Go Language Modularization
Internal
Overview
Go modularization builds upon the concepts of package and module. Packages provide a namespace for their members, and they are a way to encapsulate code, hide implementation details and only expose features, such as variables, functions or type definitions that are meant to be publicly consumed. Packages can be published as part of modules. Modules have been introduced in Go 1.11.
Packages
Standard Library
Go comes with a set of over 100 "built-in" packages, which are available as part of the locally installed Go development environment.
Standard library package documentation is available online here:
The standard library is a good source of code examples, comments and style guidance.
Standard library packages:
pkg.go.dev
The place to look for published third-party packages is
Modules
Module-Aware or GOPATH Mode
The compiler must locate packages on the local file system every time it handles an import statement.
The go tool has two modes of resolving package dependencies: module-aware mode or GOPATH mode.
In module-aware mode, the go commands use go.mod files to find versioned dependencies and typically load packages out of the module cache, downloading modules if they are missing. As of Go 1.16, the module-aware mode is enabled by default, regardless of whether go.mod is present or not. The behavior can be controlled with the GO111MODULE environment variable.
In GOPATH mode, go commands use the value of the GOPATH environment variable and vendor directories to resolve packages.
Also see:
Workspace
The workspace is a concept introduced in Go 1.18. A workspace allows organizing the code for a project that has several modules which share a common list of dependencies. The workspace maintains metadata, especially dependency metadata, in a file called go.work. The dependencies declared in this file can span modules and anything declared in go.work will override dependencies in the modules's go.mod. The packages and modules maintained in a workspace are managed with the go tool.
A workspace may contain multiple projects.
The standard workspace layout is:
. ← GOPATH should point to this directory, it contains src, pkg and bin │ ├─ src │ ├─ a │ │ └─ b │ │ └─color # "color" package directory, with the "a/b/color" import path │ │ ├─ colors.go │ │ ├─ aux.go │ │ └─ ... │ │ │ ├─ weight # "weight" package directory, with the "weight" import path │ │ ├─ weights.go │ │ ├─ aux.go │ │ └─ ... │ │ │ ├─ novaordis.com │ │ └─ tools │ │ └─ hammer # "hammer" package directory, with the "novaordis.com/tools/hammer" import path │ │ └─ ... │ │ │ └─ github.com │ └─ blue-org │ └─ tools │ └─ wrench # "wrench" package directory, with the "github.com/blue-org/tools/wrench" import path │ ├─ .git │ └─ ... │ ├─ pkg │ └─ darwin_amd64 │ ├─ weights.go │ ├─ a/b/color.a │ ├─ novaordis.com/tools/hammer.a │ └─ github.com/bue-org/tools/wrench.a └─ bin
Content
src
The src subdirectory holds source code. Each package resides in a directory whose name relative to ${GOPATH}/src represents the package's import path.
The src subdirectory may contain multiple version-control repository workareas.
pkg
The build tool stores compiled packages in the pkg directory, under ${GOOS}_${GOARCH} subdirectories.
bin
The bin directory is where the executables are stored.
vendor
TODO: expand this and link to Go_Packages#Vendoring
Relationship between Workspace and GOPATH
GOPATH should point to the root of the workspace, the directory that contains src, pkg and bin. Further research is required.
Program
Go programs are constructed by linking together packages. There must be a main package, which contains the main(), to trigger the linker.
Project
See Vendoring.
Repository
A Go repository typically contains only one module, located in the root of the repository.