On May 9, 2009, at 2:59 PM, Mark S. Miller wrote:
On Sat, May 9, 2009 at 2:32 PM, David-Sarah Hopwood
<[email protected]> wrote:
[...] but the AST should preserve the associativity defined in the
language spec.
But which language spec? Again, specs only traffic in observable
differences. Since ES5 does not define any std parse or AST API, there
is no observable difference in ES5 whether this is specified as
left-or-right associative.
Exactly.
Assuming ES6 does define such APIs, the
difference becomes observable. I see no reason why ES6 could not
compatibly specify a right associative grammar for || and &&.
Glad to hear this. I'm going with right associativity for local-
convention and lazy-hacker reasons, in the SpiderMonkey prototype.
Of course if we standardize left-associative || and &&, I will cope --
software can do anything. But there's a slight and (IMHO) winning
economy of expression, interpretation, and compilation, in right
associativity for these logical connectives.
Ritchie's original C compiler, and Thompson's implementations of B
(initially for a very limited memory machine, PDP-7), focused on one-
pass code generation. This obviously made for busy-work burdens
(forward declarations, e.g.) for the C programmer. At first (B on the
PDP-7) there was no alternative. BCPL had a "let rec" equivalent ("let
X be ... and Y be ...") but Thompson's target machine had insufficient
core to hold an intermediate representation in memory for real
programs. I'm pretty sure all these compilers treated & and | (which
split into && and || later, explaining the poor precedence for & and
|) as right-associative.
Today we have too much memory to waste, and hardly any living
programmer worries about such things as fitting a compiler in 8K.
More's the pity.
But why shouldn't we bias the AST, in this particular case, toward the
more economical associativity mode?
/be
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