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It will absolutely be open source. You just didn't read the FAQ ;)

On the mailing list Chris also mentioned that it "will likely use a browser somehow, but that might be abstracted away into a webview or some such thing so you wouldn't really know." [http://groups.google.com/group/light-table/browse_thread/thr...]

Did you even look at the page? On the page it clearly states that it will be open source and will be extendable using javascript.

at least read the description of the product you are criticising...