Go is a compiled programming language; we have the source code and code for the dependencies that we want to use in our project. In general, every time we build a binary, the compiler has to read the source code of our project and dependencies and then compile it to machine code. Compiling unchanged dependencies every time we compile our main program would lead to a very slow build process. This is the reason that object files exist; they allow us to compile dependencies into reusable machine code that can be readily included in our Go binary.

These object files are stored in $GOPATH/pkg; they follow a directory structure similar to that of src/, except that they are within a subdirectory. These directories tend to follow the naming pattern of <OS>_<CPU-Architecture>, because we can build executable binaries for multiple systems:

$ tree $GOPATH/pkg
pkg
└── linux_amd64
    ├── github.com
    │   ├── abbot
    │   │   └── go-http-auth.a
    │   ├── dimfeld
    │   │   └── httppath.a
    │   ├── oklog
    │   │   └── ulid.a
    │   ├── rcrowley
    │   │   └── go-metrics.a
    │   ├── sirupsen
    │   │   └── logrus.a
    │   ├── sony
    │   │   └── gobreaker.a
    └── golang.org
        └── x
            ├── crypto
            │   ├── bcrypt.a
            │   ├── blowfish.a
            │   └── ssh
            │       └── terminal.a
            ├── net
            │   └── context.a
            └── sys