X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=lib%2Foverload.pm;h=838c91fcee0479881aaa7909c8958eb9e2d4b6ac;hb=2f3b333f8f33a996945618e3a6de2f8c72c3ba4e;hp=20411ea5767de692e49521ad34a7190171de4c17;hpb=1fef88e72b0b21420614d87ecab0aaedf3725271;p=p5sagit%2Fp5-mst-13.2.git diff --git a/lib/overload.pm b/lib/overload.pm index 20411ea..838c91f 100644 --- a/lib/overload.pm +++ b/lib/overload.pm @@ -1,12 +1,31 @@ package overload; +our $VERSION = '1.00'; + +$overload::hint_bits = 0x20000; + +sub nil {} + sub OVERLOAD { $package = shift; my %arg = @_; - my $hash = \%{$package . "::OVERLOAD"}; + my ($sub, $fb); + $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. + *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. for (keys %arg) { - $hash->{$_} = $arg{$_}; + if ($_ eq 'fallback') { + $fb = $arg{$_}; + } else { + $sub = $arg{$_}; + if (not ref $sub and $sub !~ /::/) { + $ {$package . "::(" . $_} = $sub; + $sub = \&nil; + } + #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; + *{$package . "::(" . $_} = \&{ $sub }; + } } + ${$package . "::()"} = $fb; # Make it findable too (fallback only). } sub import { @@ -18,49 +37,139 @@ sub import { sub unimport { $package = (caller())[0]; - my $hash = \%{$package . "::OVERLOAD"}; + ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table shift; for (@_) { - delete $hash->{$_}; + if ($_ eq 'fallback') { + undef $ {$package . "::()"}; + } else { + delete $ {$package . "::"}{"(" . $_}; + } } } sub Overloaded { - ($package = ref $_[0]) and defined %{$package . "::OVERLOAD"}; + my $package = shift; + $package = ref $package if ref $package; + $package->can('()'); +} + +sub ov_method { + my $globref = shift; + return undef unless $globref; + my $sub = \&{*$globref}; + return $sub if $sub ne \&nil; + return shift->can($ {*$globref}); } sub OverloadedStringify { - ($package = ref $_[0]) and - defined %{$package . "::OVERLOAD"} and - exists $ {$package . "::OVERLOAD"}{'""'} and - defined &{$ {$package . "::OVERLOAD"}{'""'}}; + my $package = shift; + $package = ref $package if ref $package; + #$package->can('(""') + ov_method mycan($package, '(""'), $package + or ov_method mycan($package, '(0+'), $package + or ov_method mycan($package, '(bool'), $package + or ov_method mycan($package, '(nomethod'), $package; } sub Method { - ($package = ref $_[0]) and - defined %{$package . "::OVERLOAD"} and - $ {$package . "::OVERLOAD"}{$_[1]}; + my $package = shift; + $package = ref $package if ref $package; + #my $meth = $package->can('(' . shift); + ov_method mycan($package, '(' . shift), $package; + #return $meth if $meth ne \&nil; + #return $ {*{$meth}}; } sub AddrRef { - $package = ref $_[0]; - bless $_[0], Overload::Fake; # Non-overloaded package + my $package = ref $_[0]; + return "$_[0]" unless $package; + bless $_[0], overload::Fake; # Non-overloaded package my $str = "$_[0]"; bless $_[0], $package; # Back - $str; + $package . substr $str, index $str, '='; } sub StrVal { - (OverloadedStringify) ? - (AddrRef) : + (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ? + (AddrRef(shift)) : "$_[0]"; } +sub mycan { # Real can would leave stubs. + my ($package, $meth) = @_; + return \*{$package . "::$meth"} if defined &{$package . "::$meth"}; + my $p; + foreach $p (@{$package . "::ISA"}) { + my $out = mycan($p, $meth); + return $out if $out; + } + return undef; +} + +%constants = ( + 'integer' => 0x1000, + 'float' => 0x2000, + 'binary' => 0x4000, + 'q' => 0x8000, + 'qr' => 0x10000, + ); + +%ops = ( with_assign => "+ - * / % ** << >> x .", + assign => "+= -= *= /= %= **= <<= >>= x= .=", + num_comparison => "< <= > >= == !=", + '3way_comparison'=> "<=> cmp", + str_comparison => "lt le gt ge eq ne", + binary => "& | ^", + unary => "neg ! ~", + mutators => '++ --', + func => "atan2 cos sin exp abs log sqrt int", + conversion => 'bool "" 0+', + iterators => '<>', + dereferencing => '${} @{} %{} &{} *{}', + special => 'nomethod fallback ='); + +use warnings::register; +sub constant { + # Arguments: what, sub + while (@_) { + 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 above as code references can be + # blessed, and C 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; + } +} + +sub remove_constant { + # Arguments: what, sub + while (@_) { + delete $^H{$_[0]}; + $^H &= ~ $constants{$_[0]}; + shift, shift; + } +} + 1; __END__ -=head1 NAME +=head1 NAME overload - Package for overloading perl operations @@ -68,7 +177,7 @@ overload - Package for overloading perl operations package SomeThing; - use overload + use overload '+' => \&myadd, '-' => \&mysub; # etc @@ -82,13 +191,6 @@ overload - Package for overloading perl operations ... $strval = overload::StrVal $b; -=head1 CAVEAT SCRIPTOR - -Overloading of