Mutable Fields

The fields of a record can be declared as mutable, meaning their contents can be updated without constructing a new record. For example, here is a record type for two-dimensional colored points whose color field c is mutable:

# type point = {x:int; y:int; mutable c:string};;
type point = {x:int; y:int; mutable c:string; }

Note that mutable is a property of the field, rather than the type of the field. In particular, we write mutable field : type, not field : mutable type.

The operator to update a mutable field is <-:

# let p = {x=0; y=0; c="red"};;
val p : point = {x=0; y=0; c="red"}

# p.c <- "white";;
- : unit = ()

# p;;
val p : point = {x=0; y=0; c="white"}

# p.x <- 3;;
Error: The record field x is not mutable

Syntax and Semantics

The syntax and semantics of <- is similar to := but complicated by fields:

  • Syntax: e1.f <- e2

  • Dynamic semantics: To evaluate e1.f <- e2, evaluate e2 to a value v2, and e1 to a value v1, which must have a field named f. Update v1.f to v2. Return ().

  • Static semantics: e1.f <- e2 : unit if e1 : t1 and t1 = {...; mutable f : t2; ...}, and e2 : t2.

Refs and Mutable Fields

It turns out that refs are actually implemented as mutable fields. In Stdlib we find the following declaration:

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

And that's why when we create a ref it does in fact looks like a record: it is a record!

# let r = ref 3110;;
val r : int ref = {contents = 3110}

The other syntax we've seen for records is in fact equivalent to simple OCaml functions:

(* Equivalent to [fun v -> {contents=e}]. *)
val ref : 'a -> 'a ref

(* Equivalent to [fun r -> r.contents]. *)
val (!) : 'a ref -> 'a

(* Equivalent to [fun r v -> r.contents <- v]. *)
val (:=) : 'a ref -> 'a -> unit

The reason we say "equivalent" is that those functions are actually implemented not in OCaml but in the OCaml run-time, which is implemented mostly in C. But the functions do behave the same as the OCaml source given above in comments.

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