| File | /usr/local/lib/perl5/5.10.1/overload.pm |
| Statements Executed | 112 |
| Statement Execution Time | 674µs |
| Calls | P | F | Exclusive Time |
Inclusive Time |
Subroutine |
|---|---|---|---|---|---|
| 6 | 1 | 1 | 212µs | 214µs | overload::OVERLOAD |
| 6 | 6 | 6 | 81µs | 295µs | overload::import |
| 1 | 1 | 1 | 25µs | 140µs | overload::BEGIN@139 |
| 1 | 1 | 2 | 2µs | 2µs | overload::CORE:match (opcode) |
| 0 | 0 | 0 | 0s | 0s | overload::AddrRef |
| 0 | 0 | 0 | 0s | 0s | overload::Method |
| 0 | 0 | 0 | 0s | 0s | overload::Overloaded |
| 0 | 0 | 0 | 0s | 0s | overload::OverloadedStringify |
| 0 | 0 | 0 | 0s | 0s | overload::constant |
| 0 | 0 | 0 | 0s | 0s | overload::mycan |
| 0 | 0 | 0 | 0s | 0s | overload::nil |
| 0 | 0 | 0 | 0s | 0s | overload::ov_method |
| 0 | 0 | 0 | 0s | 0s | overload::remove_constant |
| 0 | 0 | 0 | 0s | 0s | overload::unimport |
| Line | State ments |
Time on line |
Calls | Time in subs |
Code |
|---|---|---|---|---|---|
| 1 | package overload; | ||||
| 2 | |||||
| 3 | 1 | 1µs | our $VERSION = '1.07'; | ||
| 4 | |||||
| 5 | sub nil {} | ||||
| 6 | |||||
| 7 | # spent 214µs (212+2) within overload::OVERLOAD which was called 6 times, avg 36µs/call:
# 6 times (212µs+2µs) by overload::import at line 33, avg 36µs/call | ||||
| 8 | 86 | 222µs | $package = shift; | ||
| 9 | my %arg = @_; | ||||
| 10 | my ($sub, $fb); | ||||
| 11 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. | ||||
| 12 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. | ||||
| 13 | for (keys %arg) { | ||||
| 14 | if ($_ eq 'fallback') { | ||||
| 15 | $fb = $arg{$_}; | ||||
| 16 | } else { | ||||
| 17 | $sub = $arg{$_}; | ||||
| 18 | if (not ref $sub and $sub !~ /::/) { # spent 2µs making 1 call to overload::CORE:match | ||||
| 19 | $ {$package . "::(" . $_} = $sub; | ||||
| 20 | $sub = \&nil; | ||||
| 21 | } | ||||
| 22 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; | ||||
| 23 | *{$package . "::(" . $_} = \&{ $sub }; | ||||
| 24 | } | ||||
| 25 | } | ||||
| 26 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). | ||||
| 27 | } | ||||
| 28 | |||||
| 29 | # spent 295µs (81+214) within overload::import which was called 6 times, avg 49µs/call:
# once (19µs+42µs) by Devel::StackTrace::BEGIN@12 at line 12 of Devel/StackTrace.pm
# once (9µs+48µs) by Exception::Class::Base::BEGIN@30 at line 31 of Exception/Class/Base.pm
# once (14µs+37µs) by URI::BEGIN@24 at line 27 of URI.pm
# once (14µs+30µs) by Class::MOP::Method::BEGIN@19 at line 19 of Class/MOP/Method.pm
# once (13µs+29µs) by Moose::Meta::TypeConstraint::BEGIN@8 at line 9 of Moose/Meta/TypeConstraint.pm
# once (13µs+28µs) by Moose::Meta::Role::Method::Required::BEGIN@8 at line 9 of Moose/Meta/Role/Method/Required.pm | ||||
| 30 | 18 | 72µs | $package = (caller())[0]; | ||
| 31 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; | ||||
| 32 | shift; | ||||
| 33 | $package->overload::OVERLOAD(@_); # spent 214µs making 6 calls to overload::OVERLOAD, avg 36µs/call | ||||
| 34 | } | ||||
| 35 | |||||
| 36 | sub unimport { | ||||
| 37 | $package = (caller())[0]; | ||||
| 38 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table | ||||
| 39 | shift; | ||||
| 40 | for (@_) { | ||||
| 41 | if ($_ eq 'fallback') { | ||||
| 42 | undef $ {$package . "::()"}; | ||||
| 43 | } else { | ||||
| 44 | delete $ {$package . "::"}{"(" . $_}; | ||||
| 45 | } | ||||
| 46 | } | ||||
| 47 | } | ||||
| 48 | |||||
| 49 | sub Overloaded { | ||||
| 50 | my $package = shift; | ||||
| 51 | $package = ref $package if ref $package; | ||||
| 52 | $package->can('()'); | ||||
| 53 | } | ||||
| 54 | |||||
| 55 | sub ov_method { | ||||
| 56 | my $globref = shift; | ||||
| 57 | return undef unless $globref; | ||||
| 58 | my $sub = \&{*$globref}; | ||||
| 59 | return $sub if $sub ne \&nil; | ||||
| 60 | return shift->can($ {*$globref}); | ||||
| 61 | } | ||||
| 62 | |||||
| 63 | sub OverloadedStringify { | ||||
| 64 | my $package = shift; | ||||
| 65 | $package = ref $package if ref $package; | ||||
| 66 | #$package->can('(""') | ||||
| 67 | ov_method mycan($package, '(""'), $package | ||||
| 68 | or ov_method mycan($package, '(0+'), $package | ||||
| 69 | or ov_method mycan($package, '(bool'), $package | ||||
| 70 | or ov_method mycan($package, '(nomethod'), $package; | ||||
| 71 | } | ||||
| 72 | |||||
| 73 | sub Method { | ||||
| 74 | my $package = shift; | ||||
| 75 | if(ref $package) { | ||||
| 76 | local $@; | ||||
| 77 | local $!; | ||||
| 78 | require Scalar::Util; | ||||
| 79 | $package = Scalar::Util::blessed($package); | ||||
| 80 | return undef if !defined $package; | ||||
| 81 | } | ||||
| 82 | #my $meth = $package->can('(' . shift); | ||||
| 83 | ov_method mycan($package, '(' . shift), $package; | ||||
| 84 | #return $meth if $meth ne \&nil; | ||||
| 85 | #return $ {*{$meth}}; | ||||
| 86 | } | ||||
| 87 | |||||
| 88 | sub AddrRef { | ||||
| 89 | my $package = ref $_[0]; | ||||
| 90 | return "$_[0]" unless $package; | ||||
| 91 | |||||
| 92 | local $@; | ||||
| 93 | local $!; | ||||
| 94 | require Scalar::Util; | ||||
| 95 | my $class = Scalar::Util::blessed($_[0]); | ||||
| 96 | my $class_prefix = defined($class) ? "$class=" : ""; | ||||
| 97 | my $type = Scalar::Util::reftype($_[0]); | ||||
| 98 | my $addr = Scalar::Util::refaddr($_[0]); | ||||
| 99 | return sprintf("$class_prefix$type(0x%x)", $addr); | ||||
| 100 | } | ||||
| 101 | |||||
| 102 | 1 | 2µs | *StrVal = *AddrRef; | ||
| 103 | |||||
| 104 | sub mycan { # Real can would leave stubs. | ||||
| 105 | my ($package, $meth) = @_; | ||||
| 106 | |||||
| 107 | my $mro = mro::get_linear_isa($package); | ||||
| 108 | foreach my $p (@$mro) { | ||||
| 109 | my $fqmeth = $p . q{::} . $meth; | ||||
| 110 | return \*{$fqmeth} if defined &{$fqmeth}; | ||||
| 111 | } | ||||
| 112 | |||||
| 113 | return undef; | ||||
| 114 | } | ||||
| 115 | |||||
| 116 | 1 | 4µs | %constants = ( | ||
| 117 | 'integer' => 0x1000, # HINT_NEW_INTEGER | ||||
| 118 | 'float' => 0x2000, # HINT_NEW_FLOAT | ||||
| 119 | 'binary' => 0x4000, # HINT_NEW_BINARY | ||||
| 120 | 'q' => 0x8000, # HINT_NEW_STRING | ||||
| 121 | 'qr' => 0x10000, # HINT_NEW_RE | ||||
| 122 | ); | ||||
| 123 | |||||
| 124 | 1 | 11µs | %ops = ( with_assign => "+ - * / % ** << >> x .", | ||
| 125 | assign => "+= -= *= /= %= **= <<= >>= x= .=", | ||||
| 126 | num_comparison => "< <= > >= == !=", | ||||
| 127 | '3way_comparison'=> "<=> cmp", | ||||
| 128 | str_comparison => "lt le gt ge eq ne", | ||||
| 129 | binary => '& &= | |= ^ ^=', | ||||
| 130 | unary => "neg ! ~", | ||||
| 131 | mutators => '++ --', | ||||
| 132 | func => "atan2 cos sin exp abs log sqrt int", | ||||
| 133 | conversion => 'bool "" 0+', | ||||
| 134 | iterators => '<>', | ||||
| 135 | dereferencing => '${} @{} %{} &{} *{}', | ||||
| 136 | matching => '~~', | ||||
| 137 | special => 'nomethod fallback ='); | ||||
| 138 | |||||
| 139 | 3 | 338µs | 2 | 254µs | # spent 140µs (25+115) within overload::BEGIN@139 which was called
# once (25µs+115µs) by Class::MOP::Method::BEGIN@19 at line 139 # spent 140µs making 1 call to overload::BEGIN@139
# spent 114µs making 1 call to warnings::register::import |
| 140 | sub constant { | ||||
| 141 | # Arguments: what, sub | ||||
| 142 | while (@_) { | ||||
| 143 | if (@_ == 1) { | ||||
| 144 | warnings::warnif ("Odd number of arguments for overload::constant"); | ||||
| 145 | last; | ||||
| 146 | } | ||||
| 147 | elsif (!exists $constants {$_ [0]}) { | ||||
| 148 | warnings::warnif ("`$_[0]' is not an overloadable type"); | ||||
| 149 | } | ||||
| 150 | elsif (!ref $_ [1] || "$_[1]" !~ /(^|=)CODE\(0x[0-9a-f]+\)$/) { | ||||
| 151 | # Can't use C<ref $_[1] eq "CODE"> above as code references can be | ||||
| 152 | # blessed, and C<ref> would return the package the ref is blessed into. | ||||
| 153 | if (warnings::enabled) { | ||||
| 154 | $_ [1] = "undef" unless defined $_ [1]; | ||||
| 155 | warnings::warn ("`$_[1]' is not a code reference"); | ||||
| 156 | } | ||||
| 157 | } | ||||
| 158 | else { | ||||
| 159 | $^H{$_[0]} = $_[1]; | ||||
| 160 | $^H |= $constants{$_[0]}; | ||||
| 161 | } | ||||
| 162 | shift, shift; | ||||
| 163 | } | ||||
| 164 | } | ||||
| 165 | |||||
| 166 | sub remove_constant { | ||||
| 167 | # Arguments: what, sub | ||||
| 168 | while (@_) { | ||||
| 169 | delete $^H{$_[0]}; | ||||
| 170 | $^H &= ~ $constants{$_[0]}; | ||||
| 171 | shift, shift; | ||||
| 172 | } | ||||
| 173 | } | ||||
| 174 | |||||
| 175 | 1 | 24µs | 1; | ||
| 176 | |||||
| 177 | __END__ | ||||
| 178 | |||||
| 179 | =head1 NAME | ||||
| 180 | |||||
| 181 | overload - Package for overloading Perl operations | ||||
| 182 | |||||
| 183 | =head1 SYNOPSIS | ||||
| 184 | |||||
| 185 | package SomeThing; | ||||
| 186 | |||||
| 187 | use overload | ||||