operators is a subject not to be taken lightly. -Neither its precise implementation, syntax, nor semantics are -100% endorsed by Larry Wall. So any of these may be changed -at some point in the future. - =head1 DESCRIPTION =head2 Declaration of overloaded functions @@ -97,17 +199,18 @@ The compilation directive package Number; use overload - "+" => \&add, + "+" => \&add, "*=" => "muas"; declares function Number::add() for addition, and method muas() in the "class" C (or one of its base classes) -for the assignment form C<*=> of multiplication. +for the assignment form C<*=> of multiplication. Arguments of this directive come in (key, value) pairs. Legal values -are values legal inside a C<&{ ... }> call, so the name of a subroutine, -a reference to a subroutine, or an anonymous subroutine will all work. -Legal keys are listed below. +are values legal inside a C<&{ ... }> call, so the name of a +subroutine, a reference to a subroutine, or an anonymous subroutine +will all work. Note that values specified as strings are +interpreted as methods, not subroutines. Legal keys are listed below. The subroutine C will be called to execute C<$a+$b> if $a is a reference to an object blessed into the package C, or if $a is @@ -117,6 +220,10 @@ C<$a+=7>, or C<$a++>. See L. (Mathemagical methods refer to methods triggered by an overloaded mathematical operator.) +Since overloading respects inheritance via the @ISA hierarchy, the +above declaration would also trigger overloading of C<+> and C<*=> in +all the packages which inherit from C. + =head2 Calling Conventions for Binary Operations The functions specified in the C directive are called @@ -145,7 +252,8 @@ the arguments are reversed. the current operation is an assignment variant (as in C<$a+=7>), but the usual function is called instead. This additional -information can be used to generate some optimizations. +information can be used to generate some optimizations. Compare +L. =back @@ -155,9 +263,67 @@ Unary operation are considered binary operations with the second argument being C. Thus the functions that overloads C<{"++"}> is called with arguments C<($a,undef,'')> when $a++ is executed. +=head2 Calling Conventions for Mutators + +Two types of mutators have different calling conventions: + +=over + +=item C<++> and C<--> + +The routines which implement these operators are expected to actually +I their arguments. So, assuming that $obj is a reference to a +number, + + sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} + +is an appropriate implementation of overloaded C<++>. Note that + + sub incr { ++$ {$_[0]} ; shift } + +is OK if used with preincrement and with postincrement. (In the case +of postincrement a copying will be performed, see L.) + +=item C and other assignment versions + +There is nothing special about these methods. They may change the +value of their arguments, and may leave it as is. The result is going +to be assigned to the value in the left-hand-side if different from +this value. + +This allows for the same method to be used as overloaded C<+=> and +C<+>. Note that this is I, but not recommended, since by the +semantic of L<"Fallback"> Perl will call the method for C<+> anyway, +if C<+=> is not overloaded. + +=back + +B Due to the presence of assignment versions of operations, +routines which may be called in assignment context may create +self-referential structures. Currently Perl will not free self-referential +structures until cycles are C broken. You may get problems +when traversing your structures too. + +Say, + + use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; + +is asking for trouble, since for code C<$obj += $foo> the subroutine +is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, +\$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 (see +L). + +Even if no I assignment-variants of operators are present in +the script, they may be generated by the optimizer. Say, C<",$obj,"> or +C<',' . $obj . ','> may be both optimized to + + my $tmp = ',' . $obj; $tmp .= ','; + =head2 Overloadable Operations -The following symbols can be specified in C: +The following symbols can be specified in C directive: =over 5 @@ -172,6 +338,10 @@ the assignment variant is not available. Methods for operations "C<+>", increment and decrement methods. The operation "C<->" can be used to autogenerate missing methods for unary minus or C. +See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and +L<"Calling Conventions for Binary Operations">) for details of these +substitutions. + =item * I "<", "<=", ">", ">=", "==", "!