Manpages - overload.3perl

Declaration

Arguments of the use overload directive are (key, value) pairs. For the full set of legal keys, see Overloadable Operations below.

Operator implementations (the values) can be subroutines, references to subroutines, or anonymous subroutines - in other words, anything legal inside a &{ ... } call. Values specified as strings are interpreted as method names. Thus

package Number; use overload “-” > "minus", "*“ => \&muas, ”“ => sub { …; };

declares that subtraction is to be implemented by method minus() in the class Number (or one of its base classes), and that the function Number::muas() is to be used for the assignment form of multiplication, *=. It also defines an anonymous subroutine to implement stringification: this is called whenever an object blessed into the package Number is used in a string context (this subroutine might, for example, return the number as a Roman numeral).

Calling Conventions and Magic Autogeneration

The following sample implementation of minus() (which assumes that Number objects are simply blessed references to scalars) illustrates the calling conventions:

package Number; sub minus { my ($self, $other, $swap) = @_; my $result = $$self - $other; # * $result = -$result if $swap; ref $result ? $result

bless \$result; } # * may recurse once - see table below

Three arguments are passed to all subroutines specified in the use overload directive (with exceptions - see below, particularly nomethod).

The first of these is the operand providing the overloaded operator implementation - in this case, the object whose minus() method is being called.

The second argument is the other operand, or undef in the case of a unary operator.

The third argument is set to TRUE if (and only if) the two operands have been swapped. Perl may do this to ensure that the first argument ($self) is an object implementing the overloaded operation, in line with general object calling conventions. For example, if $x and $y are =Number=s:

operation | generates a call to ===========|===================== $x - $y | minus($x, $y, ) $x - 7 | minus($x, 7, ) 7 - $x | minus($x, 7, 1)

Perl may also use minus() to implement other operators which have not been specified in the use overload directive, according to the rules for Magic Autogeneration described later. For example, the use overload above declared no subroutine for any of the operators --, neg (the overload key for unary minus), or -=. Thus

operation | generates a call to ===========|===================== -$x

minus($x, 0, 1) $x--

minus($x, 1, undef) $x -= 3

minus($x, 3,

undef)

Note the undef=s: where autogeneration results in the method for a standard operator which does not change either of its operands, such as =-, being used to implement an operator which changes the operand (mutators: here, -- and -=), Perl passes undef as the third argument. This still evaluates as FALSE, consistent with the fact that the operands have not been swapped, but gives the subroutine a chance to alter its behaviour in these cases.

In all the above examples, minus() is required only to return the result of the subtraction: Perl takes care of the assignment to $x. In fact, such methods should not modify their operands, even if undef is passed as the third argument (see Overloadable Operations).

The same is not true of implementations of ++ and --: these are expected to modify their operand. An appropriate implementation of -- might look like

use overload – => “decr”, # … sub decr { –${$_[0]}; }

If the bitwise feature is enabled (see feature), a fifth TRUE argument is passed to subroutines handling &, |, ^ and ~. This indicates that the caller is expecting numeric behaviour. The fourth argument will be undef, as that position ($_[3]) is reserved for use by nomethod.

Mathemagic, Mutators, and Copy Constructors

The term ’mathemagic’ describes the overloaded implementation of mathematical operators. Mathemagical operations raise an issue. Consider the code:

$a = $b; –$a;

If $a and $b are scalars then after these statements

$a == $b - 1

An object, however, is a reference to blessed data, so if $a and $b are objects then the assignment $a = $b copies only the reference, leaving $a and $b referring to the same object data. One might therefore expect the operation --$a to decrement $b as well as $a. However, this would not be consistent with how we expect the mathematical operators to work.

Perl resolves this dilemma by transparently calling a copy constructor before calling a method defined to implement a mutator (--, +=, and so on.). In the above example, when Perl reaches the decrement statement, it makes a copy of the object data in $a and assigns to $a a reference to the copied data. Only then does it call decr(), which alters the copied data, leaving $b unchanged. Thus the object metaphor is preserved as far as possible, while mathemagical operations still work according to the arithmetic metaphor.

Note: the preceding paragraph describes what happens when Perl autogenerates the copy constructor for an object based on a scalar. For other cases, see Copy Constructor.

The complete list of keys that can be specified in the use overload directive are given, separated by spaces, in the values of the hash %overload::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+ qr, iterators => <>, filetest => -X, dereferencing => ${} @{} %{} &{} *{}, matching => ~~, special => nomethod fallback =

Most of the overloadable operators map one-to-one to these keys. Exceptions, including additional overloadable operations not apparent from this hash, are included in the notes which follow. This list is subject to growth over time.

A warning is issued if an attempt is made to register an operator not found above.

