SML Mutable Ref

fun get_value_at_reference(reference : 'a ref) : 'a = 
    !reference

fun set_value_at_reference(reference : 'a ref, next_value : 'a) : 'a = 
    let
        val previous_value = !reference
        val _ = reference := next_value
    in
        previous_value
    end

(* alternate version
fun set_value_at_reference(reference : 'a ref, next_value : 'a) : 'a = 
    let
        val previous_value = !reference
    in
        ( reference := next_value
        ; previous_value )
    end
*)