Question

haskell尝试获取带有标题（类型字符串）的int的csv文件以将第2行输出到第n行到列表列表中的新功能。例如[[row，1]，[row，2] ..] 到目前为止，使用Pipes库，让它输出每一行作为列表，数字为字符串，并出于某种原因打印“正确”？例如输出：'右[“1”，“3”，“5”..]'新行'右[“32”，“38”，“45”]'。任何建议，将不胜感激。代码：

{-# LANGUAGE OverloadedStrings, ScopedTypeVariables #-}
import qualified Data.Vector as V
import Pipes
import qualified Pipes.Prelude as P
import qualified System.IO as IO
import qualified Pipes.ByteString as PB
import qualified Data.Text as Text
import qualified Pipes.Csv as PCsv
import Control.Monad (forever)

showPipe :: Proxy () (Either String (V.Vector Text.Text)) () String IO b
showPipe = forever $ do
    x::(Either String (V.Vector Text.Text)) <- await
    yield $ show x

main :: IO ()
main = do
    putStrLn "Filename? "
    fName <- getLine 
    IO.withFile fName
                IO.ReadMode
                (\handle -> do
                    let producer = (PCsv.decode PCsv.NoHeader 
(PB.fromHandle handle))
                    runEffect ( (producer)>->
                                (showPipe) >->
                                P.stdoutLn)
                )

Answer 1

我不熟悉Pipe，但您似乎正在将数据提取到Either，Right通常使用Left构造函数来处理数据和错误消息的data Either a b = Left a | Right b构造函数。

Either String (V.Vector Text.Text)

在这种情况下，您要构建:: Left String，因此您的结果将是：

:: Right (V.Vector Text.Text)

或

show

在这种情况下，您获得后者，并希望Either从either monad中取出的值。您可以使用maybe一次处理这两种可能性（与您在Maybe monad中使用showPipe :: Proxy () (Either String (V.Vector Text.Text)) () String IO b showPipe = forever $ do x::(Either String (V.Vector Text.Text)) <- await let v = either id show x yield v的方式大致相同。

这会检查Left a是Right b还是id a，并根据show b或either返回。 either :: (a -> c) -> (b -> c) -> Either a b -> c -- or, specialized for this case: either :: (String -> String) -> ((V.Vector Text.Text) -> String) -> Either String (V.Vector Text.Text) -> String是类型

String

要么（a hyuk）你应该让main回来打印create table Data (n int, name char(3), val int) insert into data values (1, 'ABC',8) insert into data values (1, 'DEF', 7) insert into data values (2 , 'ABC' , 9) insert into data values (2 , 'XYZ', 6) SELECT COALESCE(a.Name, b.Name) as Name, a.Val as A, b.Val as B FROM Data a FULL OUTER JOIN Data b on a.N = b.N - 1 and a.Name = b.Name

Haskell：带有标题列表的csv

1 个答案: