Unfortunately that syntax is a little troublesome to use. To enable it, you need to add the --compiling-fslib flag, which basically declares "hi i want to buil

Unfortunately that syntax is a little troublesome to use. To enable it, you need to add the --compiling-fslib flag, which basically declares "hi i want to build a stdlib, I know what I'm doing (the flag is undocumented btw)" and as a result it doesn't include the FSharp.Core library and all the built-in modules. So your code fails to compile because it doesn't know 'string' or 'printf', AFAIK manually adding the reference isn't enough, you would need to statically include all the FSharp.Core source code.