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parrot-0.5.2
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NAME

docs/compiler_faq.pod - Parrot FAQ for compiler writers

General Questions

Which C compilers can I use with Parrot?

Whoa, there--you're looking at the wrong FAQ. This document is for people writing compilers that target Parrot.

To answer your question, though, Parrot should theoretically work with any C89-compliant C compiler, although some features require gcc. See the README files in the root directory for more information about building Parrot.

How can I implement a compiler to use as a compiler object from within Parrot? (For example, with the compile op.)

Define a sub that takes as input a string, and returns something invokable. The easiest way to create something invokable at the moment is to use another compiler object (for example, the PIR compiler), which you can get with:

.local pmc pir_compiler
pir_compiler = compreg 'PIR'

How do I embed source locations in my code for debugging?

Use #line 42 "file.pir" for this. Note that this command must be in the first column; there must not be any leading space, otherwise the command is treated as a comment (due to the # character).

Subroutines

How do I generate a sub call in PIR?

IMCC makes this easy, handling the calling conventions for you.

$P0( $P1, $P2, $P3 )

where $P0 is the function object, and $P1, $P2, and $P3 are its parameters. You can also use a function's label in place of the object:

somefunctionlabel( $P1, $P2, $P3 )

You can also get return value(s):

($P1,$P2) = $P0( $P1, $P2, $P3 )

If the function name might collide with a Parrot opcode, quote it:

i = 'new'(42)

How do I generate a method call in PIR?

Similar to function calls, just append . and the method name to the object. You should quote a literal method name to avoid confusion.

ret_val = some_obj.'some_meth'(arg)

The method name may also be a string variable representing a method name:

.local string m
m = 'bold'
curses_obj.m()

How do I locate or create a subroutine object?

There are several ways to achieve this, depending on the location of the subroutine.

If the sub is in the same compilation unit use a Sub constant:

.const .Sub foo = 'foo'

The subroutine object is available in PASM too with a similar syntax:

.const .Sub P2 = 'foo'      # any P register will do
...
.pcc_sub foo:               # denote a Sub constant

If the PIR compiler finds a 'foo' function during compiling a file, then the syntax:

foo()

gets translated to above constant declaration.

A more dynamic way is:

.local pmc foo
foo = find_name 'foo'

This searches for a subroutine 'foo' in the current lexical pad, in the current namespace, in the global, and in the builtin namespace in that order. This opcode is generated, if foo() is used, but the compiler can't figure out, where the function is.

If the subroutine is in a different namespace, use the find_global opcode:

foo = find_global 'Foo', 'foo'

This searches the sub foo in the Foo namespace.

How do I create a Closure or Coroutine?

Closure and Coroutine carry both a dynamic state. Therefore you need to perform two steps. First use one of the above ways to locate the Sub object. Then use the op newclosure to capture the environment.

.local pmc coro
coro = find_name 'my_coro'
coro = newclosure coro

Any subroutine that contains a .yield directive is automatically created as a Coroutine PMC:

.sub my_coro             # automagically a Coroutine PMC
   ...
   .yield (result)
   ...
.end

How do I generate a tail call in PIR?

.sub foo
    # do something
    .return bar(42)           # tail call sub bar
.end

.sub bar
    # ...
.end

The sub bar will return to the caller of foo.

How do I generate a sub call with a variable-length parameter list in PIR?

If you have a variable amounts of arguments in an array, you can pass all items of that array with the :flat directive.

ar = new 'ResizablePMCArray'
push ar, "arg 1\n"
push ar, "arg 2\n"
...
foo(ar :flat)
...

How to I retrieve the contents of a variable-length parameter list being passed to me?

Use a slurpy array:

.sub mysub
  .param pmc argv      :slurpy
  .local int argc
  argc = argv
  ...

If you have a few fixed parameters too, you can use a slurpy array to get the rest of the arguments

.sub mysub
  .param pmc arg0
  .param pmc arg1
  .param pmc varargs   :slurpy
  .local int num_varargs
  num_varargs = varargs
  ...

