PerlConnect Documentation
Author: Vladimir Livshits

### PERLCONNECT AND JS.pm

USER-LEVEL DOCUMENTATION
Vladimir Livshits

1. Overview
2. Features
3. Design
4. Installation
5. Current Limitations and Futher Work

#### Overview

PerlConnect provides glue for the developer between JavaScript and Perl. It currently consists of two parts, PerlConnect implemented in C and JS.pm (tentative name), a Perl module written using XSUBs. PerlConnect and JS.pm allow calling Perl from JS and JS from Perl, respectively. Whenever possible, it is attempted to archieve the maximum level of transparency for calling one language from the other. This is done by converting values between the two languages, creating wrappers around objects, and emulating the standard language syntax.

#### PerlConnect Features

PerlConnect allows running a Perl interpreter concurrently with your JavaScript embedding and executing Perl commands from JavaScript. You usually need to create a Perl interpreter by saysing something like this:

> `p = new Perl('Sys::Hostname', 'Test::Harness')`

In addition to creating an interpreter, this will also include the libraries you pass to the Perl constructor, which is equivalent to `use Sys::Hostname; use Test::Harness`. You can always include libraries explicitly by using `p.eval('use Sys::Hostname; use Test::Harness')`. There is also another way to do this: `p.use('Sys::Hostname', 'Test::Harness')`. As you can see, TMTOWTDI. Natura lly, you will want to look at the result of your function calls and `eval` statements. Suppose, you do something like this:

> `line = p.eval("'-' x 80")`

Perl's eval returns the last statement evaluated, unless you explicitly say `return`. So now `line` contains 80 dashes. You can do similar things with non-scalar data types:

> `p.use('Time::gmtime');`
> `t = p.eval('Time::gmtime::gmtime') // returns [49,0,4,24,6,98,5,204,0]`

assigns a Perl array to `t`. You can print `t`, use the `for/in` syntax to walk through it, compute its lenght, etc. You can read and assign to individual elements using the standard syntax. However, PerlValues, that is, the value we get from Perl, don't support all the standard operations, for isntance, don't expect `t.reverse()` to work. Hashes can also be returned from Perl:

> `info=p.eval("{ver=>$], pid=>$$}")`

Now you can look at individual hash keys like this:

> `info["ver"]` or `info.pid`

Suppose you want to use Perl to perform pattern-based string replacement. Here's how you can do it from JavaScript:

> `p.eval("\`
> `sub perl_replace(){\`
> `my($string, $find, $replace) = @_;\`
> `eval(\"\\$string =~ s/$find/$replace/g;\");\`
> `return $string;\`
> `}"`
> `);`

and now

> `p.perl_replace('Quick brown fox jumped over a lazy dog', 'dog', 'rhino')`

produces what you'd expect.

You can use the same syntax to call procedures defined in modules other than`main.` The example with`gmtime` can be rewritten like this:

> `p.use('Time::gmtime');`
> `t = p.Time.gmtime.gmtime() // returns [49,0,4,24,6,98,5,204,0]`

You can reference variables exported by modules others than `main` like this:

> `a=p.Foo.$bar` or`a=p.Foo["$bar"]` or
> `a=p.Foo["@bar"]`or`a=p.Foo["%bar"]`

Each of the stetements above returns a either an immediate JS value, for scalar types, or a PerlValue for compound types. `a.type` contains the type of the PerlValue in `a`. This may change because we may end up separating Perl hashes and arrays into separate classes.

#### JS.pm Features

JS.pm is much less tested then PerlConnect. You should be able to do similar things from Perl. Just say

> `use JS;`
> `$js = new JS();`

and now you can do something like this:

> `$js->eval(q/`

> > `Object o = {};`
> > `o.a = 'p';`
> > `o.b = 'q';`
> > `return o;`

> `/);`

//TODO: more features and examples

#### PerlConnect Design

PerlConnect is written in C. It uses both JavaScript and Perl APIs and implements a mapping between the two. The are the following JavaScript objects implemented by PerlConnect:

Object | What it does
`Perl` | Perl Interpreter Object. It's prototype type is PerlModule, it corresponds to `main::`. Supports `eval`, `call`, `use`.
`PerlModule` | Implements JS-like syntax for Perl modules. Doesn't export provide any methods. `path` property shown the name of the Perl module the object represents.
`PerlValue` | Represents a value returned from `eval`, `call`, or obtained by using the subscript notation (`p.Foo["@bar"]`). Its Perl type is stored in the `type` property.

See comments in the code, `jsperl.c` and `JS.pm` for more info.

