[Haskell-cafe] implementing a csv reader

[Henning Thielemann]

On Tue, 22 Aug 2006, Tamas K Papp wrote:

> Hi,
> 
> Now that I have read the tutorials, I think that the best way to learn
> Haskell would be to use the language and write something simple yet
> useful.  I noticed that Haskell lacks a module for reading/writing csv
> (comma separated value) files, so I thought I could implement that.
> 
> Questions:
> 
> 1. Please tell me if you know of a csv module, because then I would do
>    something else.

see attachment
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module Spreadsheet where

{-
  See also
     http://www.xoltar.org/languages/haskell.html
     http://www.xoltar.org/languages/haskell/CSV.hs
-}

import Useful(chop,replace)
import Data.List(intersperse)
import Text.ParserCombinators.ReadP(ReadP)
import qualified Text.ParserCombinators.ReadP as Parser

toTable :: Char -> Char -> String -> [[String]]
toTable qm sep =
   let parseChar :: ReadP Char
       parseChar = Parser.choice
                      [Parser.satisfy (qm/=),
                       Parser.string [qm,qm] >> return qm]
       parseQuoted :: ReadP String
       parseQuoted = Parser.between (Parser.char qm) (Parser.char qm)
                                    (Parser.many parseChar)
       parseCell :: ReadP String
       parseCell = Parser.choice [parseQuoted, return ""]
       parseLine :: ReadP [String]
       parseLine = Parser.sepBy (parseCell) (Parser.char sep)
       parse str =
          fromSingleton (map fst (filter (null . snd)
                            (Parser.readP_to_S parseLine str)))
   in  map parse . lines

fromSingleton :: [a] -> a
fromSingleton [x] = x
fromSingleton []  = error "fromSingleton: empty list."
fromSingleton _ = error "fromSingleton: list must contain at most one element."

fromTable :: Char -> Char -> [[String]] -> String
fromTable qm sep =
   unlines . map (concat . intersperse [sep] .
       map (\s -> [qm] ++ replace [qm] [qm,qm] s ++ [qm]))

toTableSimple :: Char -> Char -> String -> [[String]]
toTableSimple qm sep =
   map (map (dequote qm) . chop (sep==)) . lines

fromTableSimple :: Char -> Char -> [[String]] -> String
fromTableSimple qm sep =
   unlines . map (concat . intersperse [sep] . map (\s -> [qm]++s++[qm]))

dequote :: Eq a => a -> [a] -> [a]
dequote _ [] = error "dequote: string is empty"
dequote q (x:xs) =
   if x == q && last xs == q
     then init xs
     else error "dequote: string not correctly quoted"