Ocaml how to access elements in type definitions

Question

With the following type definition in ocaml:

type yearday = YMD of int * int * int 

how would you access the different ints of the type? Like if I just wanted the value of the first int.

Answer 1

The parts of a value like this are accessed through pattern matching. Here's a function that returns the first int:

let y_of_ymd (YMD (y, _, _)) = y

Here's how it looks in the toplevel (OCaml REPL):

# let y_of_ymd (YMD (y, _, _)) = y;;
val y_of_ymd : yearday -> int = <fun>
# let myymd = YMD (2017, 4, 18);;
val myymd : yearday = YMD (2017, 4, 18)
# y_of_ymd myymd;;
- : int = 2017
#

Update

If you have more than one variant in your type you can use match to determine which kind of value is present

type yearday = YMD of int * int * int | YD of int * int

let y_of_yearday yearday =
    match yearday with
    | YMD (y, _, _) -> y
    | YD (y, _) -> y

There are more concise ways to write this, but I think this gives the best idea of what's going on.