F#: let mutable vs. ref

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 situatio

Answer 1

This article by Brian might provide an answer.

Mutables are easy to use and efficient (no wrapping), but can't be captured in lambdas. Ref cells can be captured, but are verbose and less efficient (? - not sure of this).

Answer 2

You may want to take a look at the Mutable Data section in the wikibook.

For convenience, here are some relevant quotes:

The mutable keyword is frequently used with record types to create mutable records

Mutable variables are somewhat limited: mutables are inaccessible outside of the scope of the function where they are defined. Specifically, this means its not possible to reference a mutable in a subfunction of another function.

Ref cells get around some of the limitations of mutables. In fact, ref cells are very simple data type which wrap up a mutable field in a record type.

Since ref cells are allocated on the heap, they can be shared across multiple functions

Answer 3

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!

Answer 4

As described in this MSDN Blog article in section Simplified use of mutable values, you no longer need ref cells for lambdas. So in general you no longer need them at all.

Answer 5

Related Question: "You mentioned that local mutable values cannot be captured by a closure, so you need to use ref instead. The reason for this is that mutable values captured in the closure need to be allocated on the heap (because closure is allocated on the heap)." from F# ref-mutable vars vs object fields

I think let mutable is preferred over reference cells. I personally only use reference cells when they are required.

Most code I write doesn't use mutable variables thanks to recursion and tail calls. If I have a group of mutable data I use a record. For objects I use let mutable to make private mutable variables. I only really use reference cells for closures, generally events.