Reference Cells
Reference cells are storage locations that enable you to create mutable values with reference semantics.
Syntax
ref expression
Remarks
You use the ref function to create a new reference cell with an initial value. You can then change the underlying value because it is mutable. A reference cell holds an actual value; it is not just an address.
The following code example illustrates the declaration and use of reference cells.
let xRef = ref 10
printfn "%d" xRef.Value
xRef.Value <- 11
printfn "%d" xRef.Value
The output is as follows.
10
11
Reference cells are instances of the Ref generic record type, which is declared as follows.
type Ref<'a> =
{ mutable contents: 'a }
The type 'a ref is a synonym for Ref<'a>. The compiler and IntelliSense in the IDE display the former for this type, but the underlying definition is the latter.
The ref operator creates a new reference cell. The following code is the declaration of the ref operator.
let ref x = { contents = x }
The following table shows the features that are available on the reference cell.
| Operator, member, or field | Description | Type | Definition |
|---|---|---|---|
ref (operator) |
Encapsulates a value into a new reference cell. | 'a -> 'a ref |
let ref x = { contents = x } |
Value (property) |
Gets or sets the underlying value. | unit -> 'a |
member x.Value = x.contents |
C# programmers should know that ref in C# is not the same thing as ref in F#. The equivalent constructs in F# are byrefs, which are a different concept from reference cells.
Values marked as mutable may be automatically promoted to 'a ref if captured by a closure; see Values.
Deprecated constructs
Since F# 6.0, the following operators are deprecated and their use gives informational warnings:
| Operator, member, or field | Description | Type | Definition |
|---|---|---|---|
! (dereference operator, deprecated) |
Returns the underlying value. | 'a ref -> 'a |
let (!) r = r.contents |
:= (assignment operator, deprecated) |
Changes the underlying value. | 'a ref -> 'a -> unit |
let (:=) r x = r.contents <- x |
contents (record field) |
Gets or sets the underlying value. | 'a |
let ref x = { contents = x } |
Instead, the direct use of .Value is preferred; see F# RFC FS-1111.
The field contents is provided for compatibility with other versions of ML and will produce a warning during compilation. To disable the warning, use the --mlcompatibility compiler option. For more information, see Compiler Options.
