haskell - Haskell 中的 "parse error on input"if-then-else 条件

Question

以下 do block 在我尝试编译时抛出错误“输入 `conn' 时出现解析错误”。我已经尝试了 if-then-else 语句的许多不同配置，但都无济于事。数据库逻辑在我添加条件之前工作，所以这没有问题。我在其他地方有太多行吗？有没有办法在不完全修改逻辑的情况下解决这个问题？

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

Answer 1

你需要像这样在 else 之后引入一个 do block :

  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