I see now, thanks to both of you; I think I was thrown off by the term
'generators' -- the 'refinement' provided by the second generator is more
like filter rather than map.
I guess if I actually wanted the case all the pairs were evaluated and
resulted in [Bool] where the first occurrence of an item were True, rather
than multiple generators I would have two nested list comprehensions?
Thanks everyone,
On Sun, Apr 26, 2020 at 9:39 AM Francesco Ariis
Hello Ken,
I recently came across this function which made me realize I don't understand list comprehensions well. I hope someone can help me understand them better by understanding this example better. The function takes a
On Sun, Apr 26, 2020 at 08:50:20AM -0400, Ken Overton wrote: list
of Eq and returns the list of unique elements from it:
unique :: Eq a => [a] -> [a] unique xs = [x | (x,y) <- zip xs [0..], x `notElem` (take y xs)]
[...]
So the first generator should produce [(Eq,Int)] as input to the second generator? And the second generator should produce [Bool]?
1. (x,y) <- zip xs [0..] -- generates a list of pairs. 2. x `notElem` (take y xs) -- acts like a guard to 1., so only the `x`s which are not in the first `y` elements of `xs` (in other words, the previous elements of `xs`) will be returned.
The examples on the wiki [1] show more way of using list comprehensions.
[1] https://wiki.haskell.org/List_comprehension#Examples _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
-- Ken Overton (917) 863-3937 ken.overton@gmail.com