如何在Haskell中编写嵌套的if语句？

Linda Su 发表于 Dev

苏琳达

我有此函数“路径”，它带有3个参数：

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!"

我想知道这是否是具有多个条件和多个if语句的正确方法？

ØrjanJohansen

这不是不正确的，但不是惯用语言（即习惯样式）。通常，我们喜欢使用警卫而不是if-then-else，例如@ user5402的答案。但是，在您的情况下，您还只是将常量常量与进行了比较==，这意味着最好的方法是将其进一步扩展并使用模式匹配（我对它进行了更漂亮的格式化）：

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!"

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