Question

In this sample, get_final_answer is being eagerly evaluated, and always returns 0.0. I thought expressions containing refs were treated differently (and not eagerly evaluated in this case) due to their inherently mutable characteristics. I expected it to return 7.0.

let FinalAnswer = ref 0.0
let get_final_answer = !FinalAnswer

let rec eval_expr_fail =
        FinalAnswer := 7.0
        get_final_answer        // fails, returns 0.0

let rec eval_expr_works =
        FinalAnswer := 7.0
        !FinalAnswer           // works, return 7.0

How do I dereference FinalAnswer outside the block where I updated it?

Answer 1

get_final_answer in let get_final_answer = !FinalAnswer is a float value, not a function. It is the value of 0.0, and has nothing to do with FinalAnswer once the value is assigned.

Making it as a function gets what you want:

let FinalAnswer = ref 0.0
let get_final_answer() = !FinalAnswer

let rec eval_expr_fail =
        FinalAnswer := 7.0
        get_final_answer ()       //  returns 7.0