Question

First, I acknowledge the possibility that this question could be a duplicate; just let me know.

I’m curious what the general “best practice” is for those situations when mutability is desired. F# seems to offer two facilities for this: the let mutable binding, which seems to work like variables in “most” languages, and the reference cell (created with the ref function) that requires explicit dereferencing to use.

There are a couple of cases where one is “forced” into one or the other: .NET interop tends to use mutable with <-, and in workflow computations one must use ref with :=. So those cases are pretty clear-cut, but I’m curious what to do when creating my own mutable variables outside of those scenarios. What advantage does one style have over the other? (Perhaps further insight into the implementation would help.)

Thanks!

Answer 1

I can only support what gradbot said – when I need mutation, I prefer let mutable.

Regarding the implementation and differences between the two – ref cells are essentially implemented by a very simple record that contains a mutable record field. You could write them easily yourself:

type ref<'T> =  // '
  { mutable value : 'T } // '

// the ref function, ! and := operators look like this:
let (!) (a:ref<_>) = a.value
let (:=) (a:ref<_>) v = a.value <- v
let ref v = { value = v }

A notable difference between the two approaches is that let mutable stores the mutable value on the stack (as a mutable variable in C#) while ref stores the mutable value in a field of a heap-allocated record. This may have some impact on the performance, but I don’t have any numbers…

Thanks to this, mutable values that use ref can be aliased – meaning that you can create two values that reference the same mutable value:

let a = ref 5  // allocates a new record on the heap
let b = a      // b references the same record
b := 10        // modifies the value of 'a' as well!

let mutable a = 5 // mutable value on the stack
let mutable b = a // new mutable value initialized to current value of 'a'
b <- 10           // modifies the value of 'b' only!