Function in same package undefined

Try running just go build. When you give it a go file as an argument, it will not look for other go files. You can also do go build *.go

This is an old post but it didn't answer my issue clearly, so I'm posting for the benefit of others in the future.

When run go run --help you will find this manual:

Run compiles and runs the main package comprising the named Go source files. A Go source file is defined to be a file ending in a literal ".go" suffix.

By default, 'go run' runs the compiled binary directly: 'a.out arguments...'.

go run <filename.go> is used for small programs with just a few files. With several files you will run into an issue where your main.go cannot find other files because go run doesn't compile and link them implicitly unless named. That's why go build the project works.

Alternatively, go run *.go (building all files) should work most of the time.