CODEWITHSUNDEEP
smlml
Ryan Smith
Ryan Smith

Reputation: 709

How to have more than one type in ML?

Is it possible in ML to allow a variable in ML to have more than one type?

For instance, If I want a node in a tree to only be an int or a string. 

Node of int * string

I tried this but it just results in a tuple type of (int, string). I don't want it to be a tuple, just either an int or a string. Is this allowed?

Upvotes: 3

Views: 567

Answers (2)

Yawar
Yawar

Reputation: 11627

Just to generalise @TimDestan's great answer a bit: the general idea of a value having either one type or the other can be encoded in a type. E.g., in Haskell and Scala we have the Either type which can hold values of two different types exclusively. We can define the same in SML:

datatype ('a, 'b) either = Left of 'a | Right of 'b

Now you can encode your node as:

Node of (int, string) either

And nodes can be constructed like:

Node (Left 1)
Node (Right "hello")

This is a bit more lightweight than defining a custom sum type for this case specifically.

Upvotes: 1

Tim Destan
Tim Destan

Reputation: 2028

As pointed out in a comment, the feature you're looking for is union types. I believe the syntax for them in SML is:

datatype Node = IntNode of int
              | StringNode of string

Upvotes: 4

Related Questions