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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 the builtin PIR or PASM compilers.

See languages/tcl/tcl.pir_template's .sub _tcl_compile as an example.

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

You can do this using either the setfile and setline opcodes or with C-like #line comments:

  #line 27 "my_source.file"

Simply set the source file name or line number whenever it changes. But note that currently (Parrot 0.3.0) both are ignored in the lexer.

Subroutines

How do I generate a sub call in PIR?

If you have a fixed-length parameter list, IMCC makes this blindingly easy:

   $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 . to the object and the method:

  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

Both carry dynamic state, therefore you use one of the above ways to locate the sub object, then you just clone it to get a distinct copy of it:

  .local pmc coro
  coro = find_name "my_coro"
  coro = clone 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 .ResitablePMCArray
  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.

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. Nested namespace names have a NULL char prepended to their name.

  .sub main :main
    $P0 = new .Integer
    store_global "foo", $P0
    store_global "Bar", "baz", $P0
    # ...
    .include "interpinfo.pasm"
    .local pmc ns, Bar_ns
    ns = interpinfo .INTERPINFO_NAMESPACE_ROOT
    delete ns["foo"]                # delete from top level
    Bar_ns = ns["\0Bar"]            # get Bar namespace
    delete Bar_ns["baz"]
    $P0 = find_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 t 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 __init method
    $P0 = new .Integer
    setattribute self, "legs", $P0
    ...
  .end
  .sub run method
    ...
  .end

How do I access attributes?

You can access attributes by a short name, a fully qualified name, or by an index.

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

or

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

or

  .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 define a method named __init in the namespace of the class.

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

    .namespace ["Dog"]

    .sub __init method
      # ...

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?

First, you have to retrieve the type id of class you want to instantiate:

    find_type $I0, "Dog"

Then, you can create an instance of Dog with the new op:

    new $P0, $I0    # creates a Dog object and stores it in register $P0

or for short:

    new $P0, "Dog"

During the new opcode the constructor is called.

How can I pass arguments to an 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
    clear_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 '(0,0') P0, S0      # PASM syntax
    print "Exception caught!\n"
    exit 1

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
    clear_eh
    exit 0

  handler:
    .local pmc exception
    .local string message
    .get_results (exception, message)   # PIR syntax
    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) && $(BUILD) generate $(PMCS)
      @cd $(PMCDIR) && $(BUILD) compile $(PMCS)
      @cd $(PMCDIR) && $(BUILD) linklibs $(PMCS)
      @cd $(PMCDIR) && $(BUILD) copy "--destination=$(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)

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.