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

It is a good idea to introduce a new ADT, e.g. "Summary" instead of tuples. Then, since you want to accumulate the values of Summary you came make it an istance of Data.Monoid. Then you classify each of your lines with the help of classifier functions (e.g. isAtom, isSheet, etc.) and concatenate them together using Monoid's mconcat function (as suggested by @dave4420).

Here is the code (it uses String instead of ByteString, but it is quite easy to change):

module Classifier where

import Data.List
import Data.Monoid

data Summary = Summary
  { atoms :: [String]
  , sheets :: [String]
  , digits :: [String]
  } deriving (Show)

instance Monoid Summary where
  mempty = Summary [] [] []
  Summary as1 ss1 ds1 `mappend` Summary as2 ss2 ds2 =
    Summary (as1 `mappend` as2)
            (ss1 `mappend` ss2)
            (ds1 `mappend` ds2)

classify :: [String] -> Summary
classify = mconcat  . map classifyLine

classifyLine :: String -> Summary
classifyLine line
  | isAtom line  = Summary [line] [] [] -- or "mempty { atoms = [line] }"
  | isSheet line = Summary [] [line] []
  | isDigit line = Summary [] [] [line]
  | otherwise    = mempty -- or "error" if you need this  

isAtom, isSheet, isDigit :: String -> Bool
isAtom = isPrefixOf "atom"
isSheet = isPrefixOf "sheet"
isDigit = isPrefixOf "digits"

input :: [String]
input = ["atom1", "sheet1", "sheet2", "digits1"]

test :: Summary
test = classify input