package overload;
+our $VERSION = '1.00';
+
$overload::hint_bits = 0x20000;
sub nil {}
}
%constants = (
- 'integer' => 0x1000,
+ 'integer' => 0x1000,
'float' => 0x2000,
'binary' => 0x4000,
'q' => 0x8000,
binary => "& | ^",
unary => "neg ! ~",
mutators => '++ --',
- func => "atan2 cos sin exp abs log sqrt",
+ func => "atan2 cos sin exp abs log sqrt int",
conversion => 'bool "" 0+',
iterators => '<>',
dereferencing => '${} @{} %{} &{} *{}',
special => 'nomethod fallback =');
+use warnings::register;
sub constant {
# Arguments: what, sub
while (@_) {
- $^H{$_[0]} = $_[1];
- $^H |= $constants{$_[0]} | $overload::hint_bits;
+ if (@_ == 1) {
+ warnings::warnif ("Odd number of arguments for overload::constant");
+ last;
+ }
+ elsif (!exists $constants {$_ [0]}) {
+ warnings::warnif ("`$_[0]' is not an overloadable type");
+ }
+ elsif (!ref $_ [1] || "$_[1]" !~ /CODE\(0x[\da-f]+\)$/) {
+ # Can't use C<ref $_[1] eq "CODE"> above as code references can be
+ # blessed, and C<ref> would return the package the ref is blessed into.
+ if (warnings::enabled) {
+ $_ [1] = "undef" unless defined $_ [1];
+ warnings::warn ("`$_[1]' is not a code reference");
+ }
+ }
+ else {
+ $^H{$_[0]} = $_[1];
+ $^H |= $constants{$_[0]} | $overload::hint_bits;
+ }
shift, shift;
}
}
__END__
-=head1 NAME
+=head1 NAME
overload - Package for overloading perl operations
package SomeThing;
- use overload
+ use overload
'+' => \&myadd,
'-' => \&mysub;
# etc
package Number;
use overload
- "+" => \&add,
+ "+" => \&add,
"*=" => "muas";
declares function Number::add() for addition, and method muas() in
the "class" C<Number> (or one of its base classes)
-for the assignment form C<*=> of multiplication.
+for the assignment form C<*=> of multiplication.
Arguments of this directive come in (key, value) pairs. Legal values
are values legal inside a C<&{ ... }> call, so the name of a
=back
-B<Warning.> Due to the presense of assignment versions of operations,
-routines which may be called in assignment context may create
-self-referential structures. Currently Perl will not free self-referential
+B<Warning.> Due to the presence of assignment versions of operations,
+routines which may be called in assignment context may create
+self-referential structures. Currently Perl will not free self-referential
structures until cycles are C<explicitly> broken. You may get problems
when traversing your structures too.
-Say,
+Say,
use overload '+' => sub { bless [ \$_[0], \$_[1] ] };
is asking for trouble, since for code C<$obj += $foo> the subroutine
-is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
+is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj,
\$foo]>. If using such a subroutine is an important optimization, one
can overload C<+=> explicitly by a non-"optimized" version, or switch
-to non-optimized version if C<not defined $_[2]> (see
+to non-optimized version if C<not defined $_[2]> (see
L<Calling Conventions for Binary Operations>).
Even if no I<explicit> assignment-variants of operators are present in
"**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=",
For these operations a substituted non-assignment variant can be called if
-the assignment variant is not available. Methods for operations "C<+>",
-"C<->", "C<+=>", and "C<-=>" can be called to automatically generate
-increment and decrement methods. The operation "C<->" can be used to
+the assignment variant is not available. Methods for operations C<+>,
+C<->, C<+=>, and C<-=> can be called to automatically generate
+increment and decrement methods. The operation C<-> can be used to
autogenerate missing methods for unary minus or C<abs>.
See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and
"&", "^", "|", "neg", "!", "~",
-"C<neg>" stands for unary minus. If the method for C<neg> is not
+C<neg> stands for unary minus. If the method for C<neg> is not
specified, it can be autogenerated using the method for
-subtraction. If the method for "C<!>" is not specified, it can be
-autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>".
+subtraction. If the method for C<!> is not specified, it can be
+autogenerated using the methods for C<bool>, or C<"">, or C<0+>.
=item * I<Increment and decrement>
=item * I<Transcendental functions>
- "atan2", "cos", "sin", "exp", "abs", "log", "sqrt",
+ "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int"
If C<abs> is unavailable, it can be autogenerated using methods
for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction.