=", "<=>", @@ -200,21 +370,54 @@ postfix form. =item * I - "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", + "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int" If C is unavailable, it can be autogenerated using methods for "E" or "E=E" combined with either unary minus or subtraction. +Note that traditionally the Perl function L rounds to 0, thus for +floating-point-like types one should follow the same semantic. If +C is unavailable, it can be autogenerated using the overloading of +C<0+>. + =item * I "bool", "\"\"", "0+", -If one or two of these operations are unavailable, the remaining ones can +If one or two of these operations are not overloaded, the remaining ones can be used instead. C is used in the flow control operators (like C) and for the ternary "C" operation. These functions can return any arbitrary Perl value. If the corresponding operation for this value is overloaded too, that operation will be called again with this value. +As a special case if the overload returns the object itself then it will +be used directly. An overloaded conversion returning the object is +probably a bug, because you're likely to get something that looks like +C. + +=item * I + + "<>" + +If not overloaded, the argument will be converted to a filehandle or +glob (which may require a stringification). The same overloading +happens both for the I syntax C$varE> and +I syntax C${var}E>. + +=item * I + + '${}', '@{}', '%{}', '&{}', '*{}'. + +If not overloaded, the argument will be dereferenced I, thus +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 "nomethod", "fallback", "=", @@ -223,14 +426,66 @@ see L>. =back -See L<"Fallback"> for an explanation of when a missing method can be autogenerated. +See L<"Fallback"> for an explanation of when a missing method can be +autogenerated. + +A computer-readable form of the above table is available in the hash +%overload::ops, with values being space-separated lists of names: + + with_assign => '+ - * / % ** << >> x .', + assign => '+= -= *= /= %= **= <<= >>= x= .=', + num_comparison => '< <= > >= == !=', + '3way_comparison'=> '<=> cmp', + str_comparison => 'lt le gt ge eq ne', + binary => '& | ^', + unary => 'neg ! ~', + mutators => '++ --', + func => 'atan2 cos sin exp abs log sqrt', + conversion => 'bool "" 0+', + iterators => '<>', + dereferencing => '${} @{} %{} &{} *{}', + special => 'nomethod fallback =' + +=head2 Inheritance and overloading + +Inheritance interacts with overloading in two ways. + +=over + +=item Strings as values of C directive + +If C in + + use overload key => value; + +is a string, it is interpreted as a method name. + +=item Overloading of an operation is inherited by derived classes + +Any class derived from an overloaded class is also overloaded. The +set of overloaded methods is the union of overloaded methods of all +the ancestors. If some method is overloaded in several ancestor, then +which description will be used is decided by the usual inheritance +rules: + +If C inherits from C and C (in this order), C overloads +C<+> with C<\&D::plus_sub>, and C overloads C<+> by C<"plus_meth">, +then the subroutine C will be called to implement +operation C<+> for an object in package C. + +=back + +Note that since the value of the C key is not a subroutine, +its inheritance is not governed by the above rules. In the current +implementation, the value of C in the first overloaded +ancestor is used, but this is accidental and subject to change. =head1 SPECIAL SYMBOLS FOR C Three keys are recognized by Perl that are not covered by the above description. -=head2 Last Resort +=head2 Last Resort C<"nomethod"> should be followed by a reference to a function of four parameters. If defined, it is called when the overloading mechanism @@ -245,11 +500,16 @@ the last one is used. Say, C<1-$a> can be equivalent to if the pair C<"nomethod" =E "nomethodMethod"> was specified in the C directive. +The C<"nomethod"> mechanism is I 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 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 @@ -273,10 +533,13 @@ present. =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 +B C<"fallback"> inheritance via @ISA is not carved in stone +yet, see L<"Inheritance and overloading">. + =head2 Copy Constructor The value for C<"="> is a reference to a function with three @@ -288,18 +551,18 @@ This operation is called in the situations when a mutator is applied to a reference that shares its object with some other reference, such as - $a=$b; - $a++; + $a=$b; + ++$a; To make this change $a and not change $b, a copy of C<$$a> is made, and $a is assigned a reference to this new object. This operation is -done during execution of the C<$a++>, and not during the assignment, +done during execution of the C<++$a>, and not during the assignment, (so before the increment C<$$a> coincides with C<$$b>). This