~ $ mkdir example
~ $ cd example/
example $ touch Package.swift
example $ mkdir Sources
example $ vi Sources/Example.swift
example $ cat Sources/Example.swift
func printOther() {
  print("other")
}
example $ vi Sources/Main.swift
example $ cat Sources/Main.swift
print("Hello World")
printOther()

example $ swift build
Compile Swift Module 'example' (2 sources)
/PATH/example/Sources/Main.swift:1:1: error: expressions are not allowed at the top level
print("Hello World")
^
/PATH/example/Sources/Main.swift:2:1: error: expressions are not allowed at the top level
printOther()
^
/PATH/example/Sources/Main.swift:1:1: error: expressions are not allowed at the top level
print("Hello World")
^
/PATH/example/Sources/Main.swift:2:1: error: expressions are not allowed at the top level
printOther()
^
<unknown>:0: error: build had 1 command failures
error: exit(1): /PATH-SWIFT/usr/bin/swift-build-tool -f /PATH/example/.build/debug.yaml

example $ mv Sources/Main.swift Sources/main.swift
example $ swift build
Compile Swift Module 'example' (2 sources)
Linking .build/debug/example
example $ .build/debug/example
Hello World
other
example $