How do I create an instance of a custom data type in OCaml?

If I have a custom data type ‘triple’ as follows, how would I create an instance of it that I can use?

type triple = int * int * int

Thank you!

>Solution :

Here is an instance:

let my_instance = (1, 2, 3)

For what it’s worth, your type is not a new type, it’s just a handy abbreviation for the pre-existing type that’s a tuple of 3 ints. A type declaration without a constructor is just an abbreviation, so not really what I’d call a "custom" type.

# ((1, 2, 3) : triple) = (1, 2, 3)
- : bool = true

I’m not sure this matters to you, but if you need a new type that’s not compatible with any other, you need a constructor. Something like this:

type triple2 = T of int * int * int

Then you can make an instance like this:

let my_instance2 = T (1, 2, 3)

This new type triple2 is not compatible with any other type (such as, say, triple):

# T (1, 2, 3) = (1, 2, 3)
Error: This expression has type 'a * 'b * 'c
       but an expression was expected of type triple2