2

次のプログラムをコンパイルするには？どういうわけかエラーを回避できません"No instance for (PersistBackend IO)。

私の目的は、io-streams を使用して db テーブルを効率的に埋める方法を確認することです。の型makeOutputStreamは(Maybe a -> IO ()) -> IO (OutputStream a)while をinsertWords返しm ()、戻り値の型としては受け入れませんIO ()。

(後半の追加: 回避策が見つかりましたが、質問に対する回答ではありません。以下を参照してください。)

エラーメッセージは次のとおりです。

Words_read2.hs:30:36:
    No instance for (PersistBackend IO)
      arising from a use of `insertWord'
    Possible fix: add an instance declaration for (PersistBackend IO)
    In the first argument of `Streams.makeOutputStream', namely
      `insertWord'
    In a stmt of a 'do' block:
      os <- Streams.makeOutputStream insertWord
    In the expression:
      do { is <- Streams.handleToInputStream h >>= Streams.words;
           os <- Streams.makeOutputStream insertWord;
           Streams.connect is os }

そして、このエラーを生成するコードは次のとおりです。

{-# LANGUAGE GADTs, TypeFamilies, TemplateHaskell, QuasiQuotes, FlexibleInstances, FlexibleContexts, StandaloneDeriving #-}

import qualified Data.ByteString as B
import           Data.Maybe
import           Control.Monad.IO.Class (MonadIO, liftIO)
import           Database.Groundhog.Core 
import           Database.Groundhog.TH
import           Database.Groundhog.Sqlite
import           System.IO
import           System.IO.Streams.File
import qualified System.IO.Streams as Streams

data Words = Words {word :: String} deriving (Eq, Show)

mkPersist defaultCodegenConfig [groundhog|
definitions:
  - entity: Words
|]

insertWord :: (MonadIO m, PersistBackend m) => Maybe B.ByteString -> m ()
insertWord wo = case wo of
        Just ww ->  insert_ $ Words ((show . B.unpack) ww)
        Nothing -> return ()

main = do
  withSqliteConn "words2.sqlite" $ runDbConn $ do
     runMigration defaultMigrationLogger $ migrate (undefined :: Words)
     liftIO $ withFile "web2" ReadMode $ \h -> do  -- a link to /usr/share/dict/web2 - a  list of words one per line 
        is <- Streams.handleToInputStream h >>= Streams.words 
        os <- Streams.makeOutputStream insertWord
        Streams.connect is os

回避策として、別の方法で処理を行うことができます。内部で作業するrunDbConnのではなく、接続 (のプール) にハンドルを返して渡します。このアイデアは、質問に対するSOの回答から来ています: Making Custom Instances of PersistBackend。

{-# LANGUAGE GADTs, TypeFamilies, TemplateHaskell, QuasiQuotes, FlexibleInstances, FlexibleContexts, StandaloneDeriving #-}

import qualified Data.ByteString as B
import           Data.Maybe
import qualified Data.Text as T
import qualified Data.Text.Encoding as T
import           Control.Monad.IO.Class -- (MonadIO, liftIO)
import           Control.Monad.Trans.Control
import           Database.Groundhog.Core 
import           Database.Groundhog.TH
import           Database.Groundhog.Sqlite
import           System.IO
import           System.IO.Streams.File
import qualified System.IO.Streams as Streams

data Words = Words {word :: T.Text} deriving (Eq, Show)

mkPersist defaultCodegenConfig [groundhog|
definitions:
  - entity: Words
|]

main = do
  gh <- do withSqlitePool "words5.sqlite" 5 $ \pconn -> return pconn 
  runDbConn (runMigration defaultMigrationLogger $ migrate (undefined :: Words)) gh
  withFile "web3" ReadMode $ \h -> do  -- 500 words from /usr/share/dict/web2 - a list of words one per line 
    is <- Streams.handleToInputStream h >>= Streams.words 
    os <- Streams.makeOutputStream (iw2db gh)
    Streams.connect is os

iw2db :: (MonadIO m, MonadBaseControl IO m, ConnectionManager cm Sqlite) => cm -> Maybe B.ByteString -> m()
iw2db gh (Just x) = runDbConn (insert_ $ Words (T.decodeUtf8 x)) gh
iw2db gh Nothing  = return ()
4

1 に答える 1