Source Layouts

The modules that swift build creates are determined from the filesystem layout of your source files.

For example, if you created a directory with the following layout:

example/
example/Sources/bar.swift
example/Sources/baz.swift

Running swift build within directory example would produce a single library target: example/.build/debug/example.a

To create multiple modules create multiple subdirectories:

example/Sources/foo/foo.swift
example/Sources/bar/bar.swift

Running swift build would produce two library targets:

To generate an executable module (instead of a library module) add a main.swift file to that module’s subdirectory:

example/Sources/foo/main.swift
example/Sources/bar/bar.swift

Running swift build would now produce:

Where foo is an executable and bar.a a static library.

Other Rules

Sub directories of directory named Tests become test-modules and are executed by swift test. Tests or any subdirectory can be excluded via Manifest file. The package manager will add an implicit dependency between the test suite and the target it assumes it is trying to test when the sub directory in Test and package name are the same.
Sub directories of a directory named Sources, Source, srcs or src become modules.
It is acceptable to have no Sources directory, in which case the root directory is treated as a single module (place your sources there) or sub directories of the root are considered modules. Use this layout convention for simple projects.