| 188 | '+' => \&myadd, | ||||
| 189 | '-' => \&mysub; | ||||
| 190 | # etc | ||||
| 191 | ... | ||||
| 192 | |||||
| 193 | package main; | ||||
| 194 | $a = SomeThing->new( 57 ); | ||||
| 195 | $b=5+$a; | ||||
| 196 | ... | ||||
| 197 | if (overload::Overloaded $b) {...} | ||||
| 198 | ... | ||||
| 199 | $strval = overload::StrVal $b; | ||||
| 200 | |||||
| 201 | =head1 DESCRIPTION | ||||
| 202 | |||||
| 203 | This pragma allows overloading of Perl's operators for a class. | ||||
| 204 | To overload built-in functions, see L<perlsub/Overriding Built-in Functions> instead. | ||||
| 205 | |||||
| 206 | =head2 Declaration of overloaded functions | ||||
| 207 | |||||
| 208 | The compilation directive | ||||
| 209 | |||||
| 210 | package Number; | ||||
| 211 | use overload | ||||
| 212 | "+" => \&add, | ||||
| 213 | "*=" => "muas"; | ||||
| 214 | |||||
| 215 | declares function Number::add() for addition, and method muas() in | ||||
| 216 | the "class" C<Number> (or one of its base classes) | ||||
| 217 | for the assignment form C<*=> of multiplication. | ||||
| 218 | |||||
| 219 | Arguments of this directive come in (key, value) pairs. Legal values | ||||
| 220 | are values legal inside a C<&{ ... }> call, so the name of a | ||||
| 221 | subroutine, a reference to a subroutine, or an anonymous subroutine | ||||
| 222 | will all work. Note that values specified as strings are | ||||
| 223 | interpreted as methods, not subroutines. Legal keys are listed below. | ||||
| 224 | |||||
| 225 | The subroutine C<add> will be called to execute C<$a+$b> if $a | ||||
| 226 | is a reference to an object blessed into the package C<Number>, or if $a is | ||||
| 227 | not an object from a package with defined mathemagic addition, but $b is a | ||||
| 228 | reference to a C<Number>. It can also be called in other situations, like | ||||
| 229 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical | ||||
| 230 | methods refer to methods triggered by an overloaded mathematical | ||||
| 231 | operator.) | ||||
| 232 | |||||
| 233 | Since overloading respects inheritance via the @ISA hierarchy, the | ||||
| 234 | above declaration would also trigger overloading of C<+> and C<*=> in | ||||
| 235 | all the packages which inherit from C<Number>. | ||||
| 236 | |||||
| 237 | =head2 Calling Conventions for Binary Operations | ||||
| 238 | |||||
| 239 | The functions specified in the C<use overload ...> directive are called | ||||
| 240 | with three (in one particular case with four, see L<Last Resort>) | ||||
| 241 | arguments. If the corresponding operation is binary, then the first | ||||
| 242 | two arguments are the two arguments of the operation. However, due to | ||||
| 243 | general object calling conventions, the first argument should always be | ||||
| 244 | an object in the package, so in the situation of C<7+$a>, the | ||||
| 245 | order of the arguments is interchanged. It probably does not matter | ||||
| 246 | when implementing the addition method, but whether the arguments | ||||
| 247 | are reversed is vital to the subtraction method. The method can | ||||
| 248 | query this information by examining the third argument, which can take | ||||
| 249 | three different values: | ||||
| 250 | |||||
| 251 | =over 7 | ||||
| 252 | |||||
| 253 | =item FALSE | ||||
| 254 | |||||
| 255 | the order of arguments is as in the current operation. | ||||
| 256 | |||||
| 257 | =item TRUE | ||||
| 258 | |||||
| 259 | the arguments are reversed. | ||||
| 260 | |||||
| 261 | =item C<undef> | ||||
| 262 | |||||
| 263 | the current operation is an assignment variant (as in | ||||
| 264 | C<$a+=7>), but the usual function is called instead. This additional | ||||
| 265 | information can be used to generate some optimizations. Compare | ||||
| 266 | L<Calling Conventions for Mutators>. | ||||
| 267 | |||||
| 268 | =back | ||||
| 269 | |||||
| 270 | =head2 Calling Conventions for Unary Operations | ||||
| 271 | |||||
| 272 | Unary operation are considered binary operations with the second | ||||
| 273 | argument being C<undef>. Thus the functions that overloads C<{"++"}> | ||||
| 274 | is called with arguments C<($a,undef,'')> when $a++ is executed. | ||||
| 275 | |||||
| 276 | =head2 Calling Conventions for Mutators | ||||
| 277 | |||||
| 278 | Two types of mutators have different calling conventions: | ||||
| 279 | |||||
| 280 | =over | ||||
| 281 | |||||
| 282 | =item C<++> and C<--> | ||||
| 283 | |||||
| 284 | The routines which implement these operators are expected to actually | ||||
| 285 | I<mutate> their arguments. So, assuming that $obj is a reference to a | ||||
| 286 | number, | ||||
| 287 | |||||
| 288 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} | ||||
| 289 | |||||
| 290 | is an appropriate implementation of overloaded C<++>. Note that | ||||
| 291 | |||||
| 292 | sub incr { ++$ {$_[0]} ; shift } | ||||
| 293 | |||||
| 294 | is OK if used with preincrement and with postincrement. (In the case | ||||
| 295 | of postincrement a copying will be performed, see L<Copy Constructor>.) | ||||
| 296 | |||||
| 297 | =item C<x=> and other assignment versions | ||||
| 298 | |||||
| 299 | There is nothing special about these methods. They may change the | ||||
| 300 | value of their arguments, and may leave it as is. The result is going | ||||
| 301 | to be assigned to the value in the left-hand-side if different from | ||||
| 302 | this value. | ||||
| 303 | |||||
| 304 | This allows for the same method to be used as overloaded C<+=> and | ||||
| 305 | C<+>. Note that this is I<allowed>, but not recommended, since by the | ||||
| 306 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, | ||||
| 307 | if C<+=> is not overloaded. | ||||
| 308 | |||||
| 309 | =back | ||||
| 310 | |||||
| 311 | B<Warning.> Due to the presence of assignment versions of operations, | ||||
| 312 | routines which may be called in assignment context may create | ||||
| 313 | self-referential structures. Currently Perl will not free self-referential | ||||
| 314 | structures until cycles are C<explicitly> broken. You may get problems | ||||
| 315 | when traversing your structures too. | ||||
| 316 | |||||
| 317 | Say, | ||||
| 318 | |||||
| 319 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; | ||||
| 320 | |||||
| 321 | is asking for trouble, since for code C<$obj += $foo> the subroutine | ||||
| 322 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, | ||||
| 323 | \$foo]>. If using such a subroutine is an important optimization, one | ||||
| 324 | can overload C<+=> explicitly by a non-"optimized" version, or switch | ||||
| 325 | to non-optimized version if C<not defined $_[2]> (see | ||||
| 326 | L<Calling Conventions for Binary Operations>). | ||||
| 327 | |||||
| 328 | Even if no I<explicit> assignment-variants of operators are present in | ||||
| 329 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or | ||||
| 330 | C<',' . $obj . ','> may be both optimized to | ||||
| 331 | |||||
| 332 | my $tmp = ',' . $obj; $tmp .= ','; | ||||
| 333 | |||||
| 334 | =head2 Overloadable Operations | ||||
| 335 | |||||
| 336 | The following symbols can be specified in C<use overload> directive: | ||||
| 337 | |||||
| 338 | =over 5 | ||||
| 339 | |||||
| 340 | =item * I<Arithmetic operations> | ||||
| 341 | |||||
| 342 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", | ||||
| 343 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", | ||||
| 344 | |||||
| 345 | For these operations a substituted non-assignment variant can be called if | ||||
| 346 | the assignment variant is not available. Methods for operations C<+>, | ||||
| 347 | C<->, C<+=>, and C<-=> can be called to automatically generate | ||||
| 348 | increment and decrement methods. The operation C<-> can be used to | ||||
| 349 | autogenerate missing methods for unary minus or C<abs>. | ||||
| 350 | |||||
| 351 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and | ||||
| 352 | L<"Calling Conventions for Binary Operations">) for details of these | ||||
| 353 | substitutions. | ||||
| 354 | |||||
| 355 | =item * I<Comparison operations> | ||||
| 356 | |||||
| 357 | "<", "<=", ">", ">=", "==", "!=", "<=>", | ||||
| 358 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", | ||||
| 359 | |||||
| 360 | If the corresponding "spaceship" variant is available, it can be | ||||
| 361 | used to substitute for the missing operation. During C<sort>ing | ||||
| 362 | arrays, C<cmp> is used to compare values subject to C<use overload>. | ||||
| 363 | |||||
| 364 | =item * I<Bit operations> | ||||
| 365 | |||||
| 366 | "&", "&=", "^", "^=", "|", "|=", "neg", "!", "~", | ||||
| 367 | |||||
| 368 | C<neg> stands for unary minus. If the method for C<neg> is not | ||||
