I'm trying to write a function that tells me whether one Enum is the successor of another. Here was my first attempt:
isSuccessorOf x y = x == succ y
看起来很合理。让我们尝试一下：
λ> isSuccessorOf 3 2
True
λ> isSuccessorOf 1 5
False
λ> isSuccessorOf 3 (maxBound :: Int)
*** Exception: Prelude.Enum.succ{Int}: tried to take `succ' of maxBound
Whoops. That should have been False. Let's make sure we don't try to do succ maxBound:
isSuccessorOf x y = y /= maxBound && x == succ y
让我们再试一次：
λ> isSuccessorOf 3 (maxBound :: Int)
False
λ> isSuccessorOf 3 (2 :: Integer)
<interactive>:2:1: error:
• No instance for (Bounded Integer)
arising from a use of ‘isSuccessorOf’
• In the expression: isSuccessorOf 3 (2 :: Integer)
In an equation for ‘it’: it = isSuccessorOf 3 (2 :: Integer)
Hmm, now it only works on bounded types. I'd like to avoid needing a separate function for unbounded and bounded Enums, especially if there's nothing at compile-time to keep you from using the unbounded function on a bounded type. Let's use an Ord constraint instead:
isSuccessorOf x y = x > y && x == succ y
让我们尝试一下：
λ> isSuccessorOf 3 (maxBound :: Int)
False
λ> isSuccessorOf 3 (2 :: Integer)
True
But now I'm making an unwarranted assumption. Let's try one more thing (note: this depends on Down having an Enum instance, which is new to GHC 8.10.1):
λ> import Data.Ord (Down(..))
λ> let delisleFreezing = Down 150
λ> isSuccessorOf (succ delisleFreezing) delisleFreezing
False
嗯，这不理想。
那么，没有这三个缺陷之一，有什么办法可以完成这项看似简单的任务？
- Fails to compile for types that aren't
Bounded - Bottoms for types that are
Bounded - Gives the wrong answer for types where
succ x > xdoesn't hold
Perhaps a more safe way to check this is making use of
enumFromTo, and check if the second item of the list is the successor we are looking for. We can, like you say, simply pattern match on a list with two elements, we do not need to check if that second element is indeedy:or we can, like @chi says use this to look if there is a successor: