Free software grand master Richard Stallman weighed in not long ago about Mono, the open source implementation of Microsoft's .NET. He's not against it in principle, he just doesn't feel it's a good idea to depend on it for anything, especially not the core GNU tools.
This is actually a slightly more nuanced statment than I -- and other people I know -- were expecting to hear. Stallman's stances on free software (he's not fond of the term "open source") are quite uncompromising, and so it's become something of the default expectation for him to take the hard-line free-or-nothing stance on a lot of things.
... implementing [.NET's] C# [through Mono] is [not] a bad thing. Free C# implementations permit users to run their C# programs on free platforms, which is good. (The GNU Project has an implementation of C# also, called Portable.NET.) Ideally we want to provide free implementations for all languages that programmers have used.
The problem is not in the C# implementations, but rather in Tomboy and other applications written in C#. If we lose the use of C#, we will lose them too. That doesn't make them unethical, but it means that writing them and using them is taking a gratuitous risk.
Stallman's deeper worry, shared by others, is that Microsoft has not offered people the kind of indemnification from IP backlashes that, say, Sun has with Java. Program in C#/Mono, and there's no guarantee that at some point the free implementation of same might end up being curtailed or revoked entirely due to unforeseeable patent issues.
I've mentioned before how other people manifestly do not share this point of view, and even find it faintly ridiculous -- and shook my head at the way the sourest and most divisive arguments about open source come from within its own ranks. Stallman's view is not so much divisive as it is cautionary: go ahead and use it if you want, but be warned about built anything on top of it that we have to depend on in the future.
Programming in any form is never without some element of gratuitous risk. Most of that danger doesn't come from the IP issues of the language it's written in, but from the programmers themselves. I'd maintain there is a far greater practical risk is the broad use of a program that has severe security flaws (e.g., pre-SP2 Windows XP, or some editions of Apache) than there is in this sort of issue. But if this is all about Freedom, then I say let Stallman and the other GNU folks employ their freedom to be unnerved.
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