You can keep reading here or jump to my blog to get the full experience, including the wonderful pink, blue and white palette.
Parsing CSV without header:
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy as BL
import Data.Csv
-- file.csv
-- 1,2
-- 3,4
main :: IO ()
main = do
f <- BL.readFile "file.csv"
case decode NoHeader f of
Left err -> print err
Right xs -> V.forM_ xs $ \(x :: Int, y :: Int) -> print (x, y)
-- 1,2
-- 3,4
Parsing CSV to a data type requires having an instance of FromRecord for that type:
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy as BL
import Data.Csv
import Control.Monad (mzero)
data Coords =
Coords Int Int
instance FromRecord Coords where
parseRecord xs
| length xs == 2 = Coords <$> (xs .! 0) <*> (xs .! 1)
| otherwise = mzero
-- file.csv
-- 1,2
-- 3,4
main :: IO ()
main = do
f <- BL.readFile "file.csv"
case decode NoHeader f of
Left err -> print err
Right xs -> V.forM_ xs $ \(Coords x y) -> print (x, y)
-- 1,2
-- 3,4
Or the same thing using generics:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE DeriveGeneric #-}
module Main where
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy as BL
import Data.Csv
import GHC.Generics (Generic)
data Coords =
Coords Int Int
deriving (Generic)
instance FromRecord Coords
instance ToRecord Coords
-- file.csv
-- 1,2
-- 3,4
main :: IO ()
main = do
f <- BL.readFile "file.csv"
case decode NoHeader f of
Left err -> print err
Right xs -> V.forM_ xs $ \(Coords x y) -> print (x, y)
-- 1,2
-- 3,4
Parsing CSV with explicit reference to the fields. This way the code is more robust to changes in the CSV (e.g. change order of columns):
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy as BL
import Data.Csv
data Coords =
Coords Int Int
instance FromNamedRecord Coords where
parseNamedRecord x = Coords <$> (x .: "x") <*> (x .: "y")
-- file.csv
-- x,y
-- 1,2
-- 3,4
main :: IO ()
main = do
f <- BL.readFile "file.csv"
case decodeByName f of
Left err -> print err
Right (_, xs) -> V.forM_ xs $ \(Coords x y) -> print (x, y)
-- 1,2
-- 3,4
Or the same thing using generics:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE DeriveGeneric #-}
module Main where
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy as BL
import Data.Csv
import GHC.Generics (Generic)
data Coords =
Coords { x :: Int, y :: Int }
deriving (Generic)
instance FromNamedRecord Coords
instance ToNamedRecord Coords
instance DefaultOrdered Coords
-- file.csv
-- x,y
-- 1,2
-- 3,4
main :: IO ()
main = do
f <- BL.readFile "file.csv"
case decodeByName f of
Left err -> print err
Right (_, xs) -> V.forM_ xs $ \(Coords x y) -> print (x, y)
-- 1,2
-- 3,4
Get the latest content via email from me personally. Reply with your thoughts. Let's learn from each other. Subscribe to my PinkLetter!
Top comments (0)