| 369 | specified, it can be autogenerated using the method for | ||||
| 370 | subtraction. If the method for C<!> is not specified, it can be | ||||
| 371 | autogenerated using the methods for C<bool>, or C<"">, or C<0+>. | ||||
| 372 | |||||
| 373 | The same remarks in L<"Arithmetic operations"> about | ||||
| 374 | assignment-variants and autogeneration apply for | ||||
| 375 | bit operations C<"&">, C<"^">, and C<"|"> as well. | ||||
| 376 | |||||
| 377 | =item * I<Increment and decrement> | ||||
| 378 | |||||
| 379 | "++", "--", | ||||
| 380 | |||||
| 381 | If undefined, addition and subtraction methods can be | ||||
| 382 | used instead. These operations are called both in prefix and | ||||
| 383 | postfix form. | ||||
| 384 | |||||
| 385 | =item * I<Transcendental functions> | ||||
| 386 | |||||
| 387 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", "int" | ||||
| 388 | |||||
| 389 | If C<abs> is unavailable, it can be autogenerated using methods | ||||
| 390 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. | ||||
| 391 | |||||
| 392 | Note that traditionally the Perl function L<int> rounds to 0, thus for | ||||
| 393 | floating-point-like types one should follow the same semantic. If | ||||
| 394 | C<int> is unavailable, it can be autogenerated using the overloading of | ||||
| 395 | C<0+>. | ||||
| 396 | |||||
| 397 | =item * I<Boolean, string and numeric conversion> | ||||
| 398 | |||||
| 399 | 'bool', '""', '0+', | ||||
| 400 | |||||
| 401 | If one or two of these operations are not overloaded, the remaining ones can | ||||
| 402 | be used instead. C<bool> is used in the flow control operators | ||||
| 403 | (like C<while>) and for the ternary C<?:> operation. These functions can | ||||
| 404 | return any arbitrary Perl value. If the corresponding operation for this value | ||||
| 405 | is overloaded too, that operation will be called again with this value. | ||||
| 406 | |||||
| 407 | As a special case if the overload returns the object itself then it will | ||||
| 408 | be used directly. An overloaded conversion returning the object is | ||||
| 409 | probably a bug, because you're likely to get something that looks like | ||||
| 410 | C<YourPackage=HASH(0x8172b34)>. | ||||
| 411 | |||||
| 412 | =item * I<Iteration> | ||||
| 413 | |||||
| 414 | "<>" | ||||
| 415 | |||||
| 416 | If not overloaded, the argument will be converted to a filehandle or | ||||
| 417 | glob (which may require a stringification). The same overloading | ||||
| 418 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and | ||||
| 419 | I<globbing> syntax C<E<lt>${var}E<gt>>. | ||||
| 420 | |||||
| 421 | B<BUGS> Even in list context, the iterator is currently called only | ||||
| 422 | once and with scalar context. | ||||
| 423 | |||||
| 424 | =item * I<Matching> | ||||
| 425 | |||||
| 426 | The key C<"~~"> allows you to override the smart matching logic used by | ||||
| 427 | the C<~~> operator and the switch construct (C<given>/C<when>). See | ||||
| 428 | L<perlsyn/switch> and L<feature>. | ||||
| 429 | |||||
| 430 | Unusually, overloading of the smart match operator does not automatically | ||||
| 431 | take precedence over normal smart match behaviour. In particular, in the | ||||
| 432 | following code: | ||||
| 433 | |||||
| 434 | package Foo; | ||||
| 435 | use overload '~~' => 'match'; | ||||
| 436 | |||||
| 437 | my $obj = Foo->new(); | ||||
| 438 | $obj ~~ [ 1,2,3 ]; | ||||
| 439 | |||||
| 440 | the smart match does I<not> invoke the method call like this: | ||||
| 441 | |||||
| 442 | $obj->match([1,2,3],0); | ||||
| 443 | |||||
| 444 | rather, the smart match distributive rule takes precedence, so $obj is | ||||
| 445 | smart matched against each array element in turn until a match is found, | ||||
| 446 | so you may see between one and three of these calls instead: | ||||
| 447 | |||||
| 448 | $obj->match(1,0); | ||||
| 449 | $obj->match(2,0); | ||||
| 450 | $obj->match(3,0); | ||||
| 451 | |||||
| 452 | Consult the match table in L<perlsyn/"Smart matching in detail"> for | ||||
| 453 | details of when overloading is invoked. | ||||
| 454 | |||||
| 455 | =item * I<Dereferencing> | ||||
| 456 | |||||
| 457 | '${}', '@{}', '%{}', '&{}', '*{}'. | ||||
| 458 | |||||
| 459 | If not overloaded, the argument will be dereferenced I<as is>, thus | ||||
| 460 | should be of correct type. These functions should return a reference | ||||
| 461 | of correct type, or another object with overloaded dereferencing. | ||||
| 462 | |||||
| 463 | As a special case if the overload returns the object itself then it | ||||
| 464 | will be used directly (provided it is the correct type). | ||||
| 465 | |||||
| 466 | The dereference operators must be specified explicitly they will not be passed to | ||||
| 467 | "nomethod". | ||||
| 468 | |||||
| 469 | =item * I<Special> | ||||
| 470 | |||||
| 471 | "nomethod", "fallback", "=". | ||||
| 472 | |||||
| 473 | see L<SPECIAL SYMBOLS FOR C<use overload>>. | ||||
| 474 | |||||
| 475 | =back | ||||
| 476 | |||||
| 477 | See L<"Fallback"> for an explanation of when a missing method can be | ||||
| 478 | autogenerated. | ||||
| 479 | |||||
| 480 | A computer-readable form of the above table is available in the hash | ||||
| 481 | %overload::ops, with values being space-separated lists of names: | ||||
| 482 | |||||
| 483 | with_assign => '+ - * / % ** << >> x .', | ||||
| 484 | assign => '+= -= *= /= %= **= <<= >>= x= .=', | ||||
| 485 | num_comparison => '< <= > >= == !=', | ||||
| 486 | '3way_comparison'=> '<=> cmp', | ||||
| 487 | str_comparison => 'lt le gt ge eq ne', | ||||
| 488 | binary => '& &= | |= ^ ^=', | ||||
| 489 | unary => 'neg ! ~', | ||||
| 490 | mutators => '++ --', | ||||
| 491 | func => 'atan2 cos sin exp abs log sqrt', | ||||
| 492 | conversion => 'bool "" 0+', | ||||
| 493 | iterators => '<>', | ||||
| 494 | dereferencing => '${} @{} %{} &{} *{}', | ||||
| 495 | matching => '~~', | ||||
| 496 | special => 'nomethod fallback =' | ||||
| 497 | |||||
| 498 | =head2 Inheritance and overloading | ||||
| 499 | |||||
| 500 | Inheritance interacts with overloading in two ways. | ||||
| 501 | |||||
| 502 | =over | ||||
| 503 | |||||
| 504 | =item Strings as values of C<use overload> directive | ||||
| 505 | |||||
| 506 | If C<value> in | ||||
| 507 | |||||
| 508 | use overload key => value; | ||||
| 509 | |||||
| 510 | is a string, it is interpreted as a method name. | ||||
| 511 | |||||
| 512 | =item Overloading of an operation is inherited by derived classes | ||||
| 513 | |||||
| 514 | Any class derived from an overloaded class is also overloaded. The | ||||
| 515 | set of overloaded methods is the union of overloaded methods of all | ||||
| 516 | the ancestors. If some method is overloaded in several ancestor, then | ||||
| 517 | which description will be used is decided by the usual inheritance | ||||
| 518 | rules: | ||||
| 519 | |||||
| 520 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads | ||||
| 521 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, | ||||
| 522 | then the subroutine C<D::plus_sub> will be called to implement | ||||
| 523 | operation C<+> for an object in package C<A>. | ||||
| 524 | |||||
| 525 | =back | ||||
| 526 | |||||
| 527 | Note that since the value of the C<fallback> key is not a subroutine, | ||||
| 528 | its inheritance is not governed by the above rules. In the current | ||||
| 529 | implementation, the value of C<fallback> in the first overloaded | ||||
| 530 | ancestor is used, but this is accidental and subject to change. | ||||
| 531 | |||||
| 532 | =head1 SPECIAL SYMBOLS FOR C<use overload> | ||||
| 533 | |||||
| 534 | Three keys are recognized by Perl that are not covered by the above | ||||
| 535 | description. | ||||
| 536 | |||||
| 537 | =head2 Last Resort | ||||
| 538 | |||||
| 539 | C<"nomethod"> should be followed by a reference to a function of four | ||||
| 540 | parameters. If defined, it is called when the overloading mechanism | ||||
| 541 | cannot find a method for some operation. The first three arguments of | ||||
| 542 | this function coincide with the arguments for the corresponding method if | ||||
| 543 | it were found, the fourth argument is the symbol | ||||
| 544 | corresponding to the missing method. If several methods are tried, | ||||
| 545 | the last one is used. Say, C<1-$a> can be equivalent to | ||||
| 546 | |||||
| 547 | &nomethodMethod($a,1,1,"-") | ||||
| 548 | |||||
| 549 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the | ||||
| 550 | C<use overload> directive. | ||||
| 551 | |||||
| 552 | The C<"nomethod"> mechanism is I<not> used for the dereference operators | ||||
| 553 | ( ${} @{} %{} &{} *{} ). | ||||
| 554 | |||||
| 555 | |||||
