This could have been a blog post about toothbrushes for monkeys, which I seem to have dreamed about this morning. But then I was vaguely listening to the FAIFCast in the car, and I came up with something even more esoteric to write about!

License monads would allow separating parts of code that are under different licenses, in a rigorous fashion. For example, we might have two functions in different license monads:

foo :: String -> GPL String bar :: Char -> BSD Char

Perhaps foo needs to use bar to calculate its value. It can only do so if there's a way to lift code in the BSD monad into the GPL monad. Which we can legally write, since the BSD license is upwards-compatable with the GPL:

liftGPL :: BSD a -> GPL a

On the other hand, there should be no way provided to lift the GPL monad into the BSD monad. So bar cannot be written using code from foo , which would violate foo's GPL license.

Perhaps the reason I am thinking about this is that the other day I found myself refactoring some code out of the git-annex webapp (which is AGPL licensed) and into the git-annex assistant (which is GPL licensed). Which meant I had to relicense that code.

Luckily that was easy to do legally speaking, since I am the only author of the git-annex webapp so far, and own the whole license of it. (Actually, it's only around 3 thousand lines of code, and another thousand of html.)

It also turned out to be easy to do the refactoring, technically speaking because looking at the code, I realized I had accidentially written it in the wrong monad; all the functions were in the webapp's Handler monad, but all of them used liftAnnex to actually do their work in the Annex monad. If that had not been the case, I would not have been able to refactor the code, at least not without entirely rewriting it.

It's as if I had accidentially written:

foo :: String -> GPL String foo = mapM ( liftGPL . bar )

Which can be generalized to:

foo :: String -> BSD String foo = mapM bar

I don't think that license monads can be realistically used in the current world, because lawyers and math often don't mix well. Lawyers have, after all, in the past written laws re-defining pi to be 3. Still, license monads are an interesting way to think about things for myself. They capture how my code and licenses are structured and allow me to reason about it on a more granular level than the licences of individual files.

(I have a vague feeling someone else may have written about this idea before? Perhaps I should have been blogging about monkey toothbrushes after all...)