过滤Haskell中的列表列表

Question

I want a function Int -> [[String]] , trying to filter out the elements that have a specific value on the Inth spot. I thought I could combine filter and !! but I can't get it to work. So far I have:

filter (!! (== value)) rows

where value is a String and rows is a [[String]] . I thought it would take the Int combined with a [String] from rows and check if that particular entry equals value and then keep those rows, but it gets interpreted differently. Any help would be appreciated.

Answer 1

(!! (== value)) is invalid because the right operand of !! must be an Int . (== value) is a function.

Your options are

\n -> filter (\xs -> xs !! n == value) rows

or without the explicit lambda

\n -> filter ((== value) . (!! n)) rows