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One particularly useful mutable record is the general-purpose type of mutable reference cells, or ref cells for short. These often play much the same role as pointers in other imperative programming languages. You can see how to use mutable reference cells in the following example:
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> let cell1 = ref 1;; val cell1 : int ref = {contents = 1;} > !cell1;; val it : int = 1 > cell1 := 3;; val it : unit = () > cell1;; val it : int ref = {contents = 3;} > !cell1;; val it : int = 3 The key type is 'T ref, and its main operators are ref, !, and :=. The types of these operators are as follows: val ref : 'T -> 'T ref val (:=) : 'T ref -> 'T -> unit val (!) : 'T ref -> 'T These allocate a reference cell, mutate the cell, and read the cell, respectively. The operation cell1 := 3 is the key one; after this operation, the value returned by evaluating the expression !cell1 is changed. You can also use either the contents field or the Value property to access the value of a reference cell. Both the 'T ref type and its operations are defined in the F# library as simple record data structures with a single mutable field: type 'T ref = { mutable contents: 'T } let (!) r = r.contents let (:=) r v = r.contents <- v let ref v = { contents = v } The type 'T ref is a synonym for a type Microsoft.FSharp.Core.Ref<'T> defined in this way.
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Which Data Structures Are Mutable
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It s useful to know which data structures are mutable and which aren t. If a data structure can be mutated, then this is typically evident in the types of operations you can perform on that structure. For example, if a data structure Table<'Key,'Value> has an operation like the following, then in practice you can be sure that updates to the data structure modify the data structure itself:
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val add : Table<'Key,'Value> -> 'Key -> 'Value -> unit
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That is, the updates to the data structure are destructive, and no value is returned from the operation; the result is the type unit, which is akin to void in C and many other languages. Likewise, the following member indicates the data structure is almost certainly mutable:
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member Add : 'Key * 'Value -> unit
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In both cases, the presence of unit as a return type is a sure sign that an operation performs some side effects. In contrast, operations on immutable data structures typically return a new instance of the data structure when an operation such as add is performed. For example:
val add : 'Key -> 'Value -> Table<'Key,'Value> -> Table<'Key,'Value>
Or for example:
member Add : 'Key * 'Value -> Table<'Key,'Value>
As discussed in 3, immutable data structures are also called functional or persistent. The latter name is used because the original table isn t modified when adding an element. Well-crafted persistent data structures don t duplicate the actual memory used to store the data structure every time an addition is made; instead, internal nodes can be shared between the old and new data structures. Example persistent data structures in the F# library are F# lists, options, tuples, and the types Microsoft.FSharp. Collections.Map<'Key,'Value> and Microsoft.FSharp.Collections.Set<'Key>. Most data structures in the .NET libraries aren t persistent, although if you re careful, you can use them as persistent data structures by accessing them in read-only mode and copying them where necessary.
Avoiding Aliasing
Like all mutable data structures, two mutable record values or two values of type 'T ref may refer to the same reference cell this is called aliasing. Aliasing of immutable data structures isn t a problem; no client consuming or inspecting the data values can detect that the values have been aliased. However, aliasing of mutable data can lead to problems in understanding code. In general, it s good practice to ensure that no two values currently in scope directly alias the same mutable data structures. The following example continues from earlier and shows how an update to cell1 can affect the value returned by !cell2: > let cell2 = cell1;; val cell2 : int ref = {contents = 3;} > !cell2;; val it : int = 3 > cell1 := 7;; val it : unit = () > !cell2;; val it : int 7
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