is only -done if C<++> is expressed via a method for C<'++'> or C<'+='>. Note -that if this operation is expressed via C<'+'> a nonmutator, i.e., as -in +done if C<++> is expressed via a method for C<'++'> or C<'+='> (or +C). Note that if this operation is expressed via C<'+'> +a nonmutator, i.e., as in - $a=$b; + $a=$b; $a=$a+1; then C<$a> does not reference a new copy of C<$$a>, since $$a does not @@ -313,15 +576,15 @@ string copy if the object is a plain scalar. =item B -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,""); @@ -331,6 +594,9 @@ C<'='> was overloaded with C<\&clone>. =back +Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for +C<$b = $a; ++$a>. + =head1 MAGIC AUTOGENERATION If a method for an operation is not found, and the value for C<"fallback"> is @@ -345,7 +611,7 @@ substitutions are possible for the following operations: C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> is not defined. -=item I +=item I String, numeric, and boolean conversion are calculated in terms of one another if not all of them are defined. @@ -372,7 +638,7 @@ string or numerical conversion. can be expressed in terms of string conversion. -=item I +=item I can be expressed in terms of its "spaceship" counterpart: either C=E> or C: @@ -380,6 +646,14 @@ C=E> or C: <, >, <=, >=, ==, != in terms of <=> lt, gt, le, ge, eq, ne in terms of cmp +=item I + + <> in terms of builtin operations + +=item I + + ${} @{} %{} &{} *{} in terms of builtin operations + =item I can be expressed in terms of an assignment to the dereferenced value, if this @@ -387,7 +661,7 @@ value is a scalar and not a reference. =back -=head1 WARNING +=head1 Losing overloading The restriction for the comparison operation is that even if, for example, `C' should return a blessed reference, the autogenerated `C' @@ -436,37 +710,112 @@ Returns C or a reference to the method that implements C. =back +=head1 Overloading constants + +For some application Perl parser mangles constants too much. It is possible +to hook into this process via overload::constant() and overload::remove_constant() +functions. + +These functions take a hash as an argument. The recognized keys of this hash +are + +=over 8 + +=item integer + +to overload integer constants, + +=item float + +to overload floating point constants, + +=item binary + +to overload octal and hexadecimal constants, + +=item q + +to overload C-quoted strings, constant pieces of C- and C-quoted +strings and here-documents, + +=item qr + +to overload constant pieces of regular expressions. + +=back + +The corresponding values are references to functions which take three arguments: +the first one is the I string form of the constant, the second one +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 +combinations stripped (thus the value of delimiter is not relevant for +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- and C- constants, it is C in single-quote +context (comes from strings, regular expressions, and single-quote HERE +documents), it is C for arguments of C/C operators, +it is C for right-hand side of C-operator, and it is C 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. +Similarly, negative numbers are considered as negations of positive constants. + +Note that it is probably meaningless to call the functions overload::constant() +and overload::remove_constant() from anywhere but import() and unimport() methods. +From these methods they may be called as + + sub import { + shift; + return unless @_; + die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; + overload::constant integer => sub {Math::BigInt->new(shift)}; + } + +B Currently overloaded-ness of constants does not propagate +into C. + =head1 IMPLEMENTATION What follows is subject to change RSN. -The table of methods for all operations is cached as magic in the -symbol table hash for the package. The table is rechecked for changes due to -C, C, and @ISA only during -Cing; so if they are changed dynamically, you'll need an -additional fake Cing to update the table. - -(Every SVish thing has a magic queue, and magic is an entry in that queue. -This is how a single variable may participate in multiple forms of magic -simultaneously. For instance, environment variables regularly have two -forms at once: their %ENV magic and their taint magic.) +The table of methods for all operations is cached in magic for the +symbol table hash for the package. The cache is invalidated during +processing of C, C, new function +definitions, and changes in @ISA. However, this invalidation remains +unprocessed until the next Cing into the package. Hence if you +want to change overloading structure dynamically, you'll need an +additional (fake) Cing to update