How do I pass optional arguments?

Use the :optional and :opt_flag pragmas:

.sub foo
   .param pmc arg1       :optional
   .param int has_arg1   :opt_flag
   .param pmc arg2       :optional
   .param int has_arg2   :opt_flag

   if has_arg1 goto got_arg1
   ...

How do I create nested subroutines?

Please refer to "pdds/pdd20_lexical_vars.pod" in docs for details: look for :outer.

Variables

How do I fetch a variable from the global namespace?

There are two possible ways. Either use the special PIR syntax:

$P0 = global 'name_of_the_global'

or the find_global op:

find_global $P0, 'name_of_the_global'

How can I delete a global?

You can retrieve the namespace hash and use the delete opcode.

.sub main :main
  $P0 = new 'Integer'
  store_hll_global 'foo', $P0
  store_hll_global ['Bar'], 'baz', $P0
  # ...
  .local pmc ns, Bar_ns
  ns = get_hll_namespace
  delete ns['foo']              # delete from top level
  Bar_ns = ns['Bar']            # get Bar namespace
  delete Bar_ns['baz']
  $P0 = get_hll_global ['Bar'], 'baz'
  print "never\n"
.end

How do I use lexical pads to have both a function scope and a global scope?

Please refer to "pdds/pdd20_lexical_vars.pod" in docs for details.

How can I delete a lexical variable?

You can't. You can store a Null PMC as the value though, which will catch all further access to that variable and throw an exception.

How do I resolve a variable name?

Use find_name:

$P0 = find_name '$x'
find_name $P0, 'foo'    # same thing

This will find the name foo in the lexical, global, or builtin namespace, in that order, and store it in $P0.

How do I fetch a variable from the current lexical pad?

find_lex $P0, 'foo'

or much better, if possible just use the variable defined along with the .lex definition of foo.

How do I fetch a variable from any nesting depth?

That is still the same:

find_lex $P0, 'foo'

This finds a foo variable at any outer depth starting from the top.

If your language looks up variables differently, you have to walk the 'caller' chain. See also t/dynpmc/dynlexpad.t.

How can I produce more efficient code for lexicals?

Don't emit store_lex at all. Use find_lex only if the compiler doesn't know the variable. You can always just use the register that was defined in the .lex directive as an alias to that lexical, if you are in the same scope.

Modules, Classes, and Objects

How do I create a module?

TODO

How do I create a class?

With the newclass op:

newclass $P0, 'Animal'

How do I add instance variables/attributes?

Each class knows which attributes its objects can have. You can add attributes to a class (not to individual objects) like so:

addattribute $P0, 'legs'

How do I add instance methods to a class?

Methods are declared as functions in the class namespace with the :method keyword appended to the function declaration:

.namespace [ 'Animal' ]

.sub run :method
   print "slow and steady\n"
.end

How do I override a vtable on a class?

As with methods, but note the new keyword. The vtable name specified must be an existing vtable slot.

.namespace [ 'NearlyPi' ]

.sub get_string :vtable
   .return ('three and a half')
.end

Now, given an instance of NearlyPi in $P0

$S0 = $P0
say $S0  # prints 'three and a half'

How do I access attributes?

You can access attributes by a short name:

$P0 = getattribute self, 'legs'
assign $P0, 4                   # set attribute's value

a fully qualified name:

$P0 = getattribute self, "Animal\0legs"
assign $P0, 4                   # set attribute's value

or by an index:

.local int offs
offs = classoffset 'Animal'
$I0 = offs + 0                  # 1st attribute
$P0 = getattribute self, $I0
$I0 = offs + 1                  # 2nd attribute
$P0 = getattribute self, $I0

When should I use properties vs. attributes?

Properties aren't inherited. If you have some additional data that don't fit into the class's hierarchy, you could use properties.

How do I create a class that is a subclass of another class?

