CODEWITHSUNDEEP
haskellfunctional-programming
sqd
sqd

Reputation: 1535

Haskell: Why "Nothing<(4::Maybe Int)" gets an error but "Nothing< Just 4" pass?

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

And why Nothing< Just 4 pass?

Upvotes: 0

Views: 119

Answers (2)

chi
chi

Reputation: 116174

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 Ints 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.

Upvotes: 5

Don Stewart
Don Stewart

Reputation: 137987

There's a simple type error in your expression: 4 does not have type Maybe Int.

Upvotes: 7

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