pattern matching - How can I assign a reference cell in a function and then return the value in sml -

so have function in sml takes integer value , refs it. how can assign new value variable balance , return balance? in deposit , withdraw patterns?

datatype message = getbalance | deposit of int | withdraw of int;  fun opening_account init_amt = let val balance = ref init_amt                 in fn getbalance => !balance                  | deposit x => balance = !balance + x                  | withdraw x => balance = !balance - x                end;  

to update reference cell, use :=. i.e.:

balance := !balance + x 

if wish both update balance, , return new value, 1 after other ;:

(balance := !balance + x; !balance)