OCaml

refs and mutable record fields

A record field can be designated as mutable. This allows the <- operator to be used, which modifies the value that f is pointing to. More formally:

The expression expr1 . field <- expr2 evaluates expr1 to a record value, which is then modified in-place by replacing the value associated to field in this record by the value of expr2. This operation is permitted only if field has been declared mutable in the definition of the record type.¹

type foo = { mutable alpha: int; beta: string };;
let f = { alpha = 0; beta = "abc" } in
  f.alpha <- 5;
  f;;
 
type bar = { delta: int; gamma: string };;
let b = { delta = 0; gamma = "abc" } in
  { b with delta = 5 };;

The ref type (short for “references”) is essentially syntactic sugar for a record with a single mutable field.²

#show_type ref;;
(* type 'a ref = { mutable contents : 'a; } *)
 
#show (!);;
(* ( ! ) : 'a ref -> 'a = "%field0" *)
(* Equivalent to: let ( ! ) r = r.contents *)
 
#show (:=);;
(* ( := ) : 'a ref -> 'a -> unit = "%setfield0" *)
(* Equivalent to: let ( := ) r x = r.contents <- x *)

Footnotes

1. https://ocaml.org/manual/5.3/expr.html#sss:expr-records ↩

2. https://cs3110.github.io/textbook/chapters/mut/mutable_fields.html ↩