问题
I'm still just learning the basics of Haskell, and I've tried to find an answer to this simple question, so I apologize in advance, because I'm sure it's simple.
Given:
data Fruit = Fruit| Apple | Orange
deriving (Show, Eq)
a = Apple
How do I check if some a is a Fruit?
回答1:
Assuming you really meant type comparison, the simple answer is "you can't". Haskell is statically typed, so the check is done at compile-time, not run-time. So, if you have a function like this:
foo :: Fruit -> Bool
foo Apple = True
foo x = False
The answer of whether or not x is a Fruit will always be "yes".
What you might be trying to do is find out what data constructor a given value was constructed with. To do that, use pattern matching:
fruitName :: Fruit -> String
fruitName Fruit = "Fruit"
fruitName Apple = "Apple"
fruitName Orange = "Orange"
By the way, if you're using GHCi, and you want to know the type of something, use :t
> let a = 123
> :t a
a :: Integer
>
来源：https://stackoverflow.com/questions/4273702/type-comparison-in-haskell