Usually for make program user create fsproj via dotnet new and build binary with dotnet build. But it not required because you can use fsharp compiler directly. This is demo project how to it.

Before test make initial (module-name).fsi files with compiler option --allsigs

After comment --allsigs key and add at top Boo.fsi in build-opts.txt and modify Boo.fs try access to Boo.message from App.fs

Boo.message |> printfn "%s"
Boo.hello()

dotnet fsc was print: App.fs(3,5): error FS0039: The value, constructor, namespace or type 'message' is not defined. We have protect data from access outside.