“输入解析错误”在Haskell if-then-else有条件的情况下 [英] "parse error on input" in Haskell if-then-else conditional

问题描述

当我试图编译它时，下面的块会抛出错误输入`conn'解析错误。我已经尝试了许多不同的if-then-else语句的配置，无济于事。数据库逻辑在我添加条件之前就工作了，所以这没有问题。我在其他线路上有太多线路吗？有没有什么办法可以解决这个问题，而不需要完全修改逻辑？

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

推荐答案

您需要在后面引入 do else 就像这样：

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

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