[英]Haskell cassava (Data.Csv): Ignore missing columns/fields
如何設置木薯以忽略缺失的列/字段並使用默認值填充相應的數據類型? 考慮這個例子:
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
import Data.ByteString.Lazy.Char8
import Data.Csv
import Data.Vector
import GHC.Generics
data Foo = Foo {
a :: String
, b :: Int
} deriving (Eq, Show, Generic)
instance FromNamedRecord Foo
decodeAndPrint :: ByteString -> IO ()
decodeAndPrint csv = do
print $ (decodeByName csv :: Either String (Header, Vector Foo))
main :: IO ()
main = do
decodeAndPrint "a,b,ignore\nhu,1,pu" -- [1]
decodeAndPrint "ignore,b,a\npu,1,hu" -- [2]
decodeAndPrint "ignore,b\npu,1" -- [3]
[1]
和[2]
工作得很好,但[3]
失敗了
Left "parse error (Failed reading: conversion error: no field named \"a\") at \"\""
我怎樣才能讓decodeAndPrint
能夠處理這個不完整的輸入?
我當然可以操縱輸入字節串,但也許有更優雅的解決方案。
感謝以下 Daniel Wagner 輸入的解決方案:
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
import Control.Applicative
import Data.ByteString.Lazy.Char8
import Data.Csv
import Data.Vector
import GHC.Generics
data Foo = Foo {
a :: Maybe String
, b :: Maybe Int
} deriving (Eq, Show, Generic)
instance FromNamedRecord Foo where
parseNamedRecord rec = pure Foo
<*> ((Just <$> Data.Csv.lookup rec "a") <|> pure Nothing)
<*> ((Just <$> Data.Csv.lookup rec "b") <|> pure Nothing)
decodeAndPrint :: ByteString -> IO ()
decodeAndPrint csv = do
print $ (decodeByName csv :: Either String (Header, Vector Foo))
main :: IO ()
main = do
decodeAndPrint "a,b,ignore\nhu,1,pu" -- [1]
decodeAndPrint "ignore,b,a\npu,1,hu" -- [2]
decodeAndPrint "ignore,b\npu,1" -- [3]
(警告:完全未經測試,代碼僅用於思想傳播,不適合任何用途。等等等等)
FromNamedRecord
要求的Parser
類型是Alternative
,因此只需使用(<|>)
設置默認值。
instance FromNamedRecord Foo where
parseNamedRecord rec = pure Foo
<*> (lookup rec "a" <|> pure "missing")
<*> (lookup rec "b" <|> pure 0)
如果您想稍后知道該字段是否存在,請使您的字段足夠豐富以記錄:
data RichFoo = RichFoo
{ a :: Maybe String
, b :: Maybe Int
}
instance FromNamedRecord Foo where
parseNamedRecord rec = pure RichFoo
<*> ((Just <$> lookup rec "a") <|> pure Nothing)
<*> ((Just <$> lookup rec "b") <|> pure Nothing)
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