[Caml-list] [ANN] Js_of_ocaml version 1.0
Jerome Vouillon
I'm happy to announce the first official release of Js_of_ocaml,
a compiler from OCaml bytecode to Javascript.
This tool let you write OCaml programs that run on Web browsers.
Js_of_ocaml is easy to install, and use thereafter, as it works with
an existing installation of OCaml, with no need to recompile any
library. It comes with bindings for a large part of the browser APIs.
The project page is: http://ocsigen.org/js_of_ocaml/
EXAMPLES
The compiler has been used to implement some noteworthy examples,
such as:
- an interactive 3D view of the Earth
http://ocsigen.org/js_of_ocaml/planet
- a graph viewer
http://ocsigen.org/js_of_ocaml/graph
PERFORMANCES
According to our benchmarks, with state of the art Javascript engines,
the generated programs runs typically faster than with the OCaml
bytecode interpreter ( http://ocsigen.org/js_of_ocaml/performances ).
Js_of_ocaml performs dead code elimination in order to generate
compact code: the Javascript file produced is usually smaller than
the input bytecode file, and often much smaller.
LINKS
Project home page http://ocsigen.org/js_of_ocaml/
Download http://ocsigen.org/download/js_of_ocaml-1.0.tar.gz
Get source code darcs get http://ocsigen.org/darcs/js_of_ocaml/
Documentation https://ocsigen.org/js_of_ocaml/lib/overview
FURTHER TECHNICAL DETAILS
Js_of_ocaml performs a fairly faithful translation. The order of
evaluation is preserved. Modular arithmetic is used for integers (but
with 32 bit integer). It does not support tail calls (function calls
in tail position), as this would be too expansive. However tail
recursion (self call in tail position) is properly optimized.
Explicit coercion functions can be used to convert Ocaml values to
Javascript values, and conversely (for instance, to map OCaml mutable
strings to Javascript immutable UTF-16 strings, or to map OCaml
booleans to Javascript booleans). A Camlp4 syntax extension makes it
possible to invoke Javascript methods in a type safe way.
COMPARISON TO OCAMLJS
Ocamljs is a compiler from OCaml source code to Javascript. Jake
Donham has written a fair comparison of the two tools:
http://ambassadortothecomputers.blogspot.com/2010/08/ocamljs-03.html
Ocamljs is a back-end to the existing OCaml compiler. Thus, contrary
to Js_of_ocaml, you need to perform a distinct installation of OCaml
to use Ocamljs, and you have to recompile all the libraries you may
need.
Ocamljs follows a different philosophy: it attempts to merge OCaml
datatypes with the corresponding Javascript datatypes. For instance,
OCaml objects are implemented as Javascript objects. Conversely,
Javascript objects are given an OCaml object type. A mixed
representation of strings is used: mutable OCaml-style strings and
immutable Javascript strings both have the same type. All this is
good for interoperability, but can be a source of incompatibilities
and can result in runtime errors not caught by the type checker.
Ocamljs optimizes tail recursion, but this comes at a large
performance cost.
--
Jerome Vouillon
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Yitzhak Mandelbaum
Thank you, this sounds fantastic!
One small question: could you expand on your last comment:
On Dec 13, 2010, at 8:06 AM, Jerome Vouillon wrote:
> <snip>
>
> Ocamljs optimizes tail recursion, but this comes at a large
> performance cost.
Do you mean all tail-calls come a large cost, or only those outside of plain tail-recursion? Either way, could you give us some more intuition as to why this happens, and why js_of_ocaml doesn't suffer from the same problem (assuming it applies to tail-recursion)?
Thanks,
Yitzhak
-----------------------------
Yitzhak Mandelbaum
Jerome Vouillon
> One small question: could you expand on your last comment:
>
> On Dec 13, 2010, at 8:06 AM, Jerome Vouillon wrote:
>
> > <snip>
> >
> > Ocamljs optimizes tail recursion, but this comes at a large
> > performance cost.
>
> Do you mean all tail-calls come a large cost, or only those outside
> of plain tail-recursion? Either way, could you give us some more
> intuition as to why this happens, and why js_of_ocaml doesn't suffer
> from the same problem (assuming it applies to tail-recursion)?
I meant tail calls, indeed. Tail recursion (when a function calls
itself recursively in tail position) can be optimized efficiently by
wrapping the function body inside a loop. This is what Js_of_ocaml
does.
For optimizing tail calls in general, you need to use trampolines.
Instead of calling a function in tail position, you return this
function and its arguments. Then, a piece of code called a trampoline
takes care of invoking repeately the functions it receives this way
until a final result is returned. This is expansive. You have to
make sure that this piece of code is there whenever a function is
invoked in tail position. Also, the JIT compilers cannot optimize the
trampoline, as the functions it will have to call, and their number of
arguments, are unknown.
-- Jerome