+Note that traditionally the Perl function L<int> rounds to 0, thus for
+floating-point-like types one should follow the same semantic. If
+C<int> is unavailable, it can be autogenerated using the overloading of
+C<0+>.
+
=item * I<Boolean, string and numeric conversion>
- "bool", "\"\"", "0+",
+ 'bool', '""', '0+',
If one or two of these operations are not overloaded, the remaining ones can
be used instead. C<bool> is used in the flow control operators
-(like C<while>) and for the ternary "C<?:>" operation. These functions can
+(like C<while>) and for the ternary C<?:> operation. These functions can
return any arbitrary Perl value. If the corresponding operation for this value
is overloaded too, that operation will be called again with this value.
+As a special case if the overload returns the object itself then it will
+be used directly. An overloaded conversion returning the object is
+probably a bug, because you're likely to get something that looks like
+C<YourPackage=HASH(0x8172b34)>.
+
=item * I<Iteration>
"<>"
should be of correct type. These functions should return a reference
of correct type, or another object with overloaded dereferencing.
+As a special case if the overload returns the object itself then it
+will be used directly (provided it is the correct type).
+
+The dereference operators must be specified explicitly they will not be passed to
+"nomethod".
+
=item * I<Special>
"nomethod", "fallback", "=",
if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the
C<use overload> directive.
+The C<"nomethod"> mechanism is I<not> used for the dereference operators
+( ${} @{} %{} &{} *{} ).
+
+
If some operation cannot be resolved, and there is no function
assigned to C<"nomethod">, then an exception will be raised via die()--
unless C<"fallback"> was specified as a key in C<use overload> directive.
-=head2 Fallback
+
+=head2 Fallback
The key C<"fallback"> governs what to do if a method for a particular
operation is not found. Three different cases are possible depending on
=item * defined, but FALSE
No autogeneration is tried. Perl tries to call
-C<"nomethod"> value, and if this is missing, raises an exception.
+C<"nomethod"> value, and if this is missing, raises an exception.
=back
to a reference that shares its object with some other reference, such
as
- $a=$b;
+ $a=$b;
++$a;
To make this change $a and not change $b, a copy of C<$$a> is made,
C<nomethod>). Note that if this operation is expressed via C<'+'>
a nonmutator, i.e., as in
- $a=$b;
+ $a=$b;
$a=$a+1;
then C<$a> does not reference a new copy of C<$$a>, since $$a does not
=item B<Example>
-The actually executed code for
+The actually executed code for
- $a=$b;
+ $a=$b;
Something else which does not modify $a or $b....
++$a;
may be
- $a=$b;
+ $a=$b;
Something else which does not modify $a or $b....
$a = $a->clone(undef,"");
$a->incr(undef,"");
C<$a+=$b> can use the method for C<"+"> if the method for C<"+=">
is not defined.
-=item I<Conversion operations>
+=item I<Conversion operations>
String, numeric, and boolean conversion are calculated in terms of one
another if not all of them are defined.
can be expressed in terms of string conversion.
-=item I<Comparison operations>
+=item I<Comparison operations>
can be expressed in terms of its "spaceship" counterpart: either
C<E<lt>=E<gt>> or C<cmp>:
The corresponding values are references to functions which take three arguments:
the first one is the I<initial> string form of the constant, the second one
-is how Perl interprets this constant, the third one is how the constant is used.
+is how Perl interprets this constant, the third one is how the constant is used.
Note that the initial string form does not
-contain string delimiters, and has backslashes in backslash-delimiter
+contain string delimiters, and has backslashes in backslash-delimiter
combinations stripped (thus the value of delimiter is not relevant for
-processing of this string). The return value of this function is how this
+processing of this string). The return value of this function is how this
constant is going to be interpreted by Perl. The third argument is undefined
unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote
context (comes from strings, regular expressions, and single-quote HERE
-documents), it is C<tr> for arguments of C<tr>/C<y> operators,
+documents), it is C<tr> for arguments of C<tr>/C<y> operators,
it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise.
Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>,
it is expected that overloaded constant strings are equipped with reasonable
-overloaded catenation operator, otherwise absurd results will result.
+overloaded catenation operator, otherwise absurd results will result.
Similarly, negative numbers are considered as negations of positive constants.
Note that it is probably meaningless to call the functions overload::constant()
overload::constant integer => sub {Math::BigInt->new(shift)};
}
-B<BUGS> Currently overloaded-ness of constants does not propagate
+B<BUGS> Currently overloaded-ness of constants does not propagate
into C<eval '...'>.
