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

Consider this somewhat equivalent of Maybe in C++/Java'ish psuedocode...

template
abstract class Maybe { ... }

template
class Just : Maybe {
    // constructor
    Just (T val) { ... }

    ...
}

template
class Nothing : Maybe {
    // constructor
    Nothing () { ... }

    ...
}

That is not specific to Maybe, it can be applied to any ADT. Now what exactly will

data Maybe a = a | Nothing

model into ? (assuming that its legal syntax).

If you were to write a switch statement, to 'pattern match' against types, what will u match against (the switch is on the TYPE not the value), something like this (not necessarily valid code) :

switch (typeof (x)) {
    case Just : ...
    case Nothing : ...
    default : ... // Here you dont have any 'a' to get the inner type
}