• not The operator not is not a valid key for use overload. However, if the operator ! is overloaded then the same implementation will be used for not (since the two operators differ only in precedence).
• neg The key neg is used for unary minus to disambiguate it from binary -.
• ++, -- Assuming they are to behave analogously to Perl’s ++ and --, overloaded implementations of these operators are required to mutate their operands. No distinction is made between prefix and postfix forms of the increment and decrement operators: these differ only in the point at which Perl calls the associated subroutine when evaluating an expression.
• Assignments += -= = /= %= **= <<= >>= x= .= &= |= ^= &.= |.= ^.= Simple assignment is not overloadable (the = key is used for the Copy Constructor). Perl does have a way to make assignments to an object do whatever you want, but this involves using *tie(), not overload - see tie in perlfunc and the COOKBOOK examples below. The subroutine for the assignment variant of an operator is required only to return the result of the operation. It is permitted to change the value of its operand (this is safe because Perl calls the copy constructor first), but this is optional since Perl assigns the returned value to the left-hand operand anyway. An object that overloads an assignment operator does so only in respect of assignments to that object. In other words, Perl never calls the corresponding methods with the third argument (the swap argument) set to TRUE. For example, the operation $a *= $b cannot lead to $b’s implementation of *= being called, even if $a is a scalar. (It can, however, generate a call to $b’s method for *).

• Non-mutators with a mutator variant + - * / % ** << >> x . & | ^ &.

  . ^. As described above, Perl may call methods for operators like +

  and & in the course of implementing missing operations like ++, +=, and &=. While these methods may detect this usage by testing the definedness of the third argument, they should in all cases avoid changing their operands. This is because Perl does not call the copy constructor before invoking these methods.

• int Traditionally, the Perl function int rounds to 0 (see int in perlfunc), and so for floating-point-like types one should follow the same semantic.
• String, numeric, boolean, and regexp conversions “” 0+ bool These conversions are invoked according to context as necessary. For example, the subroutine for "" (stringify) may be used where the overloaded object is passed as an argument to print, and that for bool where it is tested in the condition of a flow control statement (like while) or the ternary ?: operation. Of course, in contexts like, for example, $obj + 1, Perl will invoke $obj’s implementation of + rather than (in this example) converting $obj to a number using the numify method 0+ (an exception to this is when no method has been provided for + and fallback is set to TRUE). The subroutines for "", 0+, and bool can return any arbitrary Perl value. If the corresponding operation for this value is overloaded too, the operation will be called again with this value. As a special case if the overload returns the object itself then it will be used directly. An overloaded conversion returning the object is probably a bug, because you’re likely to get something that looks like YourPackage=HASH(0x8172b34). qr The subroutine for qr is used wherever the object is interpolated into or used as a regexp, including when it appears on the RHS of a =~ or !~ operator. qr must return a compiled regexp, or a ref to a compiled regexp (such as qr// returns), and any further overloading on the return value will be ignored.
• Iteration If <> is overloaded then the same implementation is used for both the read-filehandle syntax <$var> and globbing syntax <${var}>.
• File tests The key -X is used to specify a subroutine to handle all the filetest operators (-f, -x, and so on: see -X in perlfunc for the full list); it is not possible to overload any filetest operator individually. To distinguish them, the letter following the ’-’ is passed as the second argument (that is, in the slot that for binary operators is used to pass the second operand). Calling an overloaded filetest operator does not affect the stat value associated with the special filehandle _. It still refers to the result of the last stat, lstat or unoverloaded filetest. This overload was introduced in Perl 5.12.
• Matching The key "~~" allows you to override the smart matching logic used by the ~~ operator and the switch construct (given=/=when). See Switch Statements in perlsyn and feature. Unusually, the overloaded implementation of the smart match operator does not get full control of the smart match behaviour. In particular, in the following code: package Foo; use overload ~ => match; my $obj = Foo->new(); $obj ~ [ 1,2,3 ]; the smart match does not invoke the method call like this: $obj->match([1,2,3],0); rather, the smart match distributive rule takes precedence, so $obj is smart matched against each array element in turn until a match is found, so you may see between one and three of these calls instead: $obj->match(1,0); $obj->match(2,0); $obj->match(3,0); Consult the match table in Smartmatch Operator in perlop for details of when overloading is invoked.
• Dereferencing \({} @{} %{} &{} *{} If these operators are not explicitly overloaded then they work in the normal way, yielding the underlying scalar, array, or whatever stores the object data (or the appropriate error message if the dereference operator doesn't match it). Defining a catch-all =nomethod= (see below) makes no difference to this as the catch-all function will not be called to implement a missing dereference operator. If a dereference operator is overloaded then it must return a /reference/ of the appropriate type (for example, the subroutine for key =\){}= should return a reference to a scalar, not a scalar), or another object which overloads the operator: that is, the subroutine only determines what is dereferenced and the actual dereferencing is left to Perl. As a special case, if the subroutine returns the object itself then it will not be called again - avoiding infinite recursion.
• Special nomethod fallback = See “Special Keys for use overload”.

If a method for an operation is not found then Perl tries to autogenerate a substitute implementation from the operations that have been defined.

Note: the behaviour described in this section can be disabled by setting fallback to FALSE (see fallback).

