/ 
Inline-Lua-0.03
River stage one • 1 direct dependent • 1 total dependent
/ Inline::Lua

NAME

Inline::Lua - Perl extension for embedding Lua scripts into Perl code

SYNOPSIS

    use Inline 'Lua';
    print "The answer to life, the universe and everything is ", answer(6, 7), "\n";

    __END__
    __Lua__
    function answer (a, b)
	return a*b 
    end

DESCRIPTION

Inline::Lua allows you to write functions in Lua. Those of you who are not yet familiar with Lua should have a cursory glance at http://www.lua.org/ to get a taste of this language. In short:

Lua was designed to be embedded into other applications and not so much as a language on its own. However, despite its small set of language features, it is an extremely powerful and expressive language. Its strong areas are an elegant and yet concise syntax, good overall performance and a beautiful implementation of some concepts from the world of functional programming.

USING Inline::Lua

Lua code can be included in the usual Inline style. Pass it as string at use time:

use Inline Lua => 'function pow (a, b) return a^b end';
print pow(2, 8);  # prints 256

Heredocs may come in handy for that:

    use Inline Lua => <<EOLUA;
    function pow (a, b)
	return a^b
    end
    EOLUA

    print pow(2, 8);

Or append it to your script after the __END__ token:

    use Inline 'Lua';

    print pow(2, 8)

    __END__
    __Lua__
    function pow (a, b)
	return a^b 
    end

All of those are equivalent.

Exchanging values with Lua functions

Lua datatypes map exceptionally well onto Perl types and vice versa. Lua knows about eight distinct types:

  • nil

    This is Perl's undef

  • number

    A Perl scalar with a, guess what, number in it.

  • string

    A Perl scalar with a string in it.

  • function

    Lua functions act as first class data types. The Perl equivalent is a code-reference.

  • userdata

    Lua being an embeddable language uses this one to handle generic C types. As of now, this is not yet supported by Inline::Lua.

  • thread

    Used to implement coroutines. Not yet handled by Inline::Lua

  • table

    Lua tables act as arrays or hashes depending on what you put into them. Inline::Lua can handle that transparently.

PASSING VALUES TO LUA FUNCTIONS

Whenever you call a Lua function, Inline::Lua looks at the arguments you passed to the function and converts them accordingly before executing the Lua code.

Plain Perl scalars

Scalars either holding a number, a string or undef are converted to the corresponding Lua types. Considering that those are all very basic types, this is not a very deep concept:

    use Inline Lua => <<EOLUA;
    function luaprint (a)
	io.write(a)
    end
    EOLUA
    
    lua_print("foobar");
    lua_print(42);

Care must be taken with undef. Lua is less forgiving than Perl in this respect. In particular, nil is not silently transformed into a useful value and you'll get a fatal error from Lua when you try

lua_print(undef);

Inline::Lua offers some means to deal with this problem. See "DEALING WITH UNDEF AND NIL" further below.

Array and hash references

Those are turned into Lua tables:

    use Inline Lua => <<EOLUA;
    function print_table (t)
	table.foreach(t, print)
    end
    EOLUA
    
    print_table( [1, 2, 3] );
    print_table( { key1 => 'val1',
                   key2 => 'val2' } );

This should print:

array:
1       1
2       2
3       3
hash:
key1    val1
key2    val2

Nested Perl arrays are handled as well:

print_table( [1, 2, 3, { key1 => 'val' } ] );

will result in

1       1
2       2
3       3
4       table: 0x8148128

Function references

That's the real interesting stuff. You are allowed to call Lua functions with function references as arguments and the Lua code will do the right thing:

    use Inline Lua => EOLUA
    function map_print (func, tab)
	table.foreach(tab, func)
    end
    EOLUA

    sub dump {
        my ($key, $val) = @_;
	print "$key => $val\n";

	# table.foreach() breaks out when the return value
	# is non-nill. Hence undef must be returned.
     	return undef;
    }
    
    map_print( \&dump, { key1 => 1, key2 => 2 } );

Here's a bit of currying. The Lua code calls the code-reference passed to it. This code-reference itself returns a reference to a Perl functions which eventually is triggered by Lua and its result is printed:

    use Inline Lua => <<EOLUA;
    function lua_curry (f, a, b) 
	local g = f(a)
	io.write( g(b) )
	-- or simply: io.write( f(a)(b) )
    end
    EOLUA

    sub curry {
	my $arg = shift;
	return sub { return $arg * shift };
    }

    lua_curry( \&curry, 6, 7);	# prints 42

It should be obvious that you are also allowed to pass references to anonymous functions, so

lua_curry( sub { my $arg = shift; ... }, 6, 7);

will work just as well.

