Question

我有这个函数“path”，它有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!"

我想知道如果这是具有多个条件和多个if语句的正确方法？

Answer 1

它不正确，但它不是惯用的（即习惯风格）。通常我们更喜欢看守if-then-else，比如@ user5402的答案。但是在你的情况下，你也只是将常量文字与==进行比较，这意味着最好的方法是更进一步使用模式匹配（我将其格式化得更漂亮）：

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

考虑使用警卫，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!"

请注意，String文字用双引号括起来。

如何在haskell中编写嵌套的if语句？

2 个答案: