Haskell: Scan Through a List and Apply A Different Function for Each Element

Question

I need to scan through a document and accumulate the output of different functions for each string in the file. The function run on any given line of the file depends on what i

Answer 1

I show a solution for two types of line, but it is easily extended to five types of line by using a five-tuple instead of a two-tuple.

import Data.Monoid

eachLine :: B.ByteString -> ([Atom], [Sheet])
eachLine bs | isAnAtom bs = ([ {- calculate an Atom -} ], [])
            | isASheet bs = ([], [ {- calculate a Sheet -} ])
            | otherwise = error "eachLine"

allLines :: [B.ByteString] -> ([Atom], [Sheet])
allLines bss = mconcat (map eachLine bss)

The magic is done by mconcat from Data.Monoid (included with GHC).

(On a point of style: personally I would define a Line type, a parseLine :: B.ByteString -> Line function and write eachLine bs = case parseLine bs of .... But this is peripheral to your question.)