Skip to content
Find file
Fetching contributors…
Cannot retrieve contributors at this time
533 lines (376 sloc) 14.5 KB


perl design draft - types and const

Perl already has a type system

E.g. my Dog $wuff; stores the "Dog" stashname in comppad_names.

Only for lexicals (which is good and strict-safe). Almost nobody uses it. There are ~5 typed modules on CPAN, Net::DNS the most prominent. 5.10 broke all old type modules. Moose started afresh, but naive and different.

The biggest performance win would be to be able to mark packages and its @ISA as readonly i.e. const or type objects instances to do method_call compile-time optimizations. Also hash function optimizations (perfect), natively typed arrays and hashes, more constant folding and maybe shared strings.

    const package MyBase 0.01 {
      our @ISA = ();
      sub new { bless { @_ }, shift }
    const package MyChild 0.01 {
      our const @ISA = ('MyBase');

    my $obj = MyChild->new;
    => MyClass::new()

See "Compile-time type optimizations" in perltypes

Static compilers such as <B::C> and <B::CC> can optimize storage and run-time performance furthermore. Most of the previously dynamically allocated data can be made static (no parsing time, instant startup time), some data can use COW.

The management win would be to actually enable strict typing rules and to catch coding mistakes at compile-time. See "Compile-time type checks" in perltypes

MOP with 5.18

With the new MOP we can mark classes as closed or immutable (= optimizable), and type function arguments (optimizable method calls + optional type checks).

    package My;
    class MyBase (is_closed => 1) { sub new { bless {@_}, shift } }
    class MyDog (is_closed => 1, extends => MyBase) {
      #sub disallowed!
      method bark (Dog $dog) { print "bark ",ref $dog; }


const is added as type-qualifier to lexical declarations and to packages with the feature 'const', which was added with v5.18.

Declaration of my const variables and packages allows compile-time checks, shared strings, lexical scope instead of use constant, familiar syntax, and efficient optimizations, also for extensions, as the p5-mop, coretypes or B::C. const packages allow compile-time optimizations of method calls.

    use v5.18;
    my const $a = 0;
    my const @a = (0..9);
    my const %h = ('ok'  => '1',
                   'bad' => '2');
    const package foo {

Why not as attribute my $var :const?

1. This was my first proposal. blogs.perl Feb 2011

2. This attribute must also be special cased in core as my const. The hard part is optree support for const pads, not the keyword.

3. Third party modules cannot use this attribute, as there is no CHECK time hook for attributes yet, only at run-time. But at run-time it is too late.

4. The internal implementation look both bad, I'll try both. It will be easier to use for class, as class declarations are already overloaded with new syntax: extends, with, is, is_closed, metaclass, DEMOLISH, BUILD, FINALIZE. class NAME (is_closed => 1) {} currently defines an immutable, constant class.

5. my const looks better and familiar

  my const($a, int $b) = (0,1);
  my (const $a, int $b) = (0,1);
  my ($a:const, int $b:const) = (0,1);

p5 does NOT declare a type system

p5 only allows storing types in comppad_names and does some compile-time optimizations with const and methods. It only declares const.

p5 does not declare a type system by itself. One must use an extension which declares and handles its types. p5-mop is such a meta type system.

coretypes declares the native core types int, double and string for IV, NV and PV, for scalars, arrays and hashes. Not for functions yet, as function parameters are handled by extensions. coretypes is backwards compat.

There is no p5 super object, such as class or object. Maybe the mop needs one, but the three coretypes do not inherit. There is no $var->>print, @a->reverse and such planned. This can be done by mixins. coretypes will be slim, p5-mop will be fat. But hopefully optimizable (i.e. compile-time) in the general case.

PS: Several people at YAPC expressed their wish to make class immutable to be the new default. Then there must be a syntax to allow run-time changes (i.e. non-constant classes). class NAME (is_closed => 0) {} class NAME :mutable {} or such.

Acceptable type upgrades with const and coretypes

const variables are not purely constant, they are different from strictly typed variables. const variables may be upgraded to its string or numeric representation. They may be numified and/or stringified, strictly typed variables not.

  my const $a = 1;
  my const $s = "1";
  my int $ti = 0;
  my string $ts = "1";

  print "my $a";  # valid, upgraded from IV to PVIV
  print "my $ti"; # invalid, compile-time type violation error,
                  # the int IV cannot be stringified
You have to use:
  sprintf "my %d",$ti; # or
  print "my ",$ti;

  $g = $s + 1;  # valid, upgraded from PV to PVIV
  $g = $ts + 1; # invalid numify, compile-time type violation error
  $g = 0+$ts;   # invalid numify, compile-time type violation error

const and magic @_

How about function arguments and return value constness?