#### JS.pm Design

JSConnect is written using XSUBs, the language in which Perl extentions are implemented. The source files are `JS.xs` and `typemap`. After processing them using the XSUBs compiler, `xsubpp`, the resulting C file should be compiled into a DLL. `JS.pm` provides bootstraping mechanism to load this DLL. See `perlxs` and `perlxsstut` man pages for more info. The following Perl packages (objects) are implemented:

Package | What it contains
`JS` | Doesn't not do anything in particular at this point except defining a constuctor. So one can say `$js = new JS()`, which will create a new runtime, add a context to it and return that Context. JS also defines a bunch of private functions called from C by PerlConnect. They are not exposed by default, but pushed onto `@EXPORT_OK` array instead.
`JS::Runtime` | Corresponds to `JSRuntime*` struct. Provides a constructor and destructor. The destructor is invoked automatically, so you don't have to worry about Runtime deallocation. Constructor syntax is the following: `$rt = new JS::Runtime(10_000)`, where the parameter is the same number you pass to `JS_NewRuntime`. There are many private functions created in `JS.xs` that are not exported by default.
`JS::Context` | Corresponds to `JSContext*` struct. Provides a constructor and destructor. The destructor is invoked automatically, so you don't have to worry about Context deallocation. Constructor syntax is the following: `$rt = new JS::Context($rt, 1_000)`, where the parameter is the same number you pass to `JS_NewContext`. There are many private functions created in `JS.xs` that are not exported by default.
`JS::Object` | Corresponds to `JSObject*` struct. There is not that much here yet. This object is intended as a wrapper around the `JSObject* stuct`. Support for tying hashes and possibly arrays with `JS::Objects` is coming.
`[JS::Array]` | I am not quite sure if this is needed. One might probably get away with just `JS::Object` defined. If it's implemented, it will be very much similar to `JS::Object` above.

All the modules above follow the convention of storing the variable they return in the `$this` variable of the current class. So `$JS::Context::this` will always be the last context created. Currently, this is where JS API function that require contexts get it.

#### PerlConnect Installation

PerlConnect requires `js/src` and the Perl libraries and headers. The only `js/src` file that must be included in `jsapi.h`. You need to compile it together with Perl libraries. Referer to the `perlembed` man page for more details.
On WINNT:
There are MSDEV Workspace and project files in the main PerlConnect directory. There are two projects included in the PerlConnect workspace: PerlConnect and PerlConnect shell. You can use the latter to test PerlConnect. You will probably need to adjust the library and include paths. In particular, set PERL_SRC environment variable to point to the directory where you unpacked and compiled Perl sources.
On UNIX:
Currently, you can use PerlConnect with the `js.c` shell. To compile it on UNIX, you need to set the PERLCONNECT environment variable. Then you can use the `Makefile.ref` in `js/src` directory. The makefile should read the variable from the current environment. If this doesn't happent, you can try using `gmake PERLCONNECT=1`. This will compile PerlConnect into `js`.

#### JS.pm Installation

The following source files are distributed: `JS.pm`, `JS.xs`, `JS.c` (produced from `JS.xs` by `xsubpp`) and `JS.def` (required by the Visual C compiler).
On WINNT:
There is also a project file, `JS.dsp`. It automatically belongs to the PerlConnect workspace. This project file builds a DLL, `JS.dll` which is supposed to reside where Perl's DynaLoader can find it. On my machine I put it under `c:\perl\lib\auto\JS\JS.dll`. You can also put it in `c:\perl\lib\`. See DynaLoader documentation for more info on how dynamic libraries are found.
On UNIX:
//TODO: details of the makefiles

#### Current Limitations of the Package and Futher Work

1. Perl variables currently can't be assigned to, that is, `p["$a"]=100` doesn't do anything.
2. You can only have one interpreter running at a time. Despite the fact that you can create multiple Perl objects on JavaScript, they all share the same namespace. We can probably use `Safe.pm` to implement independent namespaces.
3. Module names resolution reports an error only when you try to evaluate the last element of the resolution chain. Here is what I mean: if you reference `p.Foo.Bar.Var` and `For` or `Bar` don't exist, it will only complain that p.Foo.Bar.Var is not a valid variable. Perl 5.005 provides `exists Foo::{Bar::}` to check if Foo::Bar is a valid package.
4. Dynamic loading of the Perl interpreter only if it is required.
5. Recursive printing of Perl's arrays and hashes can be added. See Data::Dumper.pm
6. Full support for tied hashes and arrays in Perl
7. Calling JavaScript functions and accessing variables from Perl. JavaScript calling syntax support using AUTOLOADing.
8. JS can be made a directory with `Object.pm`, `Context.pm`, etc. in it. See how C or Tk are organized on CPAN
9. Distribution model for JS.pm. Perl provides something by default. See `h2xs` man page, for example.