條件在做阻擋

Question

爲什麼下面的代碼塊：

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

拋出一個錯誤：

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 } 

儘管兩個：

main = do 
    runTestTT tests 

和

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

工作正常嗎？

Answer 1

的if then else的兩個分支必須具有相同的類型，但

runTestTT tests :: IO Counts 

和

print $ process seq :: IO() 

您可以添加一個return()到then科，現代的方式來做到這一點是使用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 

來修復它（或者你可以添加一個return some_value_of_type_Counts到else分支）。