Question

I’m currently learning SML and I have a question about something I have no name for. Lets call it “type alias” for the moment. Suppose I have the following datatype definition:

datatype 'a stack = Stack of 'a list;

I now want to add an explicit “empty stack” type. I can to this by adding it to the datatype:

datatype 'a stack = emptystack | Stack of 'a list;

Now I can pattern match a function like “push”:

fun push (emptystack) (e:'a) = Stack([e])
  | push (Stack(list):'a stack) (e:'a) = Stack(e::list);

The problem here is that Stack([]) and emptystack are different but I want them to be the same. So every time SML encounters an Stack([]) it should “know” that this is emptystack (in case of push it should then use the emptystack match).

Is there a way to achieve this?

Answer 1

The short answer is: No, it is not possible.

You can create type aliases with the code

type number = int

val foo : number -> int -> number =
 fn a => fn b => a+b

val x : int = foo 1 3;
val y : number = foo 1 3;

However, as the name says, it only works for types. Your question goes for value constructors, which there is no syntax for.