Heh, it has to do with using the 'APL standard library' and the interfaces to call in to APL. The interfaces require that anything that uses those interfaces also has to be GPL, and the standard library in APL means that code made in APL that links with it also is bound to the GPL. That is why linking exceptions exist for the standard libraries and calling interfaces for every language that I've run across. ^.^
So if anyone wanted to distribute the ability to call APL from their EVM program then their program needs to be GPL as well. If they make a program in APL that makes an APL calls in to the APL standard library that they then call directly from the EVM, then their EVM program needs to be GPL as well.
Things like that, I can go in to more detail but this is discussed to death online. ^.^
But in general, do not just pick a license randomly, actually research in to them, the GPL itself means that it is near impossible to use it with anything that is not GPL, and since most things are not GPL and cannot be GPL, it could never be used with up.
It details it, but in essence that means calling programmatic interface of the program.
No, GPL can work with GPL fine, however bash calls ls via the kernel process command (well actually you usually call the builtin in reality), that is an OS bound, not an application interface. Now if bash talked to ls via a specific protocol, that'd be different.
If this is not intended to be used with a distributed EVM program and an APL 'compiled program' (of whatever form it is) is not, then no worry. ^.^
Actually the GPL prevents users from being able to do a significant amount. If you just want to ensure that the APL and any changes to it itself are distributed as open source then there are licenses for that, however the GPL is not one. A real world scenario is a program made with apl that uses the apl standard library would not be able to be linked with a non-gpl program and distributed, ever. Another real world scenario is the apl itself being used as, say, a scripting language, in a program that is not gpl, then those could never ever be distributed as well in that form.
In essence, learn about the licenses you use. ^.^
There are cases I'd use the GPL, this is not one.