Return values are always copied without keeping constness from within a function.

  # valid
  perl -MReadonly -e'sub ret { Readonly my $i => 1; $i} my $x=ret();$x+=1'

Arguments handled by reference keep constness:

  # invalid
  perl -MReadonly -e'sub get { Readonly $_[0]; } my $x=1; get($x); $x+=1;'
  => Modification of a read-only value attempted

Arguments copied by value from @_ loose constness:

  # valid
  perl -MReadonly -e'sub get { my $x=shift; Readonly $x; } my $x=1; get($x); $x+=1;'
  perl -MReadonly -e'sub get { my $x=shift; $x+=1; } Readonly my $x=>1; get($x);'

Declare function signatures and types (target 5.20)

perlsub has this say: "Some folks would prefer full alphanumeric prototypes. Alphanumerics have been intentionally left out of prototypes for the express purpose of someday in the future adding named, formal parameters. The current mechanism's main goal is to let module writers provide better diagnostics for module users. Larry feels the notation quite understandable to Perl programmers, and that it will not intrude greatly upon the meat of the module, nor make it harder to read. The line noise is visually encapsulated into a small pill that's easy to swallow."

We want to optionally declare function parameter names and types and the return type. There is no need to come up with new keywords like fun just seperate sub prototypes from sub parameters. The simple rule is: If there is a whitespace or alphanumeric sequence in the protoype, it's no prototype. The general rule, esp. for single parameters: If there is any non-prototype character, it's an parameter declaration then.

Prototype changes the parser bindings, named function signatures avoids manual @_ extraction, function types and const declarations will catch type errors earlier and helps in compiler-time optimizations.

  sub fun ($arg1) {}

Function parameters have optional names and if so use them inside the function as such. @_ is not used then externaly, ony internaly.

  sub adder ($arg1, $arg2) { $arg1 + arg2 }

You are able to declare types of function parameters and return values.

  int sub adder (const int $arg1, const int $arg2) { $arg1 + arg2 }

You can als use types only without names. Note that the ; semicolon here denotes the 2nd argument as optional.

  int sub adder (const int; const int) { shift + shift }

With names it is better to use the = syntax for optional parameter declarations and a default value. You are able to declare optional parameter default values with using a name and = and a literal default value. Default values can be constants or variables, but no function calls.

  int sub adder (const int $arg1, const int $arg2=0) { $arg1 + arg2 }

Arguments are copied by default. To use pass by reference style as with $_[0] which changes the passed value, use the ref syntax \$name

  int sub adder (int \$arg1, const int $arg2=0) { $arg1 += arg2 }
  my $i=0; 
  print $i;
  => 2

See also Method::Signatures.

Return type declarations

The parameters were straightforward, but this is now hairy, as there are many competing syntax variants currently used.

At first: return types loose constness.

  const int
  sub ret_const { # => returns const int  
    ReadOnly my $foo => 1;
    my const $myarg = shift;

    $_[0] = 2; # run-time error byref
    $foo       # return a copy of a const

  my const $arg = 1;
  my $foo = ret_const($arg);
  $foo += 1;

Pointy Variant sub function (int $i -> int) {} Hashy Variant sub function (int $i => int) {} Old Variant (use typesafety) sub function (int; int $i) {} Modern C-like variant int sub function (int $i) {}

Changes & existing bugs

[#21979] [#todo] Can't declare subroutine entry in "my"

Multiple my declarations with types are not parsed correctly.

  perl -e'$i::x; my (j $a, i $b)=(1,2);'
  => Can't declare subroutine entry in "my" at -e line 1, near ")="

  perl -e'my (i $a)=(1);'
  => Can't declare subroutine entry in "my" at -e line 1, near ")="

This is a parser problem. A my (TYPE EXPR, ...) declaration should not be mixed with a function call, i.e. subroutine entry. my is a reserved keyword and cannot be the name of a subroutine.

The error should be the same as with perl -e'my j $a;' => No such class j at -e line 1, near "my j" and perl -e'$i::x; my i $a;' parses correctly.

[#21979][#109744] referenced constant loses readonlyness

On a threading Perl, a list-mode refgen applied to a constant will copy the constant rather than reference it, but the SVf_READONLY flag is not copied. An srefgen, however, will reference it. On a non-threading Perl constness is preserved. starts with support for const pads (preserve and use const-ness at compile-time).

See also Scalar::Construct to create a const. This module was created in response to the ticket [#109744], similar but with less features than Readonly or Const::Fast.