the table. + +(Every SVish thing has a magic queue, and magic is an entry in that +queue. This is how a single variable may participate in multiple +forms of magic simultaneously. For instance, environment variables +regularly have two forms at once: their %ENV magic and their taint +magic. However, the magic which implements overloading is applied to +the stashes, which are rarely used directly, thus should not slow down +Perl.) If an object belongs to a package using overload, it carries a special flag. Thus the only speed penalty during arithmetic operations without overloading is the checking of this flag. -In fact, if C is not present, there is almost no overhead for -overloadable operations, so most programs should not suffer measurable -performance penalties. A considerable effort was made to minimize the overhead -when overload is used and the current operation is overloadable but -the arguments in question do not belong to packages using overload. When -in doubt, test your speed with C and without it. So far there -have been no reports of substantial speed degradation if Perl is compiled -with optimization turned on. - -There is no size penalty for data if overload is not used. - -Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is +In fact, if C is not present, there is almost no overhead +for overloadable operations, so most programs should not suffer +measurable performance penalties. A considerable effort was made to +minimize the overhead when overload is used in some package, but the +arguments in question do not belong to packages using overload. When +in doubt, test your speed with C and without it. So far +there have been no reports of substantial speed degradation if Perl is +compiled with optimization turned on. + +There is no size penalty for data if overload is not used. The only +size penalty if overload is used in some package is that I the +packages acquire a magic during the next Cing into the +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 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">). @@ -474,6 +823,543 @@ behavior by defining your own copy constructor (see L<"Copy Constructor">). It is expected that arguments to methods that are not explicitly supposed to be changed are constant (but this is not enforced). +=head1 Metaphor clash + +One may wonder why the semantic of overloaded C<=> is so counter intuitive. +If it I counter intuitive to you, you are subject to a metaphor +clash. + +Here is a Perl object metaphor: + +I< object is a reference to blessed data> + +and an arithmetic metaphor: + +I< object is a thing by itself>. + +The I
problem of overloading C<=> is the fact that these metaphors +imply different actions on the assignment C<$a = $b> if $a and $b are +objects. Perl-think implies that $a becomes a reference to whatever +$b was referencing. Arithmetic-think implies that the value of "object" +$a is changed to become the value of the object $b, preserving the fact +that $a and $b are separate entities. + +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 +C<$a = $b> values of $a and $b become I. + +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 possible to freely mix two contradicting +metaphors, overloading allows the arithmetic way to write things I. The +way it is done is described in L. + +If some mutator methods are directly applied to the overloaded values, +one may need to I other values which references the +same value: + + $a = new Data 23; + ... + $b = $a; # $b is "linked" to $a + ... + $a = $a->clone; # Unlink $b from $a + $a->increment_by(4); + +Note that overloaded access makes this transparent: + + $a = new Data 23; + $b = $a; # $b is "linked" to $a + $a += 4; # would unlink $b automagically + +However, it would not make + + $a = new Data 23; + $a = 4; # Now $a is a plain 4, not 'Data' + +preserve "objectness" of $a. But Perl I a way to make assignments +to an object do whatever you want. It is just not the overload, but +tie()ing interface (see L). Adding a FETCH() method +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. + +(Note that a workaround for a bug may be needed, see L<"BUGS">.) + +=head1 Cookbook + +Please add examples to what follows! + +=head2 Two-face scalars + +Put this in F in your Perl library directory: + + package two_face; # Scalars with separate string and + # numeric values. + sub new { my $p = shift; bless [@_], $p } + use overload '""' => \&str, '0+' => \&num, fallback => 1; + sub num {shift->[1]} + sub str {shift->[0]} + +Use it as follows: + + require two_face; + my $seven = new two_face ("vii", 7); + printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; + print "seven contains `i'\n" if $seven =~ /i/; + +(The second line