[Haskell-beginners] Problem specifying fst in tuple from list in filter

[David McBride]

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']
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>