You first have to get the class PMC of the class you want to subclass. Either you use the PMC returned by the newclass op if you created the class, or use the getclass op:

getclass $P0, 'Animal'

Then you can use the subclass op to create a new class that is a subclass of this class:

subclass $P1, $P0, 'Dog'

This stores the newly created class PMC in $P1.

How do I create a class that has more than one parent class?

First, create a class without a parent class using newclass (or with only one subclass, see previous question). Then add the other parent classes to it. Please refer to the next question for an example.

How do I add another parent class to my class?

If you have a class PMC (created with newclass or by subclass), you can add more parent classes to it with the addparent op:

getclass $P1, 'Dog'
subclass $P2, $P1, 'SmallDog'
getclass $P3, 'Pet'
addparent $P2, $P3  # make "SmallDog" also a "Pet"

How can I specify the constructor of a class?

Just override the init vtable for that class.

newclass $P0, 'Dog'         # create a class named Dog
# ...

.namespace ['Dog']

.sub init :vtable
  # ...

Or you can specify the constructor method by setting the BUILD property of the class PMC:

newclass $P0, 'Dog'         # create a class named Dog
new $P1, 'String'           # create a string
set $P1, 'initialise'       # set it to the name of the constructor method
setprop $P0, 'BUILD', $P1   # set the BUILD property

How do I instantiate a class?

You can do so either with the class name:

new $P0, 'Dog'

or with the class object:

  $P1 = get_class 'Dog'   # find the 'Dog' class
  unless null $P1 goto have_dog_class
  printerr "Oops; can't find the 'Dog' class.\n"
  .return ()
have_dog_class:
  new $P0, $P1    # creates a Dog object and stores it in register $P0

The chief difference is that using a string constant will produce the specific error "Class 'Dog' not found" if that happens to be the case; the other code has to check explicitly.

During the new opcode the constructor is called.

How can I pass arguments to a constructor?

You can pass only a single argument to a constructor. By convention, a hash PMC is passed to the constructor that contains the arguments as key/value pairs:

new $P0, 'Hash'
set $P0['greeting'], 'hello'
set $P0['size'], 1.23

find_type $I0, 'Alien'
new $P1, $I0, $P0           # create an Alien object and pass
                            # the hash to the constructor

How do I add module/class methods?

TODO

How do I access module/class variables?

TODO

Exceptions

How do I throw an exception in PIR?

Create an Exception object and throw it!

$P0 = new 'Exception'
throw $P0

Not too hard, is it?

How do I throw an exception with an error message in PIR?

$P0 = new 'Exception'
$P0['_message'] = 'something happened'
throw $P0

How do I catch an exception in PIR?

Use push_eh to push an exception handler onto the stack.

push_eh handler
  $P0 = new 'Exception'       # or any other code ...
  throw $P0                   # ... that might throw
pop_eh
exit 0

An exception handler is called with two arguments: the exception and the message. More information is available inside the exception 'object' (TBD).

handler:
  .get_results ($P0, $S0)
  print 'Exception caught:'
  print $S0
  print "\n"
  exit 1

How do I let exceptions from exit pass through my handler?

Rethrow the exception if it has a severity of EXCEPT_EXIT.

handler:
  .include 'except_severity.pasm'
  .get_results ($P0, $S0)
  $I0 = $P0[2]
  if $I0 == .EXCEPT_EXIT goto unexceptional
  print "Exception caught!\n"
  exit 1
unexceptional:
  rethrow $P0

How do I access the error message of an exception I've caught?

  push_eh handler
  $P0 = new 'Exception'
  $P0['_message'] = 'something happened'
  throw $P0
  pop_eh
  exit 0

handler:
  .local pmc exception
  .local string message
  .get_results (exception, message)
  print 'Exception: '
  print message
  print "\n"
  exit 1

C Extensions

How do I create PMCs for my compiler?

Parrot supports dynamic PMCs, loadable at runtime, to allow compiler writers to extend Parrot with additional types. For more information about writing PMCs, see "build/pmc2c.pl" in tools and "pmc.pod" in docs.