creates a scalar which has both a string value, and a +numeric value.) This prints: + + seven=vii, seven=7, eight=8 + seven contains `i' + +=head2 Two-face references + +Suppose you want to create an object which is accessible as both an +array reference and a hash reference, similar to the +L +builtin Perl type. Let's make it better than a pseudo-hash by +allowing index 0 to be treated as a normal element. + + package two_refs; + use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; + sub new { + my $p = shift; + bless \ [@_], $p; + } + sub gethash { + my %h; + my $self = shift; + tie %h, ref $self, $self; + \%h; + } + + sub TIEHASH { my $p = shift; bless \ shift, $p } + my %fields; + my $i = 0; + $fields{$_} = $i++ foreach qw{zero one two three}; + sub STORE { + my $self = ${shift()}; + my $key = $fields{shift()}; + defined $key or die "Out of band access"; + $$self->[$key] = shift; + } + sub FETCH { + my $self = ${shift()}; + my $key = $fields{shift()}; + defined $key or die "Out of band access"; + $$self->[$key]; + } + +Now one can access an object using both the array and hash syntax: + + my $bar = new two_refs 3,4,5,6; + $bar->[2] = 11; + $bar->{two} == 11 or die 'bad hash fetch'; + +Note several important features of this example. First of all, the +I type of $bar is a scalar reference, and we do not overload +the scalar dereference. Thus we can get the I non-overloaded +contents of $bar by just using C<$$bar> (what we do in functions which +overload dereference). Similarly, the object returned by the +TIEHASH() method is a scalar reference. + +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, +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 as a +hash itself, the only problem one has to circumvent is how to access +this I hash (as opposed to the I 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 %{} + my $out = $self->{$key}; + bless $self, $class; # Restore overloading + $out; + } + +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; + my $a = [@_]; + my %h; + tie %h, $p, $a; + bless \ [$a, \%h], $p; + } + sub gethash { + my %h; + my $self = shift; + tie %h, ref $self, $self; + \%h; + } + + sub TIEHASH { my $p = shift; bless \ shift, $p } + my %fields; + my $i = 0; + $fields{$_} = $i++ foreach qw{zero one two three}; + sub STORE { + my $a = ${shift()}; + my $key = $fields{shift()}; + defined $key or die "Out of band access"; + $a->[$key] = shift; + } + sub FETCH { + my $a = ${shift()}; + my $key = $fields{shift()}; + defined $key or die "Out of band access"; + $a->[$key]; + } + +Now if $baz is overloaded like this, then C<$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 a reference to a reference to the actual array, so + +=over + +=item * + +There are no loops of references. + +=item * + +Both "objects" which are blessed into the class C are +references to a reference to an array, thus references to a I. +Thus the accessor expression C<$$foo-E[$ind]> involves no +overloaded operations. + +=back + +=head2 Symbolic calculator + +Put this in F in your Perl library directory: + + package symbolic; # Primitive symbolic calculator + use overload nomethod => \&wrap; + + sub new { shift; bless ['n', @_] } + sub wrap { + my ($obj, $other, $inv, $meth) = @_; + ($obj, $other) = ($other, $obj) if $inv; + bless [$meth, $obj, $other]; + } + +This module is very unusual as overloaded modules go: it does not +provide any usual overloaded operators, instead it provides the L operator C. In this example the corresponding +subroutine returns an object which encapsulates operations done over +the objects: C contains C<['n', 3]>, C<2 + new +symbolic 3> contains C<['+', 2, ['n', 3]]>. + +Here is an example of the script which "calculates" the side of +circumscribed octagon using the above package: + + require symbolic; + my $iter = 1; # 2**($iter+2) = 8 + my $side = new symbolic 1; + my $cnt = $iter; + + while ($cnt--) { + $side = (sqrt(1 + $side**2) - 1)/$side; + } + print "OK\n"; + +The value of $side is + + ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], + undef], 1], ['n', 1]] + +Note that while we obtained this value using a nice little script, +there is no simple way to I this value. In fact this value may +be inspected in debugger (see L), but ony if +C Bption is set, and not via C

