Rarely does the likes of Cython or RPython generate the same level of interest as the parent language. In Cython's case, it appeals mainly to people combining C with Python. If you're not doing that, there's little incentive to use it.
Sometimes, with supersets and subsets alike, features will bubble up (or down) into the main language. With static typing in Python, for instance, there's now a proposal in the works to add type hinting to Python 3, as a way to make it easier to profile code -- and perhaps eventually as a way to accelerate its performance overall.
Future candidates for a fork
What other widely used languages might be destined for a fork in the near future?
One candidate is Google's Go, aka Golang. High-profile projects such as Docker have been built with it, and the language has enjoyed attention and accolades. But several of its features and behaviors have as many detractors as they do adherents; Go's error-handling mechanism, for instance, is one feature that's been singled out for criticism. Lack of generics is another commonly cited shortcoming, and the Go development team has insisted that generics will not be added to the language. If Go's designers are unwilling to reconsider their stance on such facets -- all signs point to that being the case -- a fork of the language might be the only way forward for the disgruntled.
Another possibility would be variations on Microsoft's family of .Net languages, mainly C#, made more possible by Microsoft's new generation of open source compilation frameworks. This development would be distinct from projects like Mono, a separate open source implementation of C# and .Net. Rather, it would be an attempt to take C# in new directions, whether they were compatible with the original or not.
One final possibility, though it's more of a fork of a specification than a language, is the next major version of HTML. In some ways this already happened, as the WHATWG and HTML5 could be considered forks from the W3C and its version of the standard. There's no guarantee such a fork would change the landscape, even if it came with a browser to run it, but that's part of the risk -- and reward -- of forking in the first place.
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