To build dynamic PMCs, add something like the following to your Makefile:

PERL     = /usr/bin/perl
PMCBUILD = $(PERL) /path/to/parrot/tools/build/dynpmc.pl
DESTDIR  = /path/to/parrot/runtime/parrot/dynext
LOAD_EXT = .so

PMCDIR    = pmc
PMCS      = MyInteger MyFloat MyString MyObject
PMC_FILES = MyInteger.pmc MyFloat.pmc MyString.pmc MyObject.pmc

dynpmcs : $(PMC_FILES)
    @cd $(PMCDIR) && $(PMCBUILD) generate $(PMCS)
    @cd $(PMCDIR) && $(PMCBUILD) compile $(PMCS)
    @cd $(PMCDIR) && $(PMCBUILD) linklibs $(PMCS)
    @cd $(PMCDIR) && $(PMCBUILD) copy "--destination=$(DESTDIR)" $(PMCS)

How do I add another op to Parrot?

Parrot supports dynamic op libraries. These allow for ops specific to one language to be used without having to place them into the Parrot core itself. For examples of dynamic op libraries, see "dynoplibs" in src.

To build dynamic op libraries, add something like the following to your makefile:

PERL     = /usr/bin/perl
OPSBUILD = $(PERL) /path/to/parrot/tools/build/dynops.pl
DESTDIR  = /path/to/parrot/runtime/parrot/dynext
LOAD_EXT = .so

OPSDIR    = ops
OPLIBS    = myops
OPS_FILES = myops.ops

dynops : $(OPS_FILES)
    @cd $(OPSDIR) && $(BUILD) generate $(OPLIBS)
    @cd $(OPSDIR) && $(BUILD) compile $(OPLIBS)
    @cd $(OPSDIR) && $(BUILD) linklibs $(OPLIBS)
    @cd $(OPSDIR) && $(BUILD) copy "--destination=$(DESTDIR)" $(OPLIBS)

How do I use the Native Calling Interface (NCI)?

Using the NCI you can invoke functions written in C from a Parrot script. To every NCI invocation, there are two parts: the native function to be invoked, and the PIR code to do the invocation.

First the native function, to be written in C. On Windows, it is necessary to do a DLL export specification of the NCI function:

/* foo.c */

/* specify the function prototype */
#ifdef __WIN32
__declspec(dllexport) void foo(void);
#else
void foo(void);
#endif

void foo(void) {
  printf("Hello Parrot!\n");
}

Then, after having compiled the file as a shared library, the PIR code looks like this:

.sub main :main
   .local pmc lib, func

   # load the shared library
   lib = loadlib "hello" # no extension, .so or .dll is assumed

   # get a reference to the function from the library just
   # loaded, called "foo", and signature "void" (and no arguments)
   func = dlfunc lib, "foo", "v"

   # invoke
   func()

.end

If you embedded a Parrot in your C file and you want to invoke another function in that same C file, you should pass a null string to loadlib. Do that as follows:

.local pmc lib
.local string libname
null libname

lib = loadlib libname

Under Linux, the .c file must then be linked with the -export-dynamic option.

Misc

How can I access a program's environment?

Create a new Env PMC and access it like a hash.

.local pmc e
e = new 'Env'
$P0 = e['USER']      # lt

How can I access Parrot's configuration?

.include 'iglobals.pasm'
.local pmc interp, cfg
interp = getinterp
cfg = interp[.IGLOBALS_CONFIG_HASH]
$S0 = cfg['VERSION']    # "0.3.0"

See config_lib.pasm for all the keys in the config hash - or iterate over the config hash.

Languages

What files do I need to modify to add my new language compiler?

Aside from the files in your new language directory, you must modify

CREDITS
MANIFEST
config/gen/languages.pm
config/gen/makefiles/languages.in
languages/LANGUAGES.STATUS.pod

Inside your language dir, you may consider adding the following:

LICENSE
MAINTAINER
README
STATUS

Look to existing languages for some examples.