command. + +If one attempts to print this value, then the overloaded operator +C<""> will be called, which will call C operator. The +result of this operator will be stringified again, but this result is +again of type C, which will lead to an infinite loop. + +Add a pretty-printer method to the module F: + + sub pretty { + my ($meth, $a, $b) = @{+shift}; + $a = 'u' unless defined $a; + $b = 'u' unless defined $b; + $a = $a->pretty if ref $a; + $b = $b->pretty if ref $b; + "[$meth $a $b]"; + } + +Now one can finish the script by + + print "side = ", $side->pretty, "\n"; + +The method C is doing object-to-string conversion, so it +is natural to overload the operator C<""> using this method. However, +inside such a method it is not necessary to pretty-print the +I $a and $b of an object. In the above subroutine +C<"[$meth $a $b]"> is a catenation of some strings and components $a +and $b. If these components use overloading, the catenation operator +will look for an overloaded operator C<.>; if not present, it will +look for an overloaded operator C<"">. Thus it is enough to use + + use overload nomethod => \&wrap, '""' => \&str; + sub str { + my ($meth, $a, $b) = @{+shift}; + $a = 'u' unless defined $a; + $b = 'u' unless defined $b; + "[$meth $a $b]"; + } + +Now one can change the last line of the script to + + print "side = $side\n"; + +which outputs + + 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. + +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, since then the loop will not terminate. + +Indeed, to terminate the cycle, the $cnt should become false. +However, the operator C for checking falsity is overloaded (this +time via overloaded C<"">), and returns a long string, thus any object +of type C is true. To overcome this, we need a way to +compare an object to 0. In fact, it is easier to write a numeric +conversion routine. + +Here is the text of F with such a routine added (and +slightly modified str()): + + package symbolic; # Primitive symbolic calculator + use overload + nomethod => \&wrap, '""' => \&str, '0+' => \# + + sub new { shift; bless ['n', @_] } + sub wrap { + my ($obj, $other, $inv, $meth) = @_; + ($obj, $other) = ($other, $obj) if $inv; + bless [$meth, $obj, $other]; + } + sub str { + my ($meth, $a, $b) = @{+shift}; + $a = 'u' unless defined $a; + if (defined $b) { + "[$meth $a $b]"; + } else { + "[$meth $a]"; + } + } + my %subr = ( n => sub {$_[0]}, + sqrt => sub {sqrt $_[0]}, + '-' => sub {shift() - shift()}, + '+' => sub {shift() + shift()}, + '/' => sub {shift() / shift()}, + '*' => sub {shift() * shift()}, + '**' => sub {shift() ** shift()}, + ); + sub num { + my ($meth, $a, $b) = @{+shift}; + 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__; + $subr->($a,$b); + } + +All the work of numeric conversion is done in %subr and num(). Of +course, %subr is not complete, it contains only operators used in the +example below. Here is the extra-credit question: why do we need an +explicit recursion in num()? (Answer is at the end of this section.) + +Use this module like this: + + require symbolic; + my $iter = new symbolic 2; # 16-gon + my $side = new symbolic 1; + my $cnt = $iter; + + while ($cnt) { + $cnt = $cnt - 1; # Mutator `--' not implemented + $side = (sqrt(1 + $side**2) - 1)/$side; + } + printf "%s=%f\n", $side, $side; + printf "pi=%f\n", $side*(2**($iter+2)); + +It prints (without so many line breaks) + + [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] + [n 1]] 2]]] 1] + [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 + pi=3.182598 + +The above module is very primitive. It does not implement +mutator methods (C<++>, C<-=> and so on), does not do deep copying +(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 +the tables of operations, and change the code which fills %subr to + + my %subr = ( 'n' => sub {$_[0]} ); + foreach my $op (split " ", $overload::ops{with_assign}) { + $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; + } + my @bins = qw(binary 3way_comparison num_comparison str_comparison); + foreach my $op (split " ", "@overload::ops{ @bins }") { + $subr{$op} = eval "sub {shift() $op shift()}"; + } + foreach my $op (split " ", "@overload::ops{qw(unary func)}") { + print "defining `$op'\n"; + $subr{$op} = eval "sub {$op shift()}"; + } + +Due to L, we do not need anything +special to make C<+=> and friends work, except filling C<+=> entry of +%subr, and defining a copy constructor (needed since Perl has no +way to know that the implementation of C<'+='> does not mutate +the argument, compare L). + +To implement a copy constructor, add C<< '=' => \&cpy >> to C +line, and code (this code assumes that mutators change things one level +deep only, so recursive copying is not needed): + + sub cpy { + my $self = shift; + bless [@$self], ref $self; + } + +To make C<++> and C<--> work, we need to implement actual mutators, +either directly, or in C. We continue to do things inside +C, thus add + + if ($meth eq '++' or $meth eq '--') { + @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference + return $obj; + } + +after the first line of wrap(). This is not a most effective +implementation, one