**Course Title:** Functional Programming with Haskell: From Fundamentals to Advanced Concepts **Section Title:** Modules and Code Organization in Haskell **Topic:** Creating and using custom modules in Haskell **Introduction** In Haskell, modules are a crucial component of code organization. They enable developers to structure their code into logical units, making it easier to manage, maintain, and reuse. In this topic, we will explore how to create and use custom modules in Haskell, including module declarations, exports, imports, and qualified imports. **Module Declarations** A Haskell module is declared using the `module` keyword followed by the module name and a `where` clause. The module name must be a valid Haskell identifier. ```haskell module MyModule where ``` You can also add a description to the module using the `-- |` syntax. ```haskell {- | This is a description of MyModule. -} module MyModule where ``` **Module Exports** By default, all definitions within a module are exported. However, you can control what is exported by using the `module` declaration with an export list. ```haskell module MyModule (myFunction) where myFunction :: Int -> Int myFunction x = x + 1 ``` In this example, only `myFunction` is exported from `MyModule`. **Imports** To use a module, you need to import it. There are two types of imports: unqualified and qualified. * Unqualified imports: Bring all definitions from the module into scope. ```haskell import MyModule ``` * Qualified imports: Require you to prefix definitions from the module with the module name. ```haskell import qualified MyModule as M ``` You can also import specific definitions using the `import` statement with a list of imports. ```haskell import MyModule (myFunction) ``` **Qualified Imports** When using qualified imports, you need to prefix definitions from the module with the module name or alias. ```haskell import qualified MyModule as M main :: IO () main = print (M.myFunction 2) ``` **Hierarchical Module Names** Haskell allows you to use hierarchical module names, which are separated by dots. ```haskell module Control.Monad.Trans.State where ``` **Module Dependencies** When creating a custom module, you may need to depend on other modules. You can specify these dependencies using the `import` statement or by listing them in the `.cabal` file (we'll cover this in the next topic). **Best Practices** Here are some best practices for creating and using custom modules in Haskell: * Use meaningful and descriptive module names. * Keep module exports to a minimum to avoid polluting the namespace. * Use qualified imports to avoid naming conflicts. * Modularize your code into logical units to improve maintainability and reusability. **Conclusion** In this topic, we've covered how to create and use custom modules in Haskell, including module declarations, exports, imports, and qualified imports. By following best practices, you can structure your code into logical units, making it easier to manage, maintain, and reuse. **External Resources** * The Haskell Wiki: [Modules](https://wiki.haskell.org/Module) * The Haskell 2010 Report: [Modules](https://www.haskell.org/onlinereport/haskell2010/haskellch5.html) **Next Topic** Managing Dependencies with Cabal and Stack **Leave a comment below if you have any questions or need further clarification on this topic.