README

CSV Files and Haskell

CSV files are the de-facto standard in many cases of data transfer, particularly when dealing with enterprise application or disparate database systems.

While there are a number of csv libraries in Haskell, at the time of this project's start, there wasn't one that provided all of the following:

• Full flexibility in quote characters, separators, input/output
• Constant space operation
• Robust parsing and error resiliency
• Battle-tested reliability in real-world datasets
• Fast operation
• Convenient interface that supports a variety of use cases

Over time, people created other plausible CSV packages like cassava. The major benefit from this library remains to be:

• Direct participation in the conduit ecosystem, which is now quite large, and all the benefits that come with it.
• Flexibility in CSV format definition.
• Resiliency to errors in the input data.

This package

csv-conduit is a conduit-based CSV parsing library that is easy to use, flexible and fast. It leverages the conduit infrastructure to provide constant-space operation, which is quite critical in many real world use cases.

For example, you can use http-conduit to download a CSV file from the internet and plug its Source into intoCSV to stream-convert the download into the Row data type and do something with it as the data streams, that is without having to download the entire file to disk first.

Author & Contributors

• Ozgun Ataman (@ozataman)
• Daniel Bergey (@bergey)
• BJTerry (@BJTerry)
• Mike Craig (@mkscrg)
• Daniel Corson (@dancor)
• Dmitry Dzhus (@dzhus)
• Niklas Hambüchen (@nh2)
• Facundo Domínguez (@facundominguez)
• Daniel Vianna (@dmvianna)

Introduction

• The CSVeable typeclass implements the key operations.
• CSVeable is parameterized on both a stream type and a target CSV row type.
• There are 2 basic row types and they implement exactly the same operations, so you can chose the right one for the job at hand:
  • type MapRow t = Map t t
  • type Row t = [t]
• You basically use the Conduits defined in this library to do the parsing from a CSV stream and rendering back into a CSV stream.
• Use the full flexibility and modularity of conduits for sources and sinks.

Speed

While fast operation is of concern, I have so far cared more about correct operation and a flexible API. Please let me know if you notice any performance regressions or optimization opportunities.

Usage Examples

Example #1: Basics Using Convenience API

import Conduit
import Data.CSV.Conduit
import Data.Text (Text)

-- Just reverse the columns
myProcessor :: Monad m => ConduitT (Row Text) (Row Text) m ()
myProcessor = mapC reverse

test :: IO ()
test = runResourceT $
  transformCSV defCSVSettings
               (sourceFile "input.csv")
               myProcessor
               (sinkFile "output.csv")

Example #2: Basics Using Conduit API

import Conduit
import Data.CSV.Conduit
import Data.Text (Text)

myProcessor :: Monad m => ConduitT (MapRow Text) (MapRow Text) m ()
myProcessor = awaitForever yield

-- Let's simply stream from a file, parse the CSV, reserialize it
-- and push back into another file.
test :: IO ()
test = runConduitRes $
  sourceFile "test/BigFile.csv" .|
  intoCSV defCSVSettings .|
  myProcessor .|
  (writeHeaders defCSVSettings >> fromCSV defCSVSettings) .|
  sinkFile "test/BigFileOut.csv"