| 556 | If some operation cannot be resolved, and there is no function | ||||
| 557 | assigned to C<"nomethod">, then an exception will be raised via die()-- | ||||
| 558 | unless C<"fallback"> was specified as a key in C<use overload> directive. | ||||
| 559 | |||||
| 560 | |||||
| 561 | =head2 Fallback | ||||
| 562 | |||||
| 563 | The key C<"fallback"> governs what to do if a method for a particular | ||||
| 564 | operation is not found. Three different cases are possible depending on | ||||
| 565 | the value of C<"fallback">: | ||||
| 566 | |||||
| 567 | =over 16 | ||||
| 568 | |||||
| 569 | =item * C<undef> | ||||
| 570 | |||||
| 571 | Perl tries to use a | ||||
| 572 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it | ||||
| 573 | then tries to calls C<"nomethod"> value; if missing, an exception | ||||
| 574 | will be raised. | ||||
| 575 | |||||
| 576 | =item * TRUE | ||||
| 577 | |||||
| 578 | The same as for the C<undef> value, but no exception is raised. Instead, | ||||
| 579 | it silently reverts to what it would have done were there no C<use overload> | ||||
| 580 | present. | ||||
| 581 | |||||
| 582 | =item * defined, but FALSE | ||||
| 583 | |||||
| 584 | No autogeneration is tried. Perl tries to call | ||||
| 585 | C<"nomethod"> value, and if this is missing, raises an exception. | ||||
| 586 | |||||
| 587 | =back | ||||
| 588 | |||||
| 589 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone | ||||
| 590 | yet, see L<"Inheritance and overloading">. | ||||
| 591 | |||||
| 592 | =head2 Copy Constructor | ||||
| 593 | |||||
| 594 | The value for C<"="> is a reference to a function with three | ||||
| 595 | arguments, i.e., it looks like the other values in C<use | ||||
| 596 | overload>. However, it does not overload the Perl assignment | ||||
| 597 | operator. This would go against Camel hair. | ||||
| 598 | |||||
| 599 | This operation is called in the situations when a mutator is applied | ||||
| 600 | to a reference that shares its object with some other reference, such | ||||
| 601 | as | ||||
| 602 | |||||
| 603 | $a=$b; | ||||
| 604 | ++$a; | ||||
| 605 | |||||
| 606 | To make this change $a and not change $b, a copy of C<$$a> is made, | ||||
| 607 | and $a is assigned a reference to this new object. This operation is | ||||
| 608 | done during execution of the C<++$a>, and not during the assignment, | ||||
| 609 | (so before the increment C<$$a> coincides with C<$$b>). This is only | ||||
| 610 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or | ||||
| 611 | C<nomethod>). Note that if this operation is expressed via C<'+'> | ||||
| 612 | a nonmutator, i.e., as in | ||||
| 613 | |||||
| 614 | $a=$b; | ||||
| 615 | $a=$a+1; | ||||
| 616 | |||||
| 617 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not | ||||
| 618 | appear as lvalue when the above code is executed. | ||||
| 619 | |||||
| 620 | If the copy constructor is required during the execution of some mutator, | ||||
| 621 | but a method for C<'='> was not specified, it can be autogenerated as a | ||||
| 622 | string copy if the object is a plain scalar or a simple assignment if it | ||||
| 623 | is not. | ||||
| 624 | |||||
| 625 | =over 5 | ||||
| 626 | |||||
| 627 | =item B<Example> | ||||
| 628 | |||||
| 629 | The actually executed code for | ||||
| 630 | |||||
| 631 | $a=$b; | ||||
| 632 | Something else which does not modify $a or $b.... | ||||
| 633 | ++$a; | ||||
| 634 | |||||
| 635 | may be | ||||
| 636 | |||||
| 637 | $a=$b; | ||||
| 638 | Something else which does not modify $a or $b.... | ||||
| 639 | $a = $a->clone(undef,""); | ||||
| 640 | $a->incr(undef,""); | ||||
| 641 | |||||
| 642 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, | ||||
| 643 | C<'='> was overloaded with C<\&clone>. | ||||
| 644 | |||||
| 645 | =back | ||||
| 646 | |||||
| 647 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for | ||||
| 648 | C<$b = $a; ++$a>. | ||||
| 649 | |||||
| 650 | =head1 MAGIC AUTOGENERATION | ||||
| 651 | |||||
| 652 | If a method for an operation is not found, and the value for C<"fallback"> is | ||||
| 653 | TRUE or undefined, Perl tries to autogenerate a substitute method for | ||||
| 654 | the missing operation based on the defined operations. Autogenerated method | ||||
| 655 | substitutions are possible for the following operations: | ||||
| 656 | |||||
| 657 | =over 16 | ||||
| 658 | |||||
| 659 | =item I<Assignment forms of arithmetic operations> | ||||
| 660 | |||||
| 661 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> | ||||
| 662 | is not defined. | ||||
| 663 | |||||
| 664 | =item I<Conversion operations> | ||||
| 665 | |||||
| 666 | String, numeric, and boolean conversion are calculated in terms of one | ||||
| 667 | another if not all of them are defined. | ||||
| 668 | |||||
| 669 | =item I<Increment and decrement> | ||||
| 670 | |||||
| 671 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, | ||||
| 672 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. | ||||
| 673 | |||||
| 674 | =item C<abs($a)> | ||||
| 675 | |||||
| 676 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). | ||||
| 677 | |||||
| 678 | =item I<Unary minus> | ||||
| 679 | |||||
| 680 | can be expressed in terms of subtraction. | ||||
| 681 | |||||
| 682 | =item I<Negation> | ||||
| 683 | |||||
| 684 | C<!> and C<not> can be expressed in terms of boolean conversion, or | ||||
| 685 | string or numerical conversion. | ||||
| 686 | |||||
| 687 | =item I<Concatenation> | ||||
| 688 | |||||
| 689 | can be expressed in terms of string conversion. | ||||
| 690 | |||||
| 691 | =item I<Comparison operations> | ||||
| 692 | |||||
| 693 | can be expressed in terms of its "spaceship" counterpart: either | ||||
| 694 | C<E<lt>=E<gt>> or C<cmp>: | ||||
| 695 | |||||
| 696 | <, >, <=, >=, ==, != in terms of <=> | ||||
| 697 | lt, gt, le, ge, eq, ne in terms of cmp | ||||
| 698 | |||||
| 699 | =item I<Iterator> | ||||
| 700 | |||||
| 701 | <> in terms of builtin operations | ||||
| 702 | |||||
| 703 | =item I<Dereferencing> | ||||
| 704 | |||||
| 705 | ${} @{} %{} &{} *{} in terms of builtin operations | ||||
| 706 | |||||
| 707 | =item I<Copy operator> | ||||
| 708 | |||||
| 709 | can be expressed in terms of an assignment to the dereferenced value, if this | ||||
| 710 | value is a scalar and not a reference, or simply a reference assignment | ||||
| 711 | otherwise. | ||||
| 712 | |||||
| 713 | =back | ||||
| 714 | |||||
| 715 | =head1 Minimal set of overloaded operations | ||||
| 716 | |||||
| 717 | Since some operations can be automatically generated from others, there is | ||||
| 718 | a minimal set of operations that need to be overloaded in order to have | ||||
| 719 | the complete set of overloaded operations at one's disposal. | ||||
| 720 | Of course, the autogenerated operations may not do exactly what the user | ||||
| 721 | expects. See L<MAGIC AUTOGENERATION> above. The minimal set is: | ||||
| 722 | |||||
| 723 | + - * / % ** << >> x | ||||
| 724 | <=> cmp | ||||
| 725 | & | ^ ~ | ||||
| 726 | atan2 cos sin exp log sqrt int | ||||
| 727 | |||||
| 728 | Additionally, you need to define at least one of string, boolean or | ||||
| 729 | numeric conversions because any one can be used to emulate the others. | ||||
| 730 | The string conversion can also be used to emulate concatenation. | ||||
| 731 | |||||
| 732 | =head1 Losing overloading | ||||
| 733 | |||||
| 734 | The restriction for the comparison operation is that even if, for example, | ||||
| 735 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' | ||||
| 736 | function will produce only a standard logical value based on the | ||||
| 737 | numerical value of the result of `C<cmp>'. In particular, a working | ||||
| 738 | numeric conversion is needed in this case (possibly expressed in terms of | ||||
| 739 | other conversions). | ||||
| 740 | |||||
| 741 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties | ||||
| 742 | if the string conversion substitution is applied. | ||||
| 743 | |||||
| 744 | When you chop() a mathemagical object it is promoted to a string and its | ||||
| 745 | mathemagical properties are lost. The same can happen with other | ||||
| 746 | operations as well. | ||||
| 747 | |||||
| 748 | =head1 Run-time Overloading | ||||
| 749 | |||||
| 750 | Since all C<use> directives are executed at compile-time, the only way to | ||||
| 751 | change overloading during run-time is to | ||||
| 752 | |||||
| 753 | eval 'use overload "+" => \&addmethod'; | ||||
| 754 | |||||
| 755 | You can also use | ||||
| 756 | |||||
| 757 | eval 'no overload "+", "--", "<="'; | ||||
| 758 | |||||
| 759 | though the use of these constructs during run-time is questionable. | ||||
| 760 | |||||
| 761 | =head1 Public functions | ||||
