Thanks for pointing out how you came to implement an interpreter before Execscript existed. If your invented language doesn't match VFP 1:1, then it's obviouslly not as simple as throwing out your interpreter and instead do EXECSCRIPT.
I think we have a bit of a language barrier and not all I say comes over to you. I don't mind to rephrase things until you get it, no problem, Piotr.
One thing I thinkg you still don't realize correctly: When you do &X, that's running "the compiler", even though it only compiles one line of code and not a whole script. It's in the runtime, that's not illegal, otherwise macro substitution couldn't be a feature of VFP. Or the other way around, as I already said for macro substitution to work the runtime must contain the ability to turn a line of soure code into compiled code that can be executed.
Notice "the compiler" is not the same as BUILD is. It's not the reason the terminology to create an EXE from a VFP project is called building an EXE instead of compiling an EXE, but it makes things a bit clearer for VFP. Compiling itself is a broader term that covers creating EXEutables, of course, so you can say you compile an EXE. The definition Wikipedia gices for compilation is turning source code into machine readable code, be it in a lower level language (like the assembly language of a CPU aka machine code or "object code" of any kind like VFP actually does. Such object code, aka byte code isn't executed by the IDE, it's read by a runtime and that executes the corresponding implementation of that.
The way the runtime is written isn't having a loophole that allows you to do someting illegal by EULA terms of VFP, simply because you're allowed to provide the runtime (obviously otherwise you couldn't provide EXEs) and so everything it executes is legal in that sense (you could, of course, write code that does illegal things in the broader criminal sense, but we're only talking about legal issues regarding the copyright of MS on the VFP IDE, including the major feature to create EXEs from source code).
The IDE can't be provided 1:1 technically in first degree without providing VFP9.exe. And that, well, is the IDE, mainly. Aside from many more files you also need to run the IDE, but which mainly are also in the set of files you're legally allowed to redistribute referring to redist.txt in the HOME() folder.
So, indeed, MS is almost allowing to distribute VFP itself, if you'd be able to provide all the missing features of the IDE by programming them, you could easily have a VFP clone on a totally legal basis that uses MS runtime to run VFP code in exactly the same manner and performance as if it came from the original IDE. MS then could only argue on infringements copyrights it has on the FoxPro language itself. but in most commands and functions of VFP they actually give up the copyright not just because the EXE in the end runs your code on a customers PC, as is the case in any other computer language, too. What goes beyond that in VFP is the availability of EXECSCRIPT, macro substitution, the DO and the COMPILE command. It's not unheard of, though. The .NET Framework and SDK kits you also get free and are allowed to use - they contain C# and VB.NET compilers, there are lots of free C++ compilers and linkers, so it's not hard to mimic an IDE.
What you did is provide a domain specific language, maybe close to the VFP language and allowing almost any VFP constructs, too, But given EXECSCRIPT and mscro substitution exists, that's not illegal. If you manage to make it a compiler that can produce an EXE from VFP source code, that still isn't violating direct copyrights MS has on files the EULA and redist.txt doesn't allow to redistribute, they may then refer to legal rights of the language definition itself, but as said they gave up on that almost entirely, even though being able to execute code and compile code they still keep the copyright on the implementation of the VFP language in C++, that's surely theirs and allowing the runtime distribution that contains the compiled C++ code doesn't make it open source or free or public domain, of course. But even if VFP wouldn't be discontinued MS gives away more of their copyrights on the VFP language than other vendors give away from their programming language.
It's not even unusual in other programming languages or even tools like SQL Servers to need license fees for the runtime (or the database server, etc). MS gave a lot more royalty free to VFP developers than other developers get, despite the fact of the existence of free compilers of other languages not only by MS, too. The whole platform of Java is free, too, for example. It's a bit in the MS blood, isn't it? I mean they want their Windows platform to be used, so they support "developers, developers, developers".