是否可以在守卫中匹配布尔元组，如模式匹配？

Question

我尝试创建一个 returns 不同字符串的函数，具体取决于布尔元组中的值

chooseAction :: (Bool , Bool , Bool , Bool) -> String 
chooseAction (isJump ,isAcceleration ,isDeceleration ,isSpeedOk) 
        | (True ,False, False, False) = "JUMP"
        | (False ,True, False, False) = "SPEED"
        | (False ,False, True, False) = "SLOW"
        | (False ,False ,False, True) = "WAIT"

但是这不能编译：

Couldn't match expected type `Bool'
            with actual type `(Bool, Bool, Bool, Bool)'
In the expression: (True, False, False, False)
In a stmt of a pattern guard for
               an equation for `chooseAction':
  (True, False, False, False)
In an equation for `chooseAction':
    chooseAction (isJump, isAcceleration, isDeceleration, isSpeedOk)
      | (True, False, False, False) = "JUMP"
      | (False, True, False, False) = "SPEED"
      | (False, False, True, False) = "SLOW"
      | (False, False, False, True) = "WAIT"

我在定义这样一个函数时做错了什么？

Answer 1

为此使用模式匹配：

chooseAction :: (Bool , Bool , Bool , Bool) -> String 
chooseAction (True ,False, False, False) = "JUMP"
chooseAction (False ,True, False, False) = "SPEED"
chooseAction (False ,False, True, False) = "SLOW"
chooseAction (False ,False ,False, True) = "WAIT"
chooseAction _ = error "This should not happen" -- if sequence is something else

Answer 2

守卫表达式的计算结果必须是 Bool 值，但在您的情况下，您有布尔元组。这就是它抛出错误的原因

Couldn't match expected type `Bool'
            with actual type `(Bool, Bool, Bool, Bool)'

你最好像这样定义函数

chooseAction::(Bool, Bool, Bool, Bool) -> String
chooseAction (True, False, False, False) = "JUMP"
chooseAction (False, True, False, False) = "SPEED"
chooseAction (False, False, True, False) = "SLOW"
chooseAction (False, False, False, True) = "WAIT"

此外，您并未涵盖所有情况。所以，你可以这样覆盖

chooseAction tuple_of_bools = "NONE"

如果none个模式匹配，则执行最后一个得到NONE。或者，如果你希望它像现在这样定义，那么只需更改比较，就像这样

chooseAction:: (Bool, Bool, Bool, Bool) -> String
chooseAction bools
        | bools == (True ,False, False, False) = "JUMP"
        | bools == (False ,True, False, False) = "SPEED"
        | bools == (False ,False, True, False) = "SLOW"
        | bools == (False ,False ,False, True) = "WAIT"
        | otherwise = "NONE"

是否可以在守卫中匹配布尔元组，如模式匹配？

Can a tuple of booleans be matched in guards, like pattern matching?

haskell

boolean

tuples