如何在haskell中編寫嵌套的if語句？

Question

我有這個功能「路徑」這3個參數：

path::String->String->String->IO() 
path place1 dir place2 = 
    if place1 == "bedroom" && d == 'n' && place2 == "den" 
    then do 
     putStrLn "You are in a bedroom with a large, comfortable bed. It has been a long, tiresome day, and you would like nothing better than to go to sleep." 
    else 
    if place1 == "bedroom" && d == 'd' && place2 == "bed" 
     then describe "bed" 
    else 
     if place1 == "den" && d == 's' && place2 == "bedroom" 
     then describe "bedroom" 
     else 
     if place1 == "bed" && d == 'u' && place2 == "bedroom" 
      then describe "bedroom" 
     else putStrLn "Cannot go there!" 

我想知道如何，如果這是具有多個條件和多個if語句的正確方法是什麼？

Answer 1

這不是不正確，但它不是慣用（即習慣風格）。通常我們更喜歡看守，以if-then-else，就像@ user5402的答案。然而，在你的情況，你也只是比較恆定的文字與==，這意味着最好的辦法就是把它一步，使用模式匹配（我格式化有點漂亮太）：

path :: String -> String -> String -> IO() 
path "bedroom" "n" "den"  = putStrLn "You are in a bedroom with a large, comfortable bed. It has been a long, tiresome day, and you would like nothing better than to go to sleep." 
path "bedroom" "d" "bed"  = describe "bed" 
path "den"  "s" "bedroom" = describe "bedroom" 
path "bed"  "u" "bedroom" = describe "bedroom" 
path _   _ _   = putStrLn "Cannot go there!" 

Answer 2

考慮使用守衛，例如：

path :: String -> String -> String -> IO() 
path place1 d place2 
     | place1 == "bedroom" && d == "n" && place2 == "den" 
     = putStrLn "You are in a bedroom ..." 
     | place1 == "bedroom" && d == "d" && place2 == "bed" 
     = describe "bed" 
     | place1 == "den" && d == "s" && place2 == "bedroom" 
     = describe "bedroom" 
     | place1 == "bed" && d == "u" && place2 == "bedroom" 
     = describe "bedroom" 
     | otherwise = putStrLn "Cannot go there!" 

注意String文字的雙引號括起來。