=head1 IMPLEMENTATION
package. This magic is three-words-long for packages without
overloading, and carries the cache table if the package is overloaded.
-Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
+Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is
carried out before any operation that can imply an assignment to the
object $a (or $b) refers to, like C<$a++>. You can override this
behavior by defining your own copy constructor (see L<"Copy Constructor">).
=head1 Metaphor clash
One may wonder why the semantic of overloaded C<=> is so counter intuitive.
-If it I<looks> counter intuitive to you, you are subject to a metaphor
-clash.
+If it I<looks> counter intuitive to you, you are subject to a metaphor
+clash.
Here is a Perl object metaphor:
The difference is not relevant in the absence of mutators. After
a Perl-way assignment an operation which mutates the data referenced by $a
-would change the data referenced by $b too. Effectively, after
+would change the data referenced by $b too. Effectively, after
C<$a = $b> values of $a and $b become I<indistinguishable>.
-On the other hand, anyone who has used algebraic notation knows the
+On the other hand, anyone who has used algebraic notation knows the
expressive power of the arithmetic metaphor. Overloading works hard
to enable this metaphor while preserving the Perlian way as far as
-possible. Since it is not not possible to freely mix two contradicting
+possible. Since it is not possible to freely mix two contradicting
metaphors, overloading allows the arithmetic way to write things I<as
far as all the mutators are called via overloaded access only>. The
way it is done is described in L<Copy Constructor>.
If some mutator methods are directly applied to the overloaded values,
-one may need to I<explicitly unlink> other values which references the
+one may need to I<explicitly unlink> other values which references the
same value:
$a = new Data 23;
preserve "objectness" of $a. But Perl I<has> a way to make assignments
to an object do whatever you want. It is just not the overload, but
tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method
-which returns the object itself, and STORE() method which changes the
+which returns the object itself, and STORE() method which changes the
value of the object, one can reproduce the arithmetic metaphor in its
completeness, at least for variables which were tie()d from the start.
package two_refs;
use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} };
- sub new {
- my $p = shift;
+ sub new {
+ my $p = shift;
bless \ [@_], $p;
}
sub gethash {
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
- sub STORE {
+ sub STORE {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$$self->[$key] = shift;
}
- sub FETCH {
+ sub FETCH {
my $self = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
Second, we create a new tied hash each time the hash syntax is used.
This allows us not to worry about a possibility of a reference loop,
-would would lead to a memory leak.
+which would lead to a memory leak.
Both these problems can be cured. Say, if we want to overload hash
dereference on a reference to an object which is I<implemented> as a
hash itself, the only problem one has to circumvent is how to access
-this I<actual> hash (as opposed to the I<virtual> exhibited by
+this I<actual> hash (as opposed to the I<virtual> hash exhibited by the
overloaded dereference operator). Here is one possible fetching routine:
sub access_hash {
my ($self, $key) = (shift, shift);
my $class = ref $self;
- bless $self, 'overload::dummy'; # Disable overloading of %{}
+ bless $self, 'overload::dummy'; # Disable overloading of %{}
my $out = $self->{$key};
bless $self, $class; # Restore overloading
$out;
}
-To move creation of the tied hash on each access, one may an extra
+To remove creation of the tied hash on each access, one may an extra
level of indirection which allows a non-circular structure of references:
package two_refs1;
use overload '%{}' => sub { ${shift()}->[1] },
'@{}' => sub { ${shift()}->[0] };
- sub new {
- my $p = shift;
+ sub new {
+ my $p = shift;
my $a = [@_];
my %h;
tie %h, $p, $a;
my %fields;
my $i = 0;
$fields{$_} = $i++ foreach qw{zero one two three};
- sub STORE {
+ sub STORE {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key] = shift;
}
- sub FETCH {
+ sub FETCH {
my $a = ${shift()};
my $key = $fields{shift()};
defined $key or die "Out of band access";
$a->[$key];
}
-Now if $baz is overloaded like this, then C<$bar> is a reference to a
+Now if $baz is overloaded like this, then C<$baz> is a reference to a
reference to the intermediate array, which keeps a reference to an
actual array, and the access hash. The tie()ing object for the access
-hash is also a reference to a reference to the actual array, so
+hash is a reference to a reference to the actual array, so
=over
$a = $a->pretty if ref $a;
$b = $b->pretty if ref $b;
"[$meth $a $b]";
- }
+ }
Now one can finish the script by
I<components> $a and $b of an object. In the above subroutine
C<"[$meth $a $b]"> is a catenation of some strings and components $a
and $b. If these components use overloading, the catenation operator
-will look for an overloaded operator C<.>, if not present, it will
+will look for an overloaded operator C<.>; if not present, it will
look for an overloaded operator C<"">. Thus it is enough to use
use overload nomethod => \&wrap, '""' => \&str;
$a = 'u' unless defined $a;
$b = 'u' unless defined $b;
"[$meth $a $b]";
- }
+ }
Now one can change the last line of the script to
side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]]
and one can inspect the value in debugger using all the possible
-methods.
