Syntax for union types

[Brendan Eich]

As Lars suggested, we have bigger fish to fry, but we settled on  
union syntax quickly and were content to stay there. I'm not against  
| instead of , and if enough people think it's the right user  
interface, we could consider it again. I'm not saying it's a good use  
of time to fuss over this, but it's "fixable" if (T, U, ...) is not  
as good as (T | U | ...).

Recall that ES4 and indeed JavaScript do not have tuples, so we want  
to use [T, U] for the array structural type describing a tuple of at  
least index 0 of type T and index 1 of type U. If we ever did add  
tuples, then Yuh-Ruey has a point I think: we might rather use (T |  
U) for union of T and U, and (T, U) -- or possibly (T, U,) to match  
expression syntax (which would have to be (e1, e2,) to avoid  
ambiguity with comma expression) for tuple type.

/be