| 762 | |||||
| 763 | Package C<overload.pm> provides the following public functions: | ||||
| 764 | |||||
| 765 | =over 5 | ||||
| 766 | |||||
| 767 | =item overload::StrVal(arg) | ||||
| 768 | |||||
| 769 | Gives string value of C<arg> as in absence of stringify overloading. If you | ||||
| 770 | are using this to get the address of a reference (useful for checking if two | ||||
| 771 | references point to the same thing) then you may be better off using | ||||
| 772 | C<Scalar::Util::refaddr()>, which is faster. | ||||
| 773 | |||||
| 774 | =item overload::Overloaded(arg) | ||||
| 775 | |||||
| 776 | Returns true if C<arg> is subject to overloading of some operations. | ||||
| 777 | |||||
| 778 | =item overload::Method(obj,op) | ||||
| 779 | |||||
| 780 | Returns C<undef> or a reference to the method that implements C<op>. | ||||
| 781 | |||||
| 782 | =back | ||||
| 783 | |||||
| 784 | =head1 Overloading constants | ||||
| 785 | |||||
| 786 | For some applications, the Perl parser mangles constants too much. | ||||
| 787 | It is possible to hook into this process via C<overload::constant()> | ||||
| 788 | and C<overload::remove_constant()> functions. | ||||
| 789 | |||||
| 790 | These functions take a hash as an argument. The recognized keys of this hash | ||||
| 791 | are: | ||||
| 792 | |||||
| 793 | =over 8 | ||||
| 794 | |||||
| 795 | =item integer | ||||
| 796 | |||||
| 797 | to overload integer constants, | ||||
| 798 | |||||
| 799 | =item float | ||||
| 800 | |||||
| 801 | to overload floating point constants, | ||||
| 802 | |||||
| 803 | =item binary | ||||
| 804 | |||||
| 805 | to overload octal and hexadecimal constants, | ||||
| 806 | |||||
| 807 | =item q | ||||
| 808 | |||||
| 809 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted | ||||
| 810 | strings and here-documents, | ||||
| 811 | |||||
| 812 | =item qr | ||||
| 813 | |||||
| 814 | to overload constant pieces of regular expressions. | ||||
| 815 | |||||
| 816 | =back | ||||
| 817 | |||||
| 818 | The corresponding values are references to functions which take three arguments: | ||||
| 819 | the first one is the I<initial> string form of the constant, the second one | ||||
| 820 | is how Perl interprets this constant, the third one is how the constant is used. | ||||
| 821 | Note that the initial string form does not | ||||
| 822 | contain string delimiters, and has backslashes in backslash-delimiter | ||||
| 823 | combinations stripped (thus the value of delimiter is not relevant for | ||||
| 824 | processing of this string). The return value of this function is how this | ||||
| 825 | constant is going to be interpreted by Perl. The third argument is undefined | ||||
| 826 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote | ||||
| 827 | context (comes from strings, regular expressions, and single-quote HERE | ||||
| 828 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, | ||||
| 829 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. | ||||
| 830 | |||||
| 831 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, | ||||
| 832 | it is expected that overloaded constant strings are equipped with reasonable | ||||
| 833 | overloaded catenation operator, otherwise absurd results will result. | ||||
| 834 | Similarly, negative numbers are considered as negations of positive constants. | ||||
| 835 | |||||
| 836 | Note that it is probably meaningless to call the functions overload::constant() | ||||
| 837 | and overload::remove_constant() from anywhere but import() and unimport() methods. | ||||
| 838 | From these methods they may be called as | ||||
| 839 | |||||
| 840 | sub import { | ||||
| 841 | shift; | ||||
| 842 | return unless @_; | ||||
| 843 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; | ||||
| 844 | overload::constant integer => sub {Math::BigInt->new(shift)}; | ||||
| 845 | } | ||||
| 846 | |||||
| 847 | =head1 IMPLEMENTATION | ||||
| 848 | |||||
| 849 | What follows is subject to change RSN. | ||||
| 850 | |||||
| 851 | The table of methods for all operations is cached in magic for the | ||||
| 852 | symbol table hash for the package. The cache is invalidated during | ||||
| 853 | processing of C<use overload>, C<no overload>, new function | ||||
| 854 | definitions, and changes in @ISA. However, this invalidation remains | ||||
| 855 | unprocessed until the next C<bless>ing into the package. Hence if you | ||||
| 856 | want to change overloading structure dynamically, you'll need an | ||||
| 857 | additional (fake) C<bless>ing to update the table. | ||||
| 858 | |||||
| 859 | (Every SVish thing has a magic queue, and magic is an entry in that | ||||
| 860 | queue. This is how a single variable may participate in multiple | ||||
| 861 | forms of magic simultaneously. For instance, environment variables | ||||
| 862 | regularly have two forms at once: their %ENV magic and their taint | ||||
| 863 | magic. However, the magic which implements overloading is applied to | ||||
| 864 | the stashes, which are rarely used directly, thus should not slow down | ||||
| 865 | Perl.) | ||||
| 866 | |||||
| 867 | If an object belongs to a package using overload, it carries a special | ||||
| 868 | flag. Thus the only speed penalty during arithmetic operations without | ||||
| 869 | overloading is the checking of this flag. | ||||
| 870 | |||||
| 871 | In fact, if C<use overload> is not present, there is almost no overhead | ||||
| 872 | for overloadable operations, so most programs should not suffer | ||||
| 873 | measurable performance penalties. A considerable effort was made to | ||||
| 874 | minimize the overhead when overload is used in some package, but the | ||||
| 875 | arguments in question do not belong to packages using overload. When | ||||
| 876 | in doubt, test your speed with C<use overload> and without it. So far | ||||
| 877 | there have been no reports of substantial speed degradation if Perl is | ||||
| 878 | compiled with optimization turned on. | ||||
| 879 | |||||
| 880 | There is no size penalty for data if overload is not used. The only | ||||
| 881 | size penalty if overload is used in some package is that I<all> the | ||||
| 882 | packages acquire a magic during the next C<bless>ing into the | ||||
| 883 | package. This magic is three-words-long for packages without | ||||
| 884 | overloading, and carries the cache table if the package is overloaded. | ||||
| 885 | |||||
| 886 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is | ||||
| 887 | carried out before any operation that can imply an assignment to the | ||||
| 888 | object $a (or $b) refers to, like C<$a++>. You can override this | ||||
| 889 | behavior by defining your own copy constructor (see L<"Copy Constructor">). | ||||
| 890 | |||||
| 891 | It is expected that arguments to methods that are not explicitly supposed | ||||
| 892 | to be changed are constant (but this is not enforced). | ||||
| 893 | |||||
| 894 | =head1 Metaphor clash | ||||
| 895 | |||||
| 896 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. | ||||
| 897 | If it I<looks> counter intuitive to you, you are subject to a metaphor | ||||
| 898 | clash. | ||||
| 899 | |||||
| 900 | Here is a Perl object metaphor: | ||||
| 901 | |||||
| 902 | I< object is a reference to blessed data> | ||||
| 903 | |||||
| 904 | and an arithmetic metaphor: | ||||
| 905 | |||||
| 906 | I< object is a thing by itself>. | ||||
| 907 | |||||
| 908 | The I<main> problem of overloading C<=> is the fact that these metaphors | ||||
| 909 | imply different actions on the assignment C<$a = $b> if $a and $b are | ||||
| 910 | objects. Perl-think implies that $a becomes a reference to whatever | ||||
| 911 | $b was referencing. Arithmetic-think implies that the value of "object" | ||||
| 912 | $a is changed to become the value of the object $b, preserving the fact | ||||
| 913 | that $a and $b are separate entities. | ||||
| 914 | |||||
| 915 | The difference is not relevant in the absence of mutators. After | ||||
| 916 | a Perl-way assignment an operation which mutates the data referenced by $a | ||||
| 917 | would change the data referenced by $b too. Effectively, after | ||||
| 918 | C<$a = $b> values of $a and $b become I<indistinguishable>. | ||||
| 919 | |||||
| 920 | On the other hand, anyone who has used algebraic notation knows the | ||||
| 921 | expressive power of the arithmetic metaphor. Overloading works hard | ||||
| 922 | to enable this metaphor while preserving the Perlian way as far as | ||||
| 923 | possible. Since it is not possible to freely mix two contradicting | ||||
| 924 | metaphors, overloading allows the arithmetic way to write things I<as | ||||
| 925 | far as all the mutators are called via overloaded access only>. The | ||||
