“parse error on input” in Haskell if-then-else conditional

问题

The following do block throws the error "parse error on input `conn'" when I attempt to compile it. I have tried many different configurations of the if-then-else statement to no avail. The database logic worked before I added the conditional, so there isn't a problem with that. Do I have too many lines in the else? Is there any way to fix this without completely revamping the logic?

main = do
   contents <- BL.getContents
   let myData = decode contents :: Maybe Data
   if maybe True (\x -> result x /= "success") myData
      then error ("JSON download failed")
      else let myTrades = process myData
         conn <- connectSqlite3 "trades.db"
         insert <- DB.prepare conn "INSERT INTO trades VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?);"
         DB.executeMany insert $ map (\xs -> map DB.toSql xs) myTrades
         DB.commit conn
         DB.disconnect conn

回答1:

You need to introduce a do block after the else like so:

  else do let myTrades = process myData
          conn <- connectSqlite3 "trades.db"
          insert <- DB.prepare conn "INSERT INTO trades VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?);"
          DB.executeMany insert $ map (\xs -> map DB.toSql xs) myTrades
          DB.commit conn
          DB.disconnect conn

来源：https://stackoverflow.com/questions/16846584/parse-error-on-input-in-haskell-if-then-else-conditional