A module groups related Julia functions, types, and constants in a namespace. That namespace lets two projects use the same function name without making those functions interchangeable. Define one with `module Name ... end`; definitions made at the REPL instead belong to `Main` unless placed in a named module. section 16.1.2 section 9.3.2
Defining `Cylinder.volume` and `Cone.volume` in separate modules gives each calculation its own name. Call the one you want with a qualified name, such as `Cylinder.volume(2.5, 3)`. section 16.1.2
Use a ready-made module
At the REPL, run `using Statistics`, then try `mean([1, 2, 3])` or `std([3, 3, 3])`. For an overview of available names, type `Statistics.` and press Tab twice. Type `?Statistics` to open its documentation. section 4.4
A module is not the same thing as a package. A package bundles source code, resources, and metadata; it defines a module that Julia loads when you use the package. For a growing package, keep the module definition in one file and call `include` there to evaluate definitions from other source files inside that module. section 16.1.2 section 16.2.2
using Statistics
The ToyGeometry module includes separate volume and trigonometry files. Their functions become part of the same module without crowding its definition file. section 16.2.2
Module boundaries also matter when extending an existing function. Before adding angle-specific methods to Julia’s `+`, write `import Base: +`. Without the import, defining `+` creates a separate function in `Main` that can shadow `Base.+`; even `3 + 4` may then fail. To make a module function available by name, Julia uses `export sphere_volume`, not a `public` keyword before its definition. section 9.3.2 section 16.4