| 926 | way it is done is described in L<Copy Constructor>. | ||||
| 927 | |||||
| 928 | If some mutator methods are directly applied to the overloaded values, | ||||
| 929 | one may need to I<explicitly unlink> other values which references the | ||||
| 930 | same value: | ||||
| 931 | |||||
| 932 | $a = Data->new(23); | ||||
| 933 | ... | ||||
| 934 | $b = $a; # $b is "linked" to $a | ||||
| 935 | ... | ||||
| 936 | $a = $a->clone; # Unlink $b from $a | ||||
| 937 | $a->increment_by(4); | ||||
| 938 | |||||
| 939 | Note that overloaded access makes this transparent: | ||||
| 940 | |||||
| 941 | $a = Data->new(23); | ||||
| 942 | $b = $a; # $b is "linked" to $a | ||||
| 943 | $a += 4; # would unlink $b automagically | ||||
| 944 | |||||
| 945 | However, it would not make | ||||
| 946 | |||||
| 947 | $a = Data->new(23); | ||||
| 948 | $a = 4; # Now $a is a plain 4, not 'Data' | ||||
| 949 | |||||
| 950 | preserve "objectness" of $a. But Perl I<has> a way to make assignments | ||||
| 951 | to an object do whatever you want. It is just not the overload, but | ||||
| 952 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method | ||||
| 953 | which returns the object itself, and STORE() method which changes the | ||||
| 954 | value of the object, one can reproduce the arithmetic metaphor in its | ||||
| 955 | completeness, at least for variables which were tie()d from the start. | ||||
| 956 | |||||
| 957 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) | ||||
| 958 | |||||
| 959 | =head1 Cookbook | ||||
| 960 | |||||
| 961 | Please add examples to what follows! | ||||
| 962 | |||||
| 963 | =head2 Two-face scalars | ||||
| 964 | |||||
| 965 | Put this in F<two_face.pm> in your Perl library directory: | ||||
| 966 | |||||
| 967 | package two_face; # Scalars with separate string and | ||||
| 968 | # numeric values. | ||||
| 969 | sub new { my $p = shift; bless [@_], $p } | ||||
| 970 | use overload '""' => \&str, '0+' => \&num, fallback => 1; | ||||
| 971 | sub num {shift->[1]} | ||||
| 972 | sub str {shift->[0]} | ||||
| 973 | |||||
| 974 | Use it as follows: | ||||
| 975 | |||||
| 976 | require two_face; | ||||
| 977 | my $seven = two_face->new("vii", 7); | ||||
| 978 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; | ||||
| 979 | print "seven contains `i'\n" if $seven =~ /i/; | ||||
| 980 | |||||
| 981 | (The second line creates a scalar which has both a string value, and a | ||||
| 982 | numeric value.) This prints: | ||||
| 983 | |||||
| 984 | seven=vii, seven=7, eight=8 | ||||
| 985 | seven contains `i' | ||||
| 986 | |||||
| 987 | =head2 Two-face references | ||||
| 988 | |||||
| 989 | Suppose you want to create an object which is accessible as both an | ||||
| 990 | array reference and a hash reference. | ||||
| 991 | |||||
| 992 | package two_refs; | ||||
| 993 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; | ||||
| 994 | sub new { | ||||
| 995 | my $p = shift; | ||||
| 996 | bless \ [@_], $p; | ||||
| 997 | } | ||||
| 998 | sub gethash { | ||||
| 999 | my %h; | ||||
| 1000 | my $self = shift; | ||||
| 1001 | tie %h, ref $self, $self; | ||||
| 1002 | \%h; | ||||
| 1003 | } | ||||
| 1004 | |||||
| 1005 | sub TIEHASH { my $p = shift; bless \ shift, $p } | ||||
| 1006 | my %fields; | ||||
| 1007 | my $i = 0; | ||||
| 1008 | $fields{$_} = $i++ foreach qw{zero one two three}; | ||||
| 1009 | sub STORE { | ||||
| 1010 | my $self = ${shift()}; | ||||
| 1011 | my $key = $fields{shift()}; | ||||
| 1012 | defined $key or die "Out of band access"; | ||||
| 1013 | $$self->[$key] = shift; | ||||
| 1014 | } | ||||
| 1015 | sub FETCH { | ||||
| 1016 | my $self = ${shift()}; | ||||
| 1017 | my $key = $fields{shift()}; | ||||
| 1018 | defined $key or die "Out of band access"; | ||||
| 1019 | $$self->[$key]; | ||||
| 1020 | } | ||||
| 1021 | |||||
| 1022 | Now one can access an object using both the array and hash syntax: | ||||
| 1023 | |||||
| 1024 | my $bar = two_refs->new(3,4,5,6); | ||||
| 1025 | $bar->[2] = 11; | ||||
| 1026 | $bar->{two} == 11 or die 'bad hash fetch'; | ||||
| 1027 | |||||
| 1028 | Note several important features of this example. First of all, the | ||||
| 1029 | I<actual> type of $bar is a scalar reference, and we do not overload | ||||
| 1030 | the scalar dereference. Thus we can get the I<actual> non-overloaded | ||||
| 1031 | contents of $bar by just using C<$$bar> (what we do in functions which | ||||
| 1032 | overload dereference). Similarly, the object returned by the | ||||
| 1033 | TIEHASH() method is a scalar reference. | ||||
| 1034 | |||||
| 1035 | Second, we create a new tied hash each time the hash syntax is used. | ||||
| 1036 | This allows us not to worry about a possibility of a reference loop, | ||||
| 1037 | which would lead to a memory leak. | ||||
| 1038 | |||||
| 1039 | Both these problems can be cured. Say, if we want to overload hash | ||||
| 1040 | dereference on a reference to an object which is I<implemented> as a | ||||
| 1041 | hash itself, the only problem one has to circumvent is how to access | ||||
| 1042 | this I<actual> hash (as opposed to the I<virtual> hash exhibited by the | ||||
| 1043 | overloaded dereference operator). Here is one possible fetching routine: | ||||
| 1044 | |||||
| 1045 | sub access_hash { | ||||
| 1046 | my ($self, $key) = (shift, shift); | ||||
| 1047 | my $class = ref $self; | ||||
| 1048 | bless $self, 'overload::dummy'; # Disable overloading of %{} | ||||
| 1049 | my $out = $self->{$key}; | ||||
| 1050 | bless $self, $class; # Restore overloading | ||||
| 1051 | $out; | ||||
| 1052 | } | ||||
| 1053 | |||||
| 1054 | To remove creation of the tied hash on each access, one may an extra | ||||
| 1055 | level of indirection which allows a non-circular structure of references: | ||||
| 1056 | |||||
| 1057 | package two_refs1; | ||||
| 1058 | use overload '%{}' => sub { ${shift()}->[1] }, | ||||
| 1059 | '@{}' => sub { ${shift()}->[0] }; | ||||
| 1060 | sub new { | ||||
| 1061 | my $p = shift; | ||||
| 1062 | my $a = [@_]; | ||||
| 1063 | my %h; | ||||
| 1064 | tie %h, $p, $a; | ||||
| 1065 | bless \ [$a, \%h], $p; | ||||
| 1066 | } | ||||
| 1067 | sub gethash { | ||||
| 1068 | my %h; | ||||
| 1069 | my $self = shift; | ||||
| 1070 | tie %h, ref $self, $self; | ||||
| 1071 | \%h; | ||||
| 1072 | } | ||||
| 1073 | |||||
| 1074 | sub TIEHASH { my $p = shift; bless \ shift, $p } | ||||
| 1075 | my %fields; | ||||
| 1076 | my $i = 0; | ||||
| 1077 | $fields{$_} = $i++ foreach qw{zero one two three}; | ||||
| 1078 | sub STORE { | ||||
| 1079 | my $a = ${shift()}; | ||||
| 1080 | my $key = $fields{shift()}; | ||||
| 1081 | defined $key or die "Out of band access"; | ||||
| 1082 | $a->[$key] = shift; | ||||
| 1083 | } | ||||
| 1084 | sub FETCH { | ||||
| 1085 | my $a = ${shift()}; | ||||
| 1086 | my $key = $fields{shift()}; | ||||
| 1087 | defined $key or die "Out of band access"; | ||||
| 1088 | $a->[$key]; | ||||
| 1089 | } | ||||
| 1090 | |||||
| 1091 | Now if $baz is overloaded like this, then C<$baz> is a reference to a | ||||
| 1092 | reference to the intermediate array, which keeps a reference to an | ||||
| 1093 | actual array, and the access hash. The tie()ing object for the access | ||||
| 1094 | hash is a reference to a reference to the actual array, so | ||||
| 1095 | |||||
| 1096 | =over | ||||
| 1097 | |||||
| 1098 | =item * | ||||
| 1099 | |||||
| 1100 | There are no loops of references. | ||||
| 1101 | |||||
| 1102 | =item * | ||||
| 1103 | |||||
| 1104 | Both "objects" which are blessed into the class C<two_refs1> are | ||||
| 1105 | references to a reference to an array, thus references to a I<scalar>. | ||||
| 1106 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no | ||||
| 1107 | overloaded operations. | ||||
| 1108 | |||||
| 1109 | =back | ||||
| 1110 | |||||
| 1111 | =head2 Symbolic calculator | ||||
| 1112 | |||||
| 1113 | Put this in F<symbolic.pm> in your Perl library directory: | ||||
| 1114 | |||||
| 1115 | package symbolic; # Primitive symbolic calculator | ||||
| 1116 | use overload nomethod => \&wrap; | ||||
| 1117 | |||||
| 1118 | sub new { shift; bless ['n', @_] } | ||||
| 1119 | sub wrap { | ||||
| 1120 | my ($obj, $other, $inv, $meth) = @_; | ||||
| 1121 | ($obj, $other) = ($other, $obj) if $inv; | ||||
| 1122 | bless [$meth, $obj, $other]; | ||||
| 1123 | } | ||||
| 1124 | |||||
| 1125 | This module is very unusual as overloaded modules go: it does not | ||||
| 1126 | provide any usual overloaded operators, instead it provides the L<Last | ||||
| 1127 | Resort> operator C<nomethod>. In this example the corresponding | ||||
| 1128 | subroutine returns an object which encapsulates operations done over | ||||
| 1129 | the objects: C<< symbolic->new(3) >> contains C<['n', 3]>, C<< 2 + | ||||