Private implementation notes

Please ignore

optree traversal

Generally, optree traversal to do optimizations in perl5 is forward only. You do not know the previous op, even kids do not know its parents. Some ops have special pointers backwards though, such as some UNOPs and BINOPs. So to do a simple tree-optimization, such as

  my const $i = 0;

  const (IV 0)
  padsv [$a]/CONST
  sassign *

which is represented in the optree as

  5     <2> sassign vKS*/2 ->6
  3        <$> const(IV 0) s ->4
  4        <0> padsv[$a:1,2] sRM*/LVINTRO,CONSTINIT ->5

you need to step forward to sassign to see that the 2nd argument padsv is constant, so either in the case of $i = 1; # $i being a const pad

  9     <2> sassign vKS/2 ->a
  7        <$> const(IV 1) s ->8
  8        <0> padsv[$a:1,2] sRM* ->9

which is illegal and throws a compiler error.

Or in the case above as my const $i = 0; which is a valid initialization, and allows overwriting the constant $i pad.


One would like to do constant folding directly at sassign, to check if the rhs of the expression evaluates to a constant.

  my $i = 1 + ($a << 2);

constant if $a is const, otherwise not.

If so, the rhs can be shortened to CONST. No need for various compiler passes through the whole optree. "localize optimizations"

  my $i;
  my const $a = 2;
  $i = 1 + ($a << 2);
  const(IV 2)
  padsv [$a] /CONST
  sassign /CONSTINIT
  const(IV 9) <=== optimized 1 + (2 << 2)
  padsv [$i]


Or get rid of a const padsv initialization by sassign at all if the rhs is parsed already to a constant scalar.

  perl -DT -e'my const $a=0;'

 <== MY(ival=1)

 1:LEX_NORMAL/XTERM "$a=0;\n"
 <== '$'

 Pending identifier '$a'
 <== PRIVATEREF(opval=op_padany)

 <== ASSIGNOP(ival=op_null)

 Saw number in ";\n"
 <== THING(opval=op_const) IV(0)

 <== ';'

 <== ';'

 Tokener got EOF
 <== EOF

 parsed as:
   MY $a(padany) ASSIGNOP THING const(IV 0);
 compiled to:
   const(IV 0)
   sassign /CONSTINIT

Since padsv already knows that it will be assigned to a CONST, and that is const it should store the value and the READONLY flag and omit the CONST and SASSIGN ops at all.


It would be nice to use compile my const $i=1 to CONST(IV=1) instead of PADSV($i) with $i SVf_READONLY to have faster access for the optimizer.

But the compiler needs to find lexical pads in the scope upwards, and I'm not sure if a CONST->op_sv assumption pointing to a pad is a good idea. A lexical is still a lexical. Every PAD*V($i) with $i SVf_READONLY should be marked as op_private = OPpPAD_CONST when the PAD is created (looked up) to be easier and more reliable detectable by the compiler, and visible via B::Concise/Deparse.

store my const $padsv as OP_CONST with ->op_sv pointing to the pad? at all, convert early or later? how about pad_findlex and lexical scoping rules then?

  { my const $i;
    sub x {$i+20}
seems to be safe to convert early, and not waste a padsv.
not for padav and padhv.

How to pad_findlex a const $i if optimized to OP_CONST? How about dynamic scope at run-time? How about late binding CvLATE: ANON and PVFM. (delayed creation of the pad) intro_my?

if so: ck: save to strip off padsv and sassign CONST IV=1

Nope. Better add const PADSV to the optimizers. CONST wants its sv as sv.


my const $i=1; # simple scalar case

Lexer: MY(ival=2) PRIVATEREF padany ASSIGNOP Initialize my const $i const IV 1 op: CONST IV=1 PADSV OPpPAD_CONST+OPpPAD_CONSTINIT, targ -> READONLY SASSIGN OPf_SPECIAL (for const init temp. overwrite)


my const ($i)=(1); # list case

Lexer: MY(ival=2) PRIVATEREF padany ASSIGNOP Initialize my const $i const IV 1 op: CONST IV=1 PADSV OPpPAD_CONST+OPpPAD_CONSTINIT, targ -> READONLY SASSIGN OPf_SPECIAL (for const init temp. overwrite)


my const @a=(1); # PADAV

Lexer: MY(ival=2) PRIVATEREF padany ASSIGNOP Initialize my const @a const IV 1 op: pushmark CONST IV=1 pushmark PADAV OPpPAD_CONST+OPpPAD_CONSTINIT, targ -> READONLY AASSIGN OPf_SPECIAL


my const $i; ... should warn: Uninitialized const $i at -e, line 1

constant folding: my const $i=1; $x=$i+20;

const 1 padsv $i sassign => padsv $i const 21 const 20 add gvsv gvsv sassign sassign

Something went wrong with that request. Please try again.