f as alist = [ b | (a, b) <- alist, a `elem` as ]
perhaps?
I think you're looking for `mapMaybe` :-)- Lyndon_______________________________________________On Sun, May 31, 2015 at 11:30 AM, <briand@aracnet.com> wrote:Hi,
A simple example of something I was trying to do but it had some questions along the lines of "the best way to do this".
Given a list
l = [a]
and an a-list
alist = [ (a,b) ]
the idea is to find all the items in l which are in alist and then create a list [b]
so the overall function should be
[a] -> [ (a,b) ] -> [b]
the solution is straightforward:
l1 = filter (\x -> isJust (lookup x alist)) l
l2 = map (\x -> fromJust (lookup x alist)) l1
`fromJust` used in the construction of l2 won't fail, because only the elements for which the lookup succeeded are in l1.
This would be something called `filterMap` but I couldn't find such a function in the list library, but it seems like there just has to be one defined in a library somewhere.
the above seems clumsy, i'm wondering how to make it "more pretty".
generally i was also wondering if the above construction is as inefficient as it looks because of the double-lookup, or would the compiler actually be able to optimize that code into something more efficient ? this code is not being used on large sets of data so efficiency doesn't matter, I'm just curious.
Thanks,
Brian
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