If some code links to your GPL library, the whole project has to be licenced GPLv3, full stop. This does not "prevent people to use [it] at all", it just stipulates that they have to make the source available and the source of improvements they make available. Each substantial library I write in my free time is GPLv3. I want to contribute to the ecosystem and I want everyone enjoying my work contributing back to the ecosystem.

A similar licence, called LGPL, allows dynamic linking without having to make the code of the whole project available, just the code of the specific library + improvements. If for some reason you need this, I invite you to check how dynamic linking works in Pharo and read this FAQ by the FSF (and all other FAQs, it is a very clear, informative document).

First, thanks for working on Pharo, what an amazing environment.

I am not a lawyer so you should double check.

For GPL, indeed the image as a whole would be GPL. If the personal project is more of an “executable”, it might be what you are looking for, any further image modification would have to be released under the GPL as well.

If the project is more of a “library”, you might or might not prefer the LGPL. In this case I feel like https://www.gnu.org/licenses/lgpl-java.en.html would apply similarly to Java. As long as you can load the image and change the original library code, other code is not restricted on the license.

Not 100% sure on the “virus” thing you mentioned but I can make an assumption.

One license type (I think its GPL3) requires any software which includes GPL3 code must also be licensed as GPL3 and thus all sources made public. You can’t take the open code, use it and then close off your own source. If I am remembering correctly

This would mean commercial applications which want to remain closed source wouldn’t be able to use that licensed code unless they were to then opensource their own code as well. So it wouldn’t prevent other open sourced, same licensed software being able to use it

Pharo is licensed under MIT hence most of my work needs to be licensed also under MIT.

I believe that this is not true. I thought that it is not mandatory for your work to be licensed under the MIT license in this case. Can anyone confirm this?

You can release your other code under gpl 3 or agpl. If you or someone wants to use it in an mit licensed project you can license it to them under a special license that stipulates that it can only be used in that project and they must give back any changes. Lawyers, please confirm.