Filehandles

From a technical point of view, Lua doesn't have a distinct type for that. It uses the userdata type for it. If you pass a reference to a filehandle to your Lua function, Inline::Lua will turn it into the thingy that Lua can deal with:

    use Inline Lua => <<EOLUA;
    function dump_fh (fh)
	for line in fh:lines() do
	    io.write(line, "\n")
	end
    end
    EOLUA

    open F, "file" or die $!;
    dump_fh(\*F);

Things you must not pass to Lua

You must not pass a reference to a scalar to any Lua function. Lua doesn't know about call-by-reference, hence trying it doesn't make much sense. You get a fatal runtime-error when you try for instance this:

function_defined_in_lua (\$var);

RETURNING VALUES FROM LUA FUNCTIONS

Returning stuff from your inlined functions is as trivial as passing them into them.

Numbers, strings, nil and boolean values

Those can be translated 1:1 into Perl:

    use Inline Lua => <<EOLUA;
    function return_basic ()
	local num = 42
	local str = "twenty-four"
	local boo = true
	return num, str, boo, nil
    end
    EOLUA

    my ($num, $str, $boo, $undef) = return_basic();

Tables

Whenever you return a Lua table, it gets returned as either a reference to a hash or a reference to an array. This depends on the values in the table. If all keys are numbers, then an array-ref is returned. Otherwise a hash-ref:

use Data::Dumper;
use Inline Lua => <<EOLUA;
function return_tab () 
    local ary  = { 1, 2, 3, [5] = 5 }
    local hash = { 1, 2, 3, key = 5 }
    return ary, hash
end
EOLUA

my ($ary, $hash) = return_tab();
print Dumper $ary;
print Dumper $hash;

__END__
$VAR1 = [
          '1',
          '2',
          '3',
          undef,
          '5'
        ];
$VAR1 = {
          '1' => '1',
          '3' => '3',
          '2' => '2',
          'key' => 'val'
        };

A couple of things worthy mention: Lua table indexes start at 1 as opposed to 0 in Perl. Inline::Lua will substract 1 from the index if the table is returned as an array so your Perl array will be 0-based. This does not happen for tables that get returned as a hash-reference as you can see in the above example.

Another thing you have to be aware of is potential holes in the array. You can create a Lua table where only the, say, 10000th element is set. Since 10000 is a number, it gets returned as an array. This array naturally will have 9999 undefined elements. In this case it might be better to forcefully turn this key into a string:

local ary = { [ tostring(10000) ] = 1 }

The tables you return can be arbitrarily deeply nested. The returned Perl structure will then also be nested.

What you cannot do is return a Lua table which uses values other than strings or numbers as keys. In Lua, a key can be any object, including a table, a function or whatever. There is no sensible way to mimick this behaviour in Perl so you will get a runtime error if you try something like this:

return { [{1, 2, 3}] = 1 }

There is no limitation on the values you put into a Lua table, though.

Functions

If your Lua function returns a function, the function is turned into a Perl function reference. If you are tired of having Perl calulcate the n-th Fibonacci number, let Lua do the hard work. This snippet below shows how a Lua function can return a Fibonacci number generator to Perl:

use Inline Lua => <<EOLUA;
function fib ()
    local f
    f = function (n)
        if n < 2 then return 1 end
        return f(n-1) + f(n-2)
    end
    return f
end
EOLUA

my $fib = fib();
print $fib->(11);
__END__
144

You can get as fancy as you want. Return a Lua function that itself returns a Lua function that returns another Lua function and so on. There should be no limitations at all.

Filehandles

Just as you can pass filehandles to Lua functions, you may also return them:

    use Inline Lua => <<EOLUA;
    function open_file (filename)
	return io.open(filename, "r")
    end
    EOLUA

    my $fh = open_file(".bashrc");
    while (<$fh>) {
	...
    }

It's a fatal error if your Lua code tries to return a closed filehandle.

