Question

I have a Java like property text files with key value pairs. What are some good approaches for loading that data into haskell and then accessing it.

The file look likes:

XXXX=vvvvv   
YYYY=uuuuu

I want to be able to access the “XXXX” key.

Answer 1

Exploring a few details ehird didn’t mention, and a slightly different approach:

import qualified Data.Map as Map

type Key = String
type Val = String

main = do
  -- get contents of file (contents :: String)
  contents <- readFile "config.txt"

  -- split into lines (optionList :: [String])
  let optionList = lines contents

  -- parse into map (optionMap :: Map Key Val)
  let optionMap = optionMapFromList optionList

  doStuffWith optionMap

optionMapFromList :: [String] -> Map.Map Key Val
optionMapFromList = foldr step Map.empty
  where step line map = case parseOpt line of
          Just (key, val) -> Map.insert key val map
          Nothing         -> map

parseOpt :: String -> Maybe (Key, Val)
parseOpt = undefined

I’ve expressed my solution to your problem as a fold: taking the list of lines in the file, and turning it into the desired map. Each step of the fold involves inspecting a single line, attempting to parse it into a key/value pair, and when successful, inserting it into the map.

I’ve left parseOpt undefined; you could use an approach like ehird’s parseField, or whatever you like. Perhaps you would prefer to only parse specific options:

interestingOpts = ["XXXX", "YYYY"]

parseOpt line = case find (`isPrefixOf` line) interestingOpts of
  Just key -> Just (key, drop 1 $ dropWhile (/= '=') line)
  Nothing  -> Nothing

Using the prefix testing approach isn’t always the best idea, though, if you have (for example) an option “XX” and an option “XXXX”. Play around and see what approach suits your data best. If you need high performance, look into using Data.Text instead of Strings.