A module groups related types and values under a name. In F#, it is a practical home for the functions that operate on a domain model: keep a Basket record as data, for example, and place AddItem, Clear, and Checkout-style functions in a module. Each function can return an updated basket rather than modifying hidden object state. This separates data from behavior and keeps functions easier to create, test, replace, and understand. section 2.1.3 section 10.1.2
Modules and namespaces both organize code, but modules can contain values as well as types. A symbol inside a module is accessed through its qualified name, such as Module.Symbol, or made convenient with open. Modules can also contain indented submodules; a nested module can use members of its parent without opening it. section 6.2 section 6.2.2 section 6.4
A small utility module
Declare a module, then place related functions beneath it. A nested module can refine the grouping when a simple utility grows. module FsInAction.MyMaths let add x y = x + y let subtract x y = x - y module Complicated = let ten = 10 let addTogetherThenSubtractTen x y = add x y |> subtract ten The functions are available as MyMaths.add and MyMaths.subtract; Complicated is a child module. section 6.2.2
Listing 6.8 Working with modules in F#
module FsInAction.MyMaths #1
let add x y = x + y #2
let subtract x y = x - y #2
module Complicated = #3
open System
let ten = 10 #4
let addTogetherThenSubtractTen x y = add x y |> subtract ten #5
Inspect the listing to see a top-level module, module-qualified naming, and an indented child module working together. section 6.2.2
Use modules as an application grows:
- Start simply: a few modules in a few files can be enough. Group multiple related types in one file rather than enforcing one type per file. section 6.2.5
- Separate Domain types, Logic functions, and Program orchestration initially; later regroup around areas such as order management, customer management, or billing when that better fits the application. section 6.2.5
- Use namespaces when separate files need modules beneath the same parent namespace. A fully qualified module cannot be declared across multiple files. section 6.2.2 section 6.2.5
- Expect an F# module to appear as a static class to C# callers, with its functions and types exposed as members. section 11.2.1
