A namespace lets you organize code into areas of related functionality by enabling you to attach a name to a grouping of program elements.
If you want to put code in a namespace, the first declaration in the file must declare the namespace. The contents of the entire file then become part of the namespace.
Namespaces cannot directly contain values and functions. Instead, values and functions must be included in modules, and modules are included in namespaces. Namespaces can contain types, modules.
Namespaces can be declared explicitly with the namespace keyword, or implicitly when declaring a module. To declare a namespace explicitly, use the namespace keyword followed by the namespace name. The following example shows a code file that declares a namespace Widgets with a type and a module included in that namespace.
namespace Widgets type MyWidget1 = member this.WidgetName = "Widget1" module WidgetsModule = let widgetName = "Widget2"
If the entire contents of the file are in one module, you can also declare namespaces implicitly by using the
module keyword and providing the new namespace name in the fully qualified module name. The following example shows a code file that declares a namespace
Widgets and a module
WidgetsModule, which contains a function.
module Widgets.WidgetModule let widgetFunction x y = printfn "%A %A" x y
The following code is equivalent to the preceding code, but the module is a local module declaration. In that case, the namespace must appear on its own line.
namespace Widgets module WidgetModule = let widgetFunction x y = printfn "%A %A" x y
If more than one module is required in the same file in one or more namespaces, you must use local module declarations. When you use local module declarations, you cannot use the qualified namespace in the module declarations. The following code shows a file that has a namespace declaration and two local module declarations. In this case, the modules are contained directly in the namespace; there is no implicitly created module that has the same name as the file. Any other code in the file, such as a
do binding, is in the namespace but not in the inner modules, so you need to qualify the module member
widgetFunction by using the module name.
namespace Widgets module WidgetModule1 = let widgetFunction x y = printfn "Module1 %A %A" x y module WidgetModule2 = let widgetFunction x y = printfn "Module2 %A %A" x y module useWidgets = do WidgetModule1.widgetFunction 10 20 WidgetModule2.widgetFunction 5 6
The output of this example is as follows.
Module1 10 20 Module2 5 6
For more information, see Modules.
When you create a nested namespace, you must fully qualify it. Otherwise, you create a new top-level namespace. Indentation is ignored in namespace declarations.
The following example shows how to declare a nested namespace.
namespace Outer // Full name: Outer.MyClass type MyClass() = member this.X(x) = x + 1 // Fully qualify any nested namespaces. namespace Outer.Inner // Full name: Outer.Inner.MyClass type MyClass() = member this.Prop1 = "X"
Namespaces in Files and Assemblies
Namespaces can span multiple files in a single project or compilation. The term namespace fragment describes the part of a namespace that is included in one file. Namespaces can also span multiple assemblies. For example, the
System namespace includes the whole .NET Framework, which spans many assemblies and contains many nested namespaces.
You use the predefined namespace
global to put names in the .NET top-level namespace.
namespace global type SomeType() = member this.SomeMember = 0
You can also use global to reference the top-level .NET namespace, for example, to resolve name conflicts with other namespaces.
F# 4.1 introduces the notion of namespaces which allow for all contained code to be mutually recursive. This is done via
namespace rec. Use of
namespace rec can alleviate some pains in not being able to write mutually referential code between types and modules. The following is an example of this:
namespace rec MutualReferences type Orientation = Up | Down type PeelState = Peeled | Unpeeled // This exception depends on the type below. exception DontSqueezeTheBananaException of Banana type BananaPeel() = class end type Banana(orientation : Orientation) = member val IsPeeled = false with get, set member val Orientation = orientation with get, set member val Sides: PeelState list = [ Unpeeled; Unpeeled; Unpeeled; Unpeeled] with get, set member self.Peel() = BananaHelpers.peel self // Note the dependency on the BananaHelpers module. member self.SqueezeJuiceOut() = raise (DontSqueezeTheBananaException self) // This member depends on the exception above. module BananaHelpers = let peel (b: Banana) = let flip (banana: Banana) = match banana.Orientation with | Up -> banana.Orientation <- Down banana | Down -> banana let peelSides (banana: Banana) = banana.Sides |> List.map (function | Unpeeled -> Peeled | Peeled -> Peeled) match b.Orientation with | Up -> b |> flip |> peelSides | Down -> b |> peelSides
Note that the exception
DontSqueezeTheBananaException and the class
Banana both refer to each other. Additionally, the module
BananaHelpers and the class
Banana also refer to each other. This would not be possible to express in F# if you removed the
rec keyword from the
This feature is also available for top-level Modules in F# 4.1 or higher.