In the context of the F# language, a module is a grouping of F# code, such as values, types, and function values, in an F# program. Grouping code in modules helps keep related code together and helps avoid name conflicts in your program.


// Top-level module declaration.
module [accessibility-modifier] [qualified-namespace.]module-name
// Local module declaration.
module [accessibility-modifier] module-name =


An F# module is a grouping of F# code constructs such as types, values, function values, and code in do bindings. It is implemented as a common language runtime (CLR) class that has only static members. There are two types of module declarations, depending on whether the whole file is included in the module: a top-level module declaration and a local module declaration. A top-level module declaration includes the whole file in the module. A top-level module declaration can appear only as the first declaration in a file.

In the syntax for the top-level module declaration, the optional qualified-namespace is the sequence of nested namespace names that contains the module. The qualified namespace does not have to be previously declared.

You do not have to indent declarations in a top-level module. You do have to indent all declarations in local modules. In a local module declaration, only the declarations that are indented under that module declaration are part of the module.

If a code file does not begin with a top-level module declaration or a namespace declaration, the whole contents of the file, including any local modules, becomes part of an implicitly created top-level module that has the same name as the file, without the extension, with the first letter converted to uppercase. For example, consider the following file.

// In the file program.fs.
let x = 40

This file would be compiled as if it were written in this manner:

module Program
let x = 40

If you have multiple modules in a file, you must use a local module declaration for each module. If an enclosing namespace is declared, these modules are part of the enclosing namespace. If an enclosing namespace is not declared, the modules become part of the implicitly created top-level module. The following code example shows a code file that contains multiple modules. The compiler implicitly creates a top-level module named Multiplemodules, and MyModule1 and MyModule2 are nested in that top-level module.

// In the file multiplemodules.fs.
// MyModule1
module MyModule1 =
    // Indent all program elements within modules that are declared with an equal sign.
    let module1Value = 100

    let module1Function x =
        x + 10

// MyModule2
module MyModule2 =

    let module2Value = 121

    // Use a qualified name to access the function.
    // from MyModule1.
    let module2Function x =
        x * (MyModule1.module1Function module2Value)

If you have multiple files in a project or in a single compilation, or if you are building a library, you must include a namespace declaration or module declaration at the top of the file. The F# compiler only determines a module name implicitly when there is only one file in a project or compilation command line, and you are creating an application.

The accessibility-modifier can be one of the following: public, private, internal. For more information, see Access Control. The default is public.

Referencing Code in Modules

When you reference functions, types, and values from another module, you must either use a qualified name or open the module. If you use a qualified name, you must specify the namespaces, the module, and the identifier for the program element you want. You separate each part of the qualified path with a dot (.), as follows.


You can open the module or one or more of the namespaces to simplify the code. For more information about opening namespaces and modules, see Import Declarations: The open Keyword.

The following code example shows a top-level module that contains all the code up to the end of the file.

module Arithmetic

let add x y =
    x + y

let sub x y =
    x - y

To use this code from another file in the same project, you either use qualified names or you open the module before you use the functions, as shown in the following examples.

// Fully qualify the function name.
let result1 = Arithmetic.add 5 9
// Open the module.
open Arithmetic
let result2 = add 5 9

Nested Modules

Modules can be nested. Inner modules must be indented as far as outer module declarations to indicate that they are inner modules, not new modules. For example, compare the following two examples. Module Z is an inner module in the following code.

module Y =
    let x = 1

    module Z =
        let z = 5

But module Z is a sibling to module Y in the following code.

module Y =
    let x = 1

module Z =
    let z = 5

Module Z is also a sibling module in the following code, because it is not indented as far as other declarations in module Y.

module Y =
        let x = 1

    module Z =
        let z = 5

Finally, if the outer module has no declarations and is followed immediately by another module declaration, the new module declaration is assumed to be an inner module, but the compiler will warn you if the second module definition is not indented farther than the first.

// This code produces a warning, but treats Z as a inner module.
module Y =
module Z =
    let z = 5

To eliminate the warning, indent the inner module.

module Y =
    module Z =
        let z = 5

If you want all the code in a file to be in a single outer module and you want inner modules, the outer module does not require the equal sign, and the declarations, including any inner module declarations, that will go in the outer module do not have to be indented. Declarations inside the inner module declarations do have to be indented. The following code shows this case.

// The top-level module declaration can be omitted if the file is named
// TopLevel.fs or topLevel.fs, and the file is the only file in an
// application.
module TopLevel

let topLevelX = 5

module Inner1 =
    let inner1X = 1
module Inner2 =
    let inner2X = 5

Module rec: allowing mutual recursive code at the module level

F# 4.1 introduces the notion of modules which allow for all contained code to be mutually recursive. This is done via module rec. Use of module 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:

module rec RecursiveModule =
    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 private BananaHelpers =
        let peel (b : Banana) =
            let flip banana =
                match banana.Orientation with
                | Up -> 
                    banana.Orientation <- Down
                | Down -> banana

            let peelSides banana =
                for side in banana.Sides do
                    if side = Unpeeled then
                        side <- 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 RecursiveModule module.

This capability is also possible in Namespaces with F# 4.1.

See Also

F# Language Reference Namespaces F# RFC FS-1009 - Allow mutually referential types and modules over larger scopes within files