1. Example
  2. Immutable records and functional updates
  3. Mutable fields and destructive updates
  4. References

Records and references

As we've seen already, combinations of sum and product types can create powerful and expressive abstractions. However, a product type with too many fields can easily become unwieldy. It may also be difficult to remember what every item means if they are the same type.

This is where records come in handy. They have named fields, so it's easy to convey the meaning of every field.

OCaml records are similar to records in Pascal or Ada, and to C structs. They can have one or more named fields, and every field can be accessed by name.

Record types cannot be anonymous, they must always be defined before use.

Example

Let's write an example program that deals with 2d vectors defined by their components (x and y):

type vector = { x: float; y: float }

let vec = {x=1.0; y=1.0}

let length v =
  (v.x *. v.x) +. (v.y *. v.y) |> sqrt
  
let () =
  Printf.printf "%f\n" (length vec)

As you can see, fields can be accessed using a dot notation (value.field_name).

When you create a new record value, all fields must be defined. Trying to create a partially defined record will cause a compilation error.

Modifying records is a more interesting subject. Unlike other values, records in OCaml can be either immutable or mutable. Thus there are two ways

Immutable records and functional updates

In imperative languages, record fields are mutable by default and assigning a new value to a field modifies an existing record irreversibly.

Since OCaml is mainly a functional language, record fields are immutable by default. However, it's possible to create a new record without destroying the old one.

This is called “functional update”. As we have seen in previous chapters, immutable values benefit from structural sharing. Inside the program memory, a record value is a collection of pointers to its fields. Since field values are known to be immutable, it is safe only copy the pointers rather than actual values.

Functional update is done using with keyword. To demonstrate it let's define a simple type for a mock contact list and define a contact, then update the phone number.

type contact = {name: string; phone: string}

let person = {name="Boris"; phone="2128506"}

let person = {person with phone="212850A"}

It's fine to update multiple fields at once as well:

type contact = {name: string; phone: string}

let person = {name="Boris"; phone="2128506"}

let person = {person with name="Bob"; phone="212850B"}

If you change every field of a record, the compiler will warn you about it:

Warning 23: all the fields are explicitly listed in this record: the 'with' clause is useless. 

Mutable fields and destructive updates

Most values in OCaml are immutable, but records can have mutable fields.

Mutable fields are declared and updated using this syntax:

type user = {
  name: string;
  mutable password: string
}

let user = {name="root"; password=""}

let () = user.password <- "qwerty"

Unlike many operators we have seen so far, <- is special syntax rather than a normal function, so it cannot be applied partially. Field update expressions like user.password <- "qwerty" have type unit.

It's impossible to modify fields this way if they aren't declared as mutable.

References

Creating a new record type every time you need a mutable variable would quickly become annoying, so OCaml standard library includes very simple syntactic sugar for it.

There's a type 'a ref that is really a record with a single mutable field named “contents”. It comes with a function ref : 'a -> 'a ref that creates a new reference, and an assignment operator :=. There's also a dereference operator ! for getting values. For example, !(ref 0) will return 0.

If that syntactic sugar didn't exist, we could trivially define it ourselves.

type 'a ref = {mutable contents: 'a}

let ref value = {contents=value}

let (:=) reference value = reference.contents <- value

let (!) r = r.contents

Fortunately it's done for us alredy, so we can just use it right away:

let r = ref 0

let () =
  r := 1;
  Printf.printf "%d\n" !r