haskell - do block 中的条件

Question

为什么下面的代码块是这样的:

main = do
    line <- getLine
    if null line
        then runTestTT tests
        else do
            line2 <- getLine
            seq::[Int] <- return $ map read $ words line2
            print $ process seq

抛出错误:

lgis.hs:28:13:
    Couldn't match type `()' with `Counts'
    Expected type: IO Counts
      Actual type: IO ()
    In a stmt of a 'do' block: print $ process seq
    In the expression:
      do { line2 <- getLine;
           seq :: [Int] <- return $ map read $ words line2;
           print $ process seq }
    In a stmt of a 'do' block:
      if null line then
          runTestTT tests
      else
          do { line2 <- getLine;
               seq :: [Int] <- return $ map read $ words line2;
               print $ process seq }

尽管两者:

main = do
    runTestTT tests

和

main = do
    line <- getLine
    line2 <- getLine
    seq::[Int] <- return $ map read $ words line2
    print $ process seq

工作正常吗？

Answer 1

if then else 的两个分支必须具有相同的类型，但是

runTestTT tests :: IO Counts

和

print $ process seq :: IO ()

您可以将 return () 添加到 then 分支，现代的方法是使用 Control.Monad.void ,

main = do
    line <- getLine
    if null line
        then void (runTestTT tests)         -- formerly: runTestTT tests >> return ()
        else do
            line2 <- getLine
            seq::[Int] <- return $ map read $ words line2
            print $ process seq

修复它(或者您可以将return some_value_of_type_Counts添加到else分支)。