On 12/18/2012 12:38 PM, Peter Alexander wrote:
On Tuesday, 18 December 2012 at 19:25:01 UTC, Walter Bright wrote:
On 12/18/2012 10:29 AM, Peter Alexander wrote:
On Tuesday, 18 December 2012 at 18:11:37 UTC, Walter Bright wrote:
Javascript proves that bytecode is not required for "write once, run
everywhere", which was one of the pitches for bytecode.
What is required for w.o.r.e. is a specification for the source code that
precludes undefined and implementation defined behavior.
Yes, bytecode isn't strictly required, but it's certainly desirable. Bytecode is
much easier to interpret, much easier to compile to, and more compact.
Bytecode would have added nothing to js but complexity.
I think you're seriously overestimating the cost of compilation.
When I say "easier", I'm talking about implementation cost. Consider how easy it
is to write a conforming Java byte code interpreter compared to a conforming
Java interpreter/compiler. Parsing and semantic analysis are much easier to get
wrong than a byte code spec. At the bytecode level, you don't need to worry
about function overloading, symbol tables, variable scoping, type inference,
forward references etc. etc. All those things are intentional complexities
meant to make life easier for the programmer, not the computer. A bytecode
doesn't need them.
D is open source. There is little implementation cost to doing a compiler for
it. It's a solved problem.
A bytecode requires another spec to be written, and if you think it's easy to
make a conformant Java VM bytecode interpreter, think again :-)
The downside of bytecode is loss of high-level meaning... but that depends on
the bytecode. There's nothing stopping the bytecode from being a serialised AST
(actually, that would be ideal).
As I pointed out to Andrei, Java bytecode *is* a serialized AST.
It's not a lossless serialisation -- especially not after optimisation. For
example, it's non-trivial to reconstruct the AST of a for loop from optimised
bytecode (or even regular bytecode).
Yes, it is trivial. The only thing that is lost are local variable names and
comments.
