Why do Maybe/Optional types use a Just/Some type instead of the actual type?

Question

In Idris, the Maybe type is defined as followed:

data Maybe a = Just a | Nothing  

It\'s defined similarly in Haskell:

Answer 1

The benefit of the constructor (Just or Some) is that it provides a way to distinguish between the branches of the data type. That is, it avoids ambiguity.

For example if we were relying on type inference, then the type of x in the following seems fairly straightforward — String.

x = "Hello"

However, if we allowed your definition of Maybe, how would we know whether x was String, a Maybe String or a Maybe (Maybe String) etc.

Also consider a data type with more than two cases:

data Tree a
  = Empty
  | Node (Tree a) (Tree a)
  | Leaf a

If we simply removed the constructors (other than Empty), following your suggestion for Maybe, we'd end up with:

data Tree a
  = Empty
  | (Tree a) (Tree a)
  | a

I hope you can see that the ambiguity gets even worse.