naturals = [1..]
naturalslessthan1000 = [x | x <- naturals, x < 1000]Haskell list comprehension question
Author
I often want to do something by generating an infinite list where each element is always greater than or equal to the previous, and then generating a finite list from this. However, doing something like (yes, this particular case is a stupid example, but the principle is there):
A list comprehension will always evaluate every item in the list. Your specific example can easily be solved though:
naturalslessthan1000 = takeWhile (<1000) naturals
Oh, and using the sum function rather than foldr (+) 0 is much more readable.
naturalslessthan1000 = takeWhile (<1000) naturals
Oh, and using the sum function rather than foldr (+) 0 is much more readable.
Author
Quote:Original post by Luctus
A list comprehension will always evaluate every item in the list. Your specific example can easily be solved though:
naturalslessthan1000 = takeWhile (<1000) naturals
takeWhile will solve the cases that I'm thinking of, thanks.
Quote:Oh, and using the sum function rather than foldr (+) 0 is much more readable.
Great, thanks. I'm still familiarising myself with the Prelude.
Quote:Original post by LuctusI realized I formulated myself poorly here though as list comprehensions are indeed evaluated lazily. To be more correct: A list comprehension will always evaluate at least as many elements as the function consuming the list comprehension needs.
A list comprehension will always evaluate every item in the list
So for example take 1000 [x | x <- naturals] will evaluate 1000 elements from naturals, while take 1000 [x | x <- naturals, even x] will evaluate 2000 elements.
Author
I realised what you meant. :-)
My previous hackish workaround had relied on this -- I'd print (in the interpreter) the full extent of the list, wait until it hung, kill it, and then return to take just as many as had worked. I think you can understand why I thought that there had to be a better way!
My previous hackish workaround had relied on this -- I'd print (in the interpreter) the full extent of the list, wait until it hung, kill it, and then return to take just as many as had worked. I think you can understand why I thought that there had to be a better way!
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