may consider + + sub inc { $_[0] = bless ['++', shift, 1]; } + +instead. + +As a final remark, note that one can fill %subr by + + my %subr = ( 'n' => sub {$_[0]} ); + foreach my $op (split " ", $overload::ops{with_assign}) { + $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; + } + my @bins = qw(binary 3way_comparison num_comparison str_comparison); + foreach my $op (split " ", "@overload::ops{ @bins }") { + $subr{$op} = eval "sub {shift() $op shift()}"; + } + foreach my $op (split " ", "@overload::ops{qw(unary func)}") { + $subr{$op} = eval "sub {$op shift()}"; + } + $subr{'++'} = $subr{'+'}; + $subr{'--'} = $subr{'-'}; + +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 +explicit recursion since the overloaded C<.>-operator will fall back +to an existing overloaded operator C<"">. Overloaded arithmetic +operators I fall back to numeric conversion if C is +not explicitly requested. Thus without an explicit recursion num() +would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild +the argument of num(). + +If you wonder why defaults for conversion are different for str() and +num(), note how easy it was to write the symbolic calculator. This +simplicity is due to an appropriate choice of defaults. One extra +note: due to the explicit recursion num() is more fragile than sym(): +we need to explicitly check for the type of $a and $b. If components +$a and $b happen to be of some related type, this may lead to problems. + +=head2 I symbolic calculator + +One may wonder why we call the above calculator symbolic. The reason +is that the actual calculation of the value of expression is postponed +until the value is I. + +To see it in action, add a method + + sub STORE { + my $obj = shift; + $#$obj = 1; + @$obj->[0,1] = ('=', shift); + } + +to the package C. After this change one can do + + my $a = new symbolic 3; + my $b = new symbolic 4; + my $c = sqrt($a**2 + $b**2); + +and the numeric value of $c becomes 5. However, after calling + + $a->STORE(12); $b->STORE(5); + +the numeric value of $c becomes 13. There is no doubt now that the module +symbolic provides a I calculator indeed. + +To hide the rough edges under the hood, provide a tie()d interface to the +package C (compare with L). Add methods + + sub TIESCALAR { my $pack = shift; $pack->new(@_) } + sub FETCH { shift } + sub nop { } # Around a bug + +(the bug is described in L<"BUGS">). One can use this new interface as + + tie $a, 'symbolic', 3; + tie $b, 'symbolic', 4; + $a->nop; $b->nop; # Around a bug + + my $c = sqrt($a**2 + $b**2); + +Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value +of $c becomes 13. To insulate the user of the module add a method + + sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } + +Now + + my ($a, $b); + symbolic->vars($a, $b); + my $c = sqrt($a**2 + $b**2); + + $a = 3; $b = 4; + printf "c5 %s=%f\n", $c, $c; + + $a = 12; $b = 5; + printf "c13 %s=%f\n", $c, $c; + +shows that the numeric value of $c follows changes to the values of $a +and $b. + =head1 AUTHOR Ilya Zakharevich EFE. @@ -483,14 +1369,61 @@ Ilya Zakharevich EFE. When Perl is run with the B<-Do> switch or its equivalent, overloading induces diagnostic messages. +Using the C command of Perl debugger (see L) one can +deduce which operations are overloaded (and which ancestor triggers +this overloading). Say, if C is overloaded, then the method C<(eq> +is shown by debugger. The method C<()> corresponds to the C +key (in fact a presence of this method shows that this package has +overloading enabled, and it is what is used by the C +function of module C). + +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 associative array -%OVERLOAD now has a special meaning in Perl. +Because it is used for overloading, the per-package hash %OVERLOAD now +has a special meaning in Perl. The symbol table is filled with names +looking like line-noise. + +For the purpose of inheritance every overloaded package behaves as if +C is present (possibly undefined). This may create +interesting effects if some package is not overloaded, but inherits +from two overloaded packages. + +Relation between overloading and tie()ing is broken. Overloading is +triggered or not basing on the I class of tie()d value. + +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 +immediately after tie()ing, so that after this call the I class +coincides with the current one. + +B a way to fix this without a speed penalty. -As shipped, mathemagical properties are not inherited via the @ISA tree. +Barewords are not covered by overloaded string constants. -This document is confusing. +This document is confusing. There are grammos and misleading language +used in places. It would seem a total rewrite is needed. =cut