Lazy evaluation and list comprehensions
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 :)
Just use GHCi's :print command =). Prelude> let xs = [1..42] :: [Int]; x = 17; ys = [ y | y <- xs, y >= x ] Prelude> :print xs xs = (_t1::[Int]) Prelude> :print ys ys = (_t2::[Int]) Initially nothing is evaluated (as expected). Let's try forcing ys to WHNF: Prelude> ys `seq` () -- forcing to WHNF () Prelude> :print xs xs = 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 : 11 : 12 : 13 : 14 : 15 : 16 : 17 : (_t3::[Int]) Prelude> :print ys ys = 17 : (_t4::[Int]) So xs had to be forced until the first element of ys appeared (since WHNF decides between [] and _:_). We didn't explicitly evaluate ys's first element but as it needed to be evaluated before it already appears evaluated now. Same thing if you continue forcing ys: Prelude> tail ys `seq` () -- forcing tail to WHNF () Prelude> :print xs xs = 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 10 : 11 : 12 : 13 : 14 : 15 : 16 : 17 : 18 : (_t5::[Int]) Prelude> :print ys ys = 17 : 18 : (_t6::[Int]) Cheers, -- Felipe.
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Felipe Almeida Lessa -
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