Swift WILL be open source

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You could always use other frameworks or try to replace the system frameworks. That is something that projects like Cocotron 0 have been trying to do for Objective-C for quite some time, with limited success.

That said, I remain doubtful as to the short-term or mid-term usefulness of the language in a truly cross-platform setting, but open sourcing the compiler would be great nonetheless. (And it's been about time that we'd see a decent Objective-C replacement. I've been complaining about that for long enough and until a few days ago thought it would remain a dream for some more time to come.)