Haskell：为什么“ Nothing <（4 :: Maybe Int）”得到一个错误，但是“ Nothing <Just 4”通过？

Question

Nothing 's type could be Maybe Int and why couldn't it be compared with another Maybe Int ?

And why Nothing< Just 4 pass?

Answer 1

表达式中有一个简单的类型错误： 4没有类型Maybe Int 。

Answer 2

Yes, Nothing can be of type Maybe Int (or Maybe String , or Maybe AnythingElse ).

Any two values of type Maybe Int can be compared.

Equality == is defined as one might expect: Nothing is only equal to Nothing , and Just x is only equal to Just y if x==y . This is defined in the Eq instance for Maybe a , which is automatically imported since it is in the Haskell Prelude .

Similarly, there is an Ord (Maybe a) instance as well in the Prelude . This instance defines < between Maybe Int s so that Nothing is the minimum element ( < anything else), while two values Just x and Just y are compared according to whether x < y .

So, Nothing < Just 4 is true.

---

The code Nothing<(4::Maybe Int) gives you an error since 4 is an Int (technically, it is any type in the Num type class), but 4 is not a Maybe Int . If you write 4 :: String or 4 :: [Int] or 4 :: Int -> Int you will get a similar type error.

The Maybe Int type contains only the values

Nothing
Just 0
Just 1
Just -1
Just 2
Just -2
...

(and some bottoms, which I omit.)

The value 4 is not a value of type Maybe Int . Just 4 is instead one of such values.