| 1130 | symbolic->new(3) >> contains C<['+', 2, ['n', 3]]>. | ||||
| 1131 | |||||
| 1132 | Here is an example of the script which "calculates" the side of | ||||
| 1133 | circumscribed octagon using the above package: | ||||
| 1134 | |||||
| 1135 | require symbolic; | ||||
| 1136 | my $iter = 1; # 2**($iter+2) = 8 | ||||
| 1137 | my $side = symbolic->new(1); | ||||
| 1138 | my $cnt = $iter; | ||||
| 1139 | |||||
| 1140 | while ($cnt--) { | ||||
| 1141 | $side = (sqrt(1 + $side**2) - 1)/$side; | ||||
| 1142 | } | ||||
| 1143 | print "OK\n"; | ||||
| 1144 | |||||
| 1145 | The value of $side is | ||||
| 1146 | |||||
| 1147 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], | ||||
| 1148 | undef], 1], ['n', 1]] | ||||
| 1149 | |||||
| 1150 | Note that while we obtained this value using a nice little script, | ||||
| 1151 | there is no simple way to I<use> this value. In fact this value may | ||||
| 1152 | be inspected in debugger (see L<perldebug>), but only if | ||||
| 1153 | C<bareStringify> B<O>ption is set, and not via C<p> command. | ||||
| 1154 | |||||
| 1155 | If one attempts to print this value, then the overloaded operator | ||||
| 1156 | C<""> will be called, which will call C<nomethod> operator. The | ||||
| 1157 | result of this operator will be stringified again, but this result is | ||||
| 1158 | again of type C<symbolic>, which will lead to an infinite loop. | ||||
| 1159 | |||||
| 1160 | Add a pretty-printer method to the module F<symbolic.pm>: | ||||
| 1161 | |||||
| 1162 | sub pretty { | ||||
| 1163 | my ($meth, $a, $b) = @{+shift}; | ||||
| 1164 | $a = 'u' unless defined $a; | ||||
| 1165 | $b = 'u' unless defined $b; | ||||
| 1166 | $a = $a->pretty if ref $a; | ||||
| 1167 | $b = $b->pretty if ref $b; | ||||
| 1168 | "[$meth $a $b]"; | ||||
| 1169 | } | ||||
| 1170 | |||||
| 1171 | Now one can finish the script by | ||||
| 1172 | |||||
| 1173 | print "side = ", $side->pretty, "\n"; | ||||
| 1174 | |||||
| 1175 | The method C<pretty> is doing object-to-string conversion, so it | ||||
| 1176 | is natural to overload the operator C<""> using this method. However, | ||||
| 1177 | inside such a method it is not necessary to pretty-print the | ||||
| 1178 | I<components> $a and $b of an object. In the above subroutine | ||||
| 1179 | C<"[$meth $a $b]"> is a catenation of some strings and components $a | ||||
| 1180 | and $b. If these components use overloading, the catenation operator | ||||
| 1181 | will look for an overloaded operator C<.>; if not present, it will | ||||
| 1182 | look for an overloaded operator C<"">. Thus it is enough to use | ||||
| 1183 | |||||
| 1184 | use overload nomethod => \&wrap, '""' => \&str; | ||||
| 1185 | sub str { | ||||
| 1186 | my ($meth, $a, $b) = @{+shift}; | ||||
| 1187 | $a = 'u' unless defined $a; | ||||
| 1188 | $b = 'u' unless defined $b; | ||||
| 1189 | "[$meth $a $b]"; | ||||
| 1190 | } | ||||
| 1191 | |||||
| 1192 | Now one can change the last line of the script to | ||||
| 1193 | |||||
| 1194 | print "side = $side\n"; | ||||
| 1195 | |||||
| 1196 | which outputs | ||||
| 1197 | |||||
| 1198 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] | ||||
| 1199 | |||||
| 1200 | and one can inspect the value in debugger using all the possible | ||||
| 1201 | methods. | ||||
| 1202 | |||||
| 1203 | Something is still amiss: consider the loop variable $cnt of the | ||||
| 1204 | script. It was a number, not an object. We cannot make this value of | ||||
| 1205 | type C<symbolic>, since then the loop will not terminate. | ||||
| 1206 | |||||
| 1207 | Indeed, to terminate the cycle, the $cnt should become false. | ||||
| 1208 | However, the operator C<bool> for checking falsity is overloaded (this | ||||
| 1209 | time via overloaded C<"">), and returns a long string, thus any object | ||||
| 1210 | of type C<symbolic> is true. To overcome this, we need a way to | ||||
| 1211 | compare an object to 0. In fact, it is easier to write a numeric | ||||
| 1212 | conversion routine. | ||||
| 1213 | |||||
| 1214 | Here is the text of F<symbolic.pm> with such a routine added (and | ||||
| 1215 | slightly modified str()): | ||||
| 1216 | |||||
| 1217 | package symbolic; # Primitive symbolic calculator | ||||
| 1218 | use overload | ||||
| 1219 | nomethod => \&wrap, '""' => \&str, '0+' => \# | ||||
| 1220 | |||||
| 1221 | sub new { shift; bless ['n', @_] } | ||||
| 1222 | sub wrap { | ||||
| 1223 | my ($obj, $other, $inv, $meth) = @_; | ||||
| 1224 | ($obj, $other) = ($other, $obj) if $inv; | ||||
| 1225 | bless [$meth, $obj, $other]; | ||||
| 1226 | } | ||||
| 1227 | sub str { | ||||
| 1228 | my ($meth, $a, $b) = @{+shift}; | ||||
| 1229 | $a = 'u' unless defined $a; | ||||
| 1230 | if (defined $b) { | ||||
| 1231 | "[$meth $a $b]"; | ||||
| 1232 | } else { | ||||
| 1233 | "[$meth $a]"; | ||||
| 1234 | } | ||||
| 1235 | } | ||||
| 1236 | my %subr = ( n => sub {$_[0]}, | ||||
| 1237 | sqrt => sub {sqrt $_[0]}, | ||||
| 1238 | '-' => sub {shift() - shift()}, | ||||
| 1239 | '+' => sub {shift() + shift()}, | ||||
| 1240 | '/' => sub {shift() / shift()}, | ||||
| 1241 | '*' => sub {shift() * shift()}, | ||||
| 1242 | '**' => sub {shift() ** shift()}, | ||||
| 1243 | ); | ||||
| 1244 | sub num { | ||||
| 1245 | my ($meth, $a, $b) = @{+shift}; | ||||
| 1246 | my $subr = $subr{$meth} | ||||
| 1247 | or die "Do not know how to ($meth) in symbolic"; | ||||
| 1248 | $a = $a->num if ref $a eq __PACKAGE__; | ||||
| 1249 | $b = $b->num if ref $b eq __PACKAGE__; | ||||
| 1250 | $subr->($a,$b); | ||||
| 1251 | } | ||||
| 1252 | |||||
| 1253 | All the work of numeric conversion is done in %subr and num(). Of | ||||
| 1254 | course, %subr is not complete, it contains only operators used in the | ||||
| 1255 | example below. Here is the extra-credit question: why do we need an | ||||
| 1256 | explicit recursion in num()? (Answer is at the end of this section.) | ||||
| 1257 | |||||
| 1258 | Use this module like this: | ||||
| 1259 | |||||
| 1260 | require symbolic; | ||||
| 1261 | my $iter = symbolic->new(2); # 16-gon | ||||
| 1262 | my $side = symbolic->new(1); | ||||
| 1263 | my $cnt = $iter; | ||||
| 1264 | |||||
| 1265 | while ($cnt) { | ||||
| 1266 | $cnt = $cnt - 1; # Mutator `--' not implemented | ||||
| 1267 | $side = (sqrt(1 + $side**2) - 1)/$side; | ||||
| 1268 | } | ||||
| 1269 | printf "%s=%f\n", $side, $side; | ||||
| 1270 | printf "pi=%f\n", $side*(2**($iter+2)); | ||||
| 1271 | |||||
| 1272 | It prints (without so many line breaks) | ||||
| 1273 | |||||
| 1274 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] | ||||
| 1275 | [n 1]] 2]]] 1] | ||||
| 1276 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 | ||||
| 1277 | pi=3.182598 | ||||
| 1278 | |||||
| 1279 | The above module is very primitive. It does not implement | ||||
| 1280 | mutator methods (C<++>, C<-=> and so on), does not do deep copying | ||||
| 1281 | (not required without mutators!), and implements only those arithmetic | ||||
| 1282 | operations which are used in the example. | ||||
| 1283 | |||||
| 1284 | To implement most arithmetic operations is easy; one should just use | ||||
| 1285 | the tables of operations, and change the code which fills %subr to | ||||
| 1286 | |||||
| 1287 | my %subr = ( 'n' => sub {$_[0]} ); | ||||
| 1288 | foreach my $op (split " ", $overload::ops{with_assign}) { | ||||
| 1289 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | ||||
| 1290 | } | ||||
| 1291 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | ||||
| 1292 | foreach my $op (split " ", "@overload::ops{ @bins }") { | ||||
| 1293 | $subr{$op} = eval "sub {shift() $op shift()}"; | ||||
| 1294 | } | ||||
| 1295 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | ||||
| 1296 | print "defining `$op'\n"; | ||||
| 1297 | $subr{$op} = eval "sub {$op shift()}"; | ||||
| 1298 | } | ||||
| 1299 | |||||
| 1300 | Due to L<Calling Conventions for Mutators>, we do not need anything | ||||
| 1301 | special to make C<+=> and friends work, except filling C<+=> entry of | ||||
| 1302 | %subr, and defining a copy constructor (needed since Perl has no | ||||
| 1303 | way to know that the implementation of C<'+='> does not mutate | ||||
| 1304 | the argument, compare L<Copy Constructor>). | ||||
| 1305 | |||||
| 1306 | To implement a copy constructor, add C<< '=' => \&cpy >> to C<use overload> | ||||
| 1307 | line, and code (this code assumes that mutators change things one level | ||||
| 1308 | deep only, so recursive copying is not needed): | ||||
| 1309 | |||||
| 1310 | sub cpy { | ||||
| 1311 | my $self = shift; | ||||
| 1312 | bless [@$self], ref $self; | ||||
| 1313 | } | ||||
| 1314 | |||||
| 1315 | To make C<++> and C<--> work, we need to implement actual mutators, | ||||
