next, we let x and ys be

 > x  = 17 -- also somewhat arbitrarily
 > ys = [ y | y <- xs, y >= x ]

My question: in which WHNF state is ys? y and x are evaluated in
applying >= to them - at least in this particular instance of Ord
(Int). Are these evaluated values then copied to the new list ys, or
are they just stored as thunks? I would guess that they are just
thunked, and that the state of ys is [*thunk*,*thunk*,...,*thunk*]
since it cannot be known beforehand to what extent the predicate
reduces its arguments (in this case >= ), but I don't know for sure.

I thought I had sufficient knowledge about thunks, WHNF, NF and lazy
evaluation, but this one does me in. I guess it is because I haven't
trawled through Okasaki's book yet :)

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