DEALING WITH UNDEF AND NIL

You can change undef's default conversion so that Inline::Lua wont transform it to nil when passing the value to Lua:

    use Inline Lua	=> 'DATA',	# source code after the __END__ token
	       Undef	=> 0;

With the above, every undef value is turned into a Lua number with the value 0. Likewise

    use Inline Lua      => 'DATA', 
	       Undef	=> '';

This will turn undef into the empty string. Any valid Perl scalar can be specified for Undef, this includes references to hashes, arrays, functions etc. A basic example:

    use Inline Lua   => 'DATA',
	       Undef => 'Undefined value';
    
    print_values(1, 2, 3, undef, 4, 5);

    __END__
    __Lua__
    function print_values (...) 
	table.foreachi(arg, print)
    end

This would come out as

1       1
2       2
3       3
4       Undefined value
5       5
6       6

Sometimes however it is important to return a real nil to Lua. The foreach iterator for example breaks out of the iteration when non-nill has been returned. If you want to use a Perl function as foreach callback, simply having it return undef will therefore not work when Undef was set to some value. Inline::Lua provides a Perl value which is always converted to nil: $Inline::Lua::Nil. Here's an example:

    use Inline Lua   => 'DATA',
               Undef => 'Undefined value';
	       
    map_print( [1, 2, 3], sub { print "@_\n"; return $Inline::Lua::Nil } );

    __END__
    __Lua__
    function map_print (tab, func)
	table.foreach(tab, func)
    end

Results in

1 1
2 2
3 3

LUA FUNCTION PROTOTYPES

Lua functions have prototypes. When compiling those functions to bytecode, Inline::Lua looks at their prototype. When calling one of those functions later, it makes sure that the function arguments are padded with undef if you supply less arguments than mentioned in the prototype:

    use Inline Lua => <<EOLUA;
    function foo (a, b, c, ...)
	print(a, b, c)
    end
    EOLUA

    foo(1);	# actually: foo(1, undef, undef)

Those padded undefs are also handled accordingly to the value of Undef. Also note that ... in a prototype is never padded (as you can see in the above).

LUA SCRIPTS AS INLINE CODE

You are allowed to provide whole Lua scripts in your Inline section. Anything outside a function is then run at compile-time:

use Inline 'Lua';

__END__
__Lua__
print(1, 2)

Moreover, Lua scripts may return values to their caller. You can get these values at any point with "Inline::Lua-main_returns">:

use Inline 'Lua';

my @ret = Inline::Lua->main_returns;

__END__
__Lua__

print("I return a list of values")
return 1, 2, 3

Note that a Lua script's return value is only retrieved once at compile-time. Hence something like this might not do what you expect:

    use Inline 'Lua';

    print join "+", Inline::Lua->main_returns;
    
    luafunc();
    
    print "\n";
    print join "+", Inline::Lua->main_returns;

    __END__
    __Lua__
    a = 1
    b = 2
    
    function luafunc ()
	a = a + 1
	b = b + 1
    end

    return a, b

This will print

1+2
1+2

and not

1+2
2+3

as you might expect.

BUGS

There must be some. My first suspicion is memory leaks that may hide somewhere in the code. Checking for memory leaks is on my agenda for the next release.

Other than that, you might enjoy an occasional segfault.

If you encounter any of the above, please report it to me.

TODO

  • Check for memory leaks.

  • Find a smart way to handle objects elegantly.

  • Look closer at the thread type and figure out whether a sensible conversion exists.

  • Improve error messages. So far you get messages such as

    Attempt to pass unsupported reference type (SCALAR) to Lua at (eval 3) line 6.

  • In general: Have Inline::Lua croak less often.

SEE ALSO

Inline

Lua's home can be found at http://www.lua.org/.

AUTHOR

Tassilo von Parseval, <tassilo.von.parseval@rwth-aachen.de>

COPYRIGHT AND LICENSE

Copyright (C) 2004-2005 by Tassilo von Parseval

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.4 or, at your option, any later version of Perl 5 you may have available.

Lua 5.0 Copyright (C) 2003-2004 Tecgraf, PUC-Rio.