[Haskell-beginners] Problem specifying fst in tuple from list in filter
toad3k at gmail.com
Thu Jan 2 16:04:15 UTC 2014
You can go:
filter ((==0) . fst)
which is equivalent to
filter (\x -> fst x == 0)
On Thu, Jan 2, 2014 at 10:52 AM, Angus Comber <anguscomber at gmail.com> wrote:
> I can generate a tuple of (index,character) like this:
> zip [0..] ['A'..'Z']
> To filter on an element in the list I can do:
> filter (== (0,'A')) $ zip [0..] ['A'..'Z']
> > [(0,'A')]
> But that is very basic, I really want to select on index value (ie first
> item in pair). How do I specify the first part of tuple:
> filter (fst == 0) $ zip [0..] ['A'..'Z']
> this doesn't work.
> Couldn't match expected type `(a1, Char) -> Bool'
> with actual type `Bool'
> In the first argument of `filter', namely `(fst == 0)'
> In the expression: filter (fst == 0)
> In the expression: filter (fst == 0) $ zip [0 .. ] ['A' .. 'Z']
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