Hello, I work on Pharo, an open source derivative of Smalltalk. Pharo is licensed under MIT hence most of my work needs to be licensed also under MIT.
However, time to time I have some projects in my free time that I made for my personal usage or for friends, and in those cases I am not OK with my work being used by for-profit project not giving anything back. I would very much prefer to use GPLv3 on those cases, but my understanding of licensing is very poor and I have been told there is a “virus” behavior on GPLv3 that may prevent people to use at all what I do, and that’s not my intention.
Do you have any advice how to handle this?
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).