+methods.
-Something is is still amiss: consider the loop variable $cnt of the
+Something is still amiss: consider the loop variable $cnt of the
script. It was a number, not an object. We cannot make this value of
type C<symbolic>, since then the loop will not terminate.
} else {
"[$meth $a]";
}
- }
- my %subr = ( n => sub {$_[0]},
- sqrt => sub {sqrt $_[0]},
+ }
+ my %subr = ( n => sub {$_[0]},
+ sqrt => sub {sqrt $_[0]},
'-' => sub {shift() - shift()},
'+' => sub {shift() + shift()},
'/' => sub {shift() / shift()},
);
sub num {
my ($meth, $a, $b) = @{+shift};
- my $subr = $subr{$meth}
+ my $subr = $subr{$meth}
or die "Do not know how to ($meth) in symbolic";
$a = $a->num if ref $a eq __PACKAGE__;
$b = $b->num if ref $b eq __PACKAGE__;
(not required without mutators!), and implements only those arithmetic
operations which are used in the example.
-To implement most arithmetic operations is easy, one should just use
+To implement most arithmetic operations is easy; one should just use
the tables of operations, and change the code which fills %subr to
my %subr = ( 'n' => sub {$_[0]} );
way to know that the implementation of C<'+='> does not mutate
the argument, compare L<Copy Constructor>).
-To implement a copy constructor, add C<'=' => \&cpy> to C<use overload>
+To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload>
line, and code (this code assumes that mutators change things one level
deep only, so recursive copying is not needed):
bless [@$self], ref $self;
}
-To make C<++> and C<--> work, we need to implement actual mutators,
+To make C<++> and C<--> work, we need to implement actual mutators,
either directly, or in C<nomethod>. We continue to do things inside
C<nomethod>, thus add
return $obj;
}
-after the first line of wrap(). This is not a most effective
+after the first line of wrap(). This is not a most effective
implementation, one may consider
sub inc { $_[0] = bless ['++', shift, 1]; }
$subr{'++'} = $subr{'+'};
$subr{'--'} = $subr{'-'};
-This finishes implementation of a primitive symbolic calculator in
-50 lines of Perl code. Since the numeric values of subexpressions
+This finishes implementation of a primitive symbolic calculator in
+50 lines of Perl code. Since the numeric values of subexpressions
are not cached, the calculator is very slow.
Here is the answer for the exercise: In the case of str(), we need no
To see it in action, add a method
- sub STORE {
- my $obj = shift;
- $#$obj = 1;
+ sub STORE {
+ my $obj = shift;
+ $#$obj = 1;
@$obj->[0,1] = ('=', shift);
}
overloading enabled, and it is what is used by the C<Overloaded>
function of module C<overload>).
+The module might issue the following warnings:
+
+=over 4
+
+=item Odd number of arguments for overload::constant
+
+(W) The call to overload::constant contained an odd number of arguments.
+The arguments should come in pairs.
+
+=item `%s' is not an overloadable type
+
+(W) You tried to overload a constant type the overload package is unaware of.
+
+=item `%s' is not a code reference
+
+(W) The second (fourth, sixth, ...) argument of overload::constant needs
+to be a code reference. Either an anonymous subroutine, or a reference
+to a subroutine.
+
+=back
+
=head1 BUGS
Because it is used for overloading, the per-package hash %OVERLOAD now
interesting effects if some package is not overloaded, but inherits
from two overloaded packages.
-Relation between overloading and tie()ing is broken. Overloading is
+Relation between overloading and tie()ing is broken. Overloading is
triggered or not basing on the I<previous> class of tie()d value.
-This happens because the presence of overloading is checked too early,
+This happens because the presence of overloading is checked too early,
before any tie()d access is attempted. If the FETCH()ed class of the
-tie()d value does not change, a simple workaround is to access the value
+tie()d value does not change, a simple workaround is to access the value
immediately after tie()ing, so that after this call the I<previous> class
coincides with the current one.
https://perl5.git.perl.org / perl5.git / blobdiff