| 1316 | either directly, or in C<nomethod>. We continue to do things inside | ||||
| 1317 | C<nomethod>, thus add | ||||
| 1318 | |||||
| 1319 | if ($meth eq '++' or $meth eq '--') { | ||||
| 1320 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference | ||||
| 1321 | return $obj; | ||||
| 1322 | } | ||||
| 1323 | |||||
| 1324 | after the first line of wrap(). This is not a most effective | ||||
| 1325 | implementation, one may consider | ||||
| 1326 | |||||
| 1327 | sub inc { $_[0] = bless ['++', shift, 1]; } | ||||
| 1328 | |||||
| 1329 | instead. | ||||
| 1330 | |||||
| 1331 | As a final remark, note that one can fill %subr by | ||||
| 1332 | |||||
| 1333 | my %subr = ( 'n' => sub {$_[0]} ); | ||||
| 1334 | foreach my $op (split " ", $overload::ops{with_assign}) { | ||||
| 1335 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; | ||||
| 1336 | } | ||||
| 1337 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); | ||||
| 1338 | foreach my $op (split " ", "@overload::ops{ @bins }") { | ||||
| 1339 | $subr{$op} = eval "sub {shift() $op shift()}"; | ||||
| 1340 | } | ||||
| 1341 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { | ||||
| 1342 | $subr{$op} = eval "sub {$op shift()}"; | ||||
| 1343 | } | ||||
| 1344 | $subr{'++'} = $subr{'+'}; | ||||
| 1345 | $subr{'--'} = $subr{'-'}; | ||||
| 1346 | |||||
| 1347 | This finishes implementation of a primitive symbolic calculator in | ||||
| 1348 | 50 lines of Perl code. Since the numeric values of subexpressions | ||||
| 1349 | are not cached, the calculator is very slow. | ||||
| 1350 | |||||
| 1351 | Here is the answer for the exercise: In the case of str(), we need no | ||||
| 1352 | explicit recursion since the overloaded C<.>-operator will fall back | ||||
| 1353 | to an existing overloaded operator C<"">. Overloaded arithmetic | ||||
| 1354 | operators I<do not> fall back to numeric conversion if C<fallback> is | ||||
| 1355 | not explicitly requested. Thus without an explicit recursion num() | ||||
| 1356 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild | ||||
| 1357 | the argument of num(). | ||||
| 1358 | |||||
| 1359 | If you wonder why defaults for conversion are different for str() and | ||||
| 1360 | num(), note how easy it was to write the symbolic calculator. This | ||||
| 1361 | simplicity is due to an appropriate choice of defaults. One extra | ||||
| 1362 | note: due to the explicit recursion num() is more fragile than sym(): | ||||
| 1363 | we need to explicitly check for the type of $a and $b. If components | ||||
| 1364 | $a and $b happen to be of some related type, this may lead to problems. | ||||
| 1365 | |||||
| 1366 | =head2 I<Really> symbolic calculator | ||||
| 1367 | |||||
| 1368 | One may wonder why we call the above calculator symbolic. The reason | ||||
| 1369 | is that the actual calculation of the value of expression is postponed | ||||
| 1370 | until the value is I<used>. | ||||
| 1371 | |||||
| 1372 | To see it in action, add a method | ||||
| 1373 | |||||
| 1374 | sub STORE { | ||||
| 1375 | my $obj = shift; | ||||
| 1376 | $#$obj = 1; | ||||
| 1377 | @$obj->[0,1] = ('=', shift); | ||||
| 1378 | } | ||||
| 1379 | |||||
| 1380 | to the package C<symbolic>. After this change one can do | ||||
| 1381 | |||||
| 1382 | my $a = symbolic->new(3); | ||||
| 1383 | my $b = symbolic->new(4); | ||||
| 1384 | my $c = sqrt($a**2 + $b**2); | ||||
| 1385 | |||||
| 1386 | and the numeric value of $c becomes 5. However, after calling | ||||
| 1387 | |||||
| 1388 | $a->STORE(12); $b->STORE(5); | ||||
| 1389 | |||||
| 1390 | the numeric value of $c becomes 13. There is no doubt now that the module | ||||
| 1391 | symbolic provides a I<symbolic> calculator indeed. | ||||
| 1392 | |||||
| 1393 | To hide the rough edges under the hood, provide a tie()d interface to the | ||||
| 1394 | package C<symbolic> (compare with L<Metaphor clash>). Add methods | ||||
| 1395 | |||||
| 1396 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } | ||||
| 1397 | sub FETCH { shift } | ||||
| 1398 | sub nop { } # Around a bug | ||||
| 1399 | |||||
| 1400 | (the bug is described in L<"BUGS">). One can use this new interface as | ||||
| 1401 | |||||
| 1402 | tie $a, 'symbolic', 3; | ||||
| 1403 | tie $b, 'symbolic', 4; | ||||
| 1404 | $a->nop; $b->nop; # Around a bug | ||||
| 1405 | |||||
| 1406 | my $c = sqrt($a**2 + $b**2); | ||||
| 1407 | |||||
| 1408 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value | ||||
| 1409 | of $c becomes 13. To insulate the user of the module add a method | ||||
| 1410 | |||||
| 1411 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } | ||||
| 1412 | |||||
| 1413 | Now | ||||
| 1414 | |||||
| 1415 | my ($a, $b); | ||||
| 1416 | symbolic->vars($a, $b); | ||||
| 1417 | my $c = sqrt($a**2 + $b**2); | ||||
| 1418 | |||||
| 1419 | $a = 3; $b = 4; | ||||
| 1420 | printf "c5 %s=%f\n", $c, $c; | ||||
| 1421 | |||||
| 1422 | $a = 12; $b = 5; | ||||
| 1423 | printf "c13 %s=%f\n", $c, $c; | ||||
| 1424 | |||||
| 1425 | shows that the numeric value of $c follows changes to the values of $a | ||||
| 1426 | and $b. | ||||
| 1427 | |||||
| 1428 | =head1 AUTHOR | ||||
| 1429 | |||||
| 1430 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. | ||||
| 1431 | |||||
| 1432 | =head1 SEE ALSO | ||||
| 1433 | |||||
| 1434 | The L<overloading> pragma can be used to enable or disable overloaded | ||||
| 1435 | operations within a lexical scope. | ||||
| 1436 | |||||
| 1437 | =head1 DIAGNOSTICS | ||||
| 1438 | |||||
| 1439 | When Perl is run with the B<-Do> switch or its equivalent, overloading | ||||
| 1440 | induces diagnostic messages. | ||||
| 1441 | |||||
| 1442 | Using the C<m> command of Perl debugger (see L<perldebug>) one can | ||||
| 1443 | deduce which operations are overloaded (and which ancestor triggers | ||||
| 1444 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> | ||||
| 1445 | is shown by debugger. The method C<()> corresponds to the C<fallback> | ||||
| 1446 | key (in fact a presence of this method shows that this package has | ||||
| 1447 | overloading enabled, and it is what is used by the C<Overloaded> | ||||
| 1448 | function of module C<overload>). | ||||
| 1449 | |||||
| 1450 | The module might issue the following warnings: | ||||
| 1451 | |||||
| 1452 | =over 4 | ||||
| 1453 | |||||
| 1454 | =item Odd number of arguments for overload::constant | ||||
| 1455 | |||||
| 1456 | (W) The call to overload::constant contained an odd number of arguments. | ||||
| 1457 | The arguments should come in pairs. | ||||
| 1458 | |||||
| 1459 | =item `%s' is not an overloadable type | ||||
| 1460 | |||||
| 1461 | (W) You tried to overload a constant type the overload package is unaware of. | ||||
| 1462 | |||||
| 1463 | =item `%s' is not a code reference | ||||
| 1464 | |||||
| 1465 | (W) The second (fourth, sixth, ...) argument of overload::constant needs | ||||
| 1466 | to be a code reference. Either an anonymous subroutine, or a reference | ||||
| 1467 | to a subroutine. | ||||
| 1468 | |||||
| 1469 | =back | ||||
| 1470 | |||||
| 1471 | =head1 BUGS | ||||
| 1472 | |||||
| 1473 | Because it is used for overloading, the per-package hash %OVERLOAD now | ||||
| 1474 | has a special meaning in Perl. The symbol table is filled with names | ||||
| 1475 | looking like line-noise. | ||||
| 1476 | |||||
| 1477 | For the purpose of inheritance every overloaded package behaves as if | ||||
| 1478 | C<fallback> is present (possibly undefined). This may create | ||||
| 1479 | interesting effects if some package is not overloaded, but inherits | ||||
| 1480 | from two overloaded packages. | ||||
| 1481 | |||||
| 1482 | Relation between overloading and tie()ing is broken. Overloading is | ||||
| 1483 | triggered or not basing on the I<previous> class of tie()d value. | ||||
| 1484 | |||||
| 1485 | This happens because the presence of overloading is checked too early, | ||||
| 1486 | before any tie()d access is attempted. If the FETCH()ed class of the | ||||
| 1487 | tie()d value does not change, a simple workaround is to access the value | ||||
| 1488 | immediately after tie()ing, so that after this call the I<previous> class | ||||
| 1489 | coincides with the current one. | ||||
| 1490 | |||||
| 1491 | B<Needed:> a way to fix this without a speed penalty. | ||||
| 1492 | |||||
| 1493 | Barewords are not covered by overloaded string constants. | ||||
| 1494 | |||||
| 1495 | This document is confusing. There are grammos and misleading language | ||||
| 1496 | used in places. It would seem a total rewrite is needed. | ||||
| 1497 | |||||
| 1498 | =cut | ||||
| 1499 | |||||
# spent 2µs within overload::CORE:match which was called
# once (2µs+0s) by overload::OVERLOAD at line 18 of overload.pm |