In gren 0.6 instead of doing this: gren make src/Main.gren, you just specify the module name: gren make Main
That means if your application module has the same name as a module in an installed package, it will be ambiguous. I'm encountering this most frequently when trying to make and run a Test module (the workaround is to namespace it or call it Tests).
The bigger problem is when this happens, the backend crashes with the cryptic "I ran into something that bypassed the normal error reporting process!" message from haskell.