CODEWITHSUNDEEP
global-variablesocamlcaml
Sara4391
Sara4391

Reputation: 121

Global variables in Ocaml

I am looking for a way to define global variables in ocaml so that i can change their value inside the program. The global variable that I want to user is:

type state = {connected : bool ; currentUser : string};;
let currentstate = {connected = false ; currentUser = ""};;

How can I change the value of connected and currentUser and save the new value in the same variable currentstae for the whole program?

Upvotes: 9

Views: 10692

Answers (2)

cyc115
cyc115

Reputation: 645

type state = {connected : bool ; currentUser : string};; let currentstate = {connected = false ; currentUser = ""};;

can be translated to :

type state = {connected : bool ref ; currentUser : string ref };;
let currentstate = {connected = ref false ; currentUser = ref ""};;

to assign value :

(currentstate.connected) := true ;;
- : unit = ()

to get value : 

!(currentstate.connected) ;;
- : bool = true

you can also pattern match on its content.

read more about ref here

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Either declare a mutable record type:

type state = 
  { mutable connected : bool; mutable currentUser : string };;

Or declare a global reference

let currentstateref = ref { connected = false; currentUser = "" };;

(then access it with !currentstateref.connected ...)

Both do different things. Mutable fields can be mutated (e.g. state.connected <- true; ... but the record containing them stays the same value). References can be updated (they "points to" some newer value).

You need to take hours to read a lot more your Ocaml book (or its reference manual). We don't have time to teach most of it to you.

A reference is really like

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

but with syntactic sugar (i.e. infix functions) for dereferencing (!) and updating (:=)

Upvotes: 5

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