16 Records and Tuples

Records allow us to aggregate data together.

We can define a record type:

type student = {
	name : string;
	year : int; (* graduation year*)
}

type student = { name : string; year : int; }

We can then create values of the record type

let rbg = {
	name = "Ruth Bader";
	year = 1954;
}

val rbg : student = {name = "Ruth Bader"; year = 1954}

Fun fact! If we want to include code we've written in a file in utop via the use directive: #use "filename.ml".

We can access a field in a record with the . operator

rbg.name;;

- : string = "Ruth Bader"

---

Another basic kind of data type is a tuple. They're like records; however, they're unnamed.

(10, 10, "am");;

- : int * int * string = (10, 10, "am")

This is a tuple with 3 values. We can even give this tuple type a name.

type time = int * int * string;;

type time = int * int * string

(10, 10, "am");;

- : int * int * string = (10, 10, "am")

let t : time = (10, 10, "am");;

val t : time = (10, 10, "am")

These two values ([6] and [7]) are the same.

type point = float * float;;

type point = float * float

let p = (5., 3.5);;

val p : float * float = (5., 3.5)

Say we wanted to access the elements of this tuple. OCaml defines the fst and snd methods that get the first and second items from a tuple respectively.

fst;;

- : 'a * 'b -> 'a = <fun>

snd;;

- : 'a * 'b -> 'b = <fun>

fst p;;

- : float = 5.

snd p;;

- : float = 3.5

It's important to note that these only work on pairs. OCaml will throw an error if we don't have a pair

fst t;; (* our time tuple from earlier *)

File "[14]", line 1, characters 4-5:
1 | fst t;; (* our time tuple from earlier *)
        ^
Error: This expression has type time = int * int * string
       but an expression was expected of type 'a * 'b