iteration order for Object

Brendan Eich brendan at mozilla.com
Thu Mar 10 19:27:05 PST 2011


On Mar 10, 2011, at 7:18 PM, Brendan Eich wrote:

> Harmony is *not* a clean break. I don't know why you wrote that.

Perhaps Charles meant "a clean break in for-in semantics".

That's what I propose via my index-finger compatibility break:


> Harmony has some room to break compatibility, especially if the old forms are caught by early errors. I've spoken of one hand's worth of fingers to count these breaks (the "five fingers of fate"). The valuable and opposable thumb is for removing the global object from the scope chain. The index finger on my hand is paren-free relaxation of syntax combined with better for-in semantics.

The idea (http://brendaneich.com/2010/11/paren-free/, followups in http://brendaneich.com/2011/01/harmony-of-my-dreams/) is to relax the grammar so that parentheses may be omitted from statement heads where the parenthesized form is an expression, but to remove parens around the for loop heads (a la Go, for the three-part for ;; head). This requires that the body or then-clause be braced unless it is a simple statement that starts with an unconditionally reserved identifier.

The benefit for code migration is that for (x in o) ...; code will not compile in such a Harmony. Migrators will have to rewrite and (if necessary) choose a custom iterator to get the desired order and values or keys: for x in keys(o) {...;}, e.g. where keys is imported from a standard enumerators module.

This is less likely to break code than a subtle, runtime-only, and therefore "unclean" break in the meaning of for (x in o).

/be



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