On Aug 25, 2008, at 8:51 PM, Mark S. Miller wrote: > How important is it that yield be an expression rather than a > statement? It seems like a sibling of return and throw, so I don't > think anyone coming to them afresh would be surprised if yield were a > statement.
This is and was not a clean-slate exercise. We both followed and gave feedback to Python in doing this work, because the Python and JS communities overlap significantly, and the languages share certain stylistic and substantial features (not all, but enough). The goals include reusing the Python open-source/python-dev experience and the developer brainprint. yield as an expression form is important for so-called "co-routines", where you can call gen.send(val) to resume a generator the val becoming the result of the last yield expression; you can also gen.throw(exc). Finally, and this is possibly the most useful, gen.close() will be called in JS when a generator used in a for-in construct. With close called from for-in, cleanup can be automated that otherwise can't be done without Python-style ref-counting/GC, Java-style finalization, or worse -- complexities we wish to avoid in any ES standard. Calling gen.close() on an open generator forces a return from the last yield. This has the desirable property that if the yield is in a try, any relevant finally clauses will be run. (Return from generator throws StopIteration, as usual for iterators; a generator can't return a value.) Background reading: http://mail.python.org/pipermail/python-dev/2006-August/068497.html http://mail.python.org/pipermail/python-dev/2006-August/068498.html http://mail.python.org/pipermail/python-dev/2006-August/068450.html One use of generators, which Neil Mix wrote: "Threads" in JS: http://www.neilmix.com/2007/02/07/threading-in-javascript-17/ Source to read: http://www.neilmix.com/demos/js17threading/example.js http://www.neilmix.com/demos/js17threading/Thread.js These use send and yield expressions heavily. The es- [EMAIL PROTECTED] archives at https://mail.mozilla.org/pipermail/es-discuss/ best searched with Google site:mail.mozilla.org search, have a number of threads tracing the evolution of generators. Besides Igor Bukanov's work to simplify the close mechanism by eliminating GeneratorExit, Chris Hansen of Google made the crucial proposal to automate close only from for-in constructs (not from GC). Generators are in JS1.7 (Firefox 2). Sugar in the form of generator expressions, as in Python 2.4, are in JS1.8 (Firefox 3). My port of Peter Norvig's Sudoku solver, which uses genexps: https://bugzilla.mozilla.org/attachment.cgi?id=266577 I believe Pythonic generators have been the most useful, if not most widely used, of the extensions that we have shipped since JS1.5 (ES3 + getters and setters). /be _______________________________________________ Es-discuss mailing list Es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