In the following tables, numbers indicate priority. For example, the table below states that, if no implementation for ! has been defined then Perl will implement it using bool (that is, by inverting the value returned by the method for bool); if boolean conversion is also unimplemented then Perl will use 0+ or, failing that, "".

operator | can be autogenerated from | | 0+ “” bool . x ========|========================= 0+ | 1 2 “” | 1 2 bool | 1 2 int | 1 2 3 ! | 2 3 1 qr | 2 1 3 . | 2 1 3 x | 2 1 3 .= | 3 2 4 1 x= | 3 2 4 1 <> | 2 1 3 -X | 2 1 3

Note: The iterator (<>) and file test (-X) operators work as normal: if the operand is not a blessed glob or IO reference then it is converted to a string (using the method for "", 0+, or bool) to be interpreted as a glob or filename.

operator | can be autogenerated from | | < <=> neg -= - ========|========================= neg | 1 -= | 1 – | 1 2 abs | a1 a2 b1 b2 [*] < | 1 <= | 1 > | 1 >= | 1 = | 1 ! | 1 * one from [a1, a2] and one from [b1, b2]

Just as numeric comparisons can be autogenerated from the method for <=>, string comparisons can be autogenerated from that for cmp:

operators | can be autogenerated from ===================|========================== lt gt le ge eq ne | cmp

Similarly, autogeneration for keys += and ++ is analogous to -= and -- above:

operator | can be autogenerated from | | = + ========|========================= += | 1 + | 1 2

And other assignment variations are analogous to += and -= (and similar to .= and x= above):

operator || = /= %= **= <<= >>= &= ^= |= &.= ^.= |.= --------------–—||--------------------------------------–— autogenerated from || * / % * << >> & ^ | &. ^. |.

Note also that the copy constructor (key =) may be autogenerated, but only for objects based on scalars. See Copy Constructor.

Minimal Set of Overloaded Operations

Since some operations can be automatically generated from others, there is a minimal set of operations that need to be overloaded in order to have the complete set of overloaded operations at one’s disposal. Of course, the autogenerated operations may not do exactly what the user expects. The minimal set is:

• - * / % ** << >> x <=> cmp & | ^ ~ &. |. ^. ~. atan2 cos sin exp log

sqrt int “” 0+ bool ~~

Of the conversions, only one of string, boolean or numeric is needed because each can be generated from either of the other two.

nomethod

The nomethod key is used to specify a catch-all function to be called for any operator that is not individually overloaded. The specified function will be passed four parameters. The first three arguments coincide with those that would have been passed to the corresponding method if it had been defined. The fourth argument is the use overload key for that missing method. If the bitwise feature is enabled (see feature), a fifth TRUE argument is passed to subroutines handling &, |, ^ and ~ to indicate that the caller is expecting numeric behaviour.

For example, if $a is an object blessed into a package declaring

use overload nomethod => catch_all, # …

then the operation

3 + $a

could (unless a method is specifically declared for the key +) result in a call

catch_all($a, 3, 1, +)

See How Perl Chooses an Operator Implementation.

fallback

The value assigned to the key fallback tells Perl how hard it should try to find an alternative way to implement a missing operator.

• defined, but FALSE use overload “fallback” => 0, # … ; This disables Magic Autogeneration.
• undef In the default case where no value is explicitly assigned to fallback, magic autogeneration is enabled.
• TRUE The same as for undef, but if a missing operator cannot be autogenerated then, instead of issuing an error message, Perl is allowed to revert to what it would have done for that operator if there had been no use overload directive. Note: in most cases, particularly the Copy Constructor, this is unlikely to be appropriate behaviour.

See How Perl Chooses an Operator Implementation.

Copy Constructor

As mentioned above, this operation is called when a mutator is applied to a reference that shares its object with some other reference. For example, if $b is mathemagical, and ++ is overloaded with incr, and = is overloaded with clone, then the code

$a = $b; # … (other code which does not modify $a or $b) … ++$b;

would be executed in a manner equivalent to

$a = $b; # … $b = $b->clone(undef, “”); $b->incr(undef, “”);

Note:

• The subroutine for = does not overload the Perl assignment operator: it is used only to allow mutators to work as described here. (See Assignments above.)
• As for other operations, the subroutine implementing ’' is passed three arguments, though the last two are always =undef and .
• The copy constructor is called only before a call to a function declared to implement a mutator, for example, if ++$b; in the code above is effected via a method declared for key ++ (or ’nomethod’, passed ++ as the fourth argument) or, by autogeneration, +=. It is not called if the increment operation is effected by a call to the method for + since, in the equivalent code, $a = $b; $b = $b + 1; the data referred to by $a is unchanged by the assignment to $b of a reference to new object data.
• The copy constructor is not called if Perl determines that it is unnecessary because there is no other reference to the data being modified.
• If fallback is undefined or TRUE then a copy constructor can be autogenerated, but only for objects based on scalars. In other cases it needs to be defined explicitly. Where an object’s data is stored as, for example, an array of scalars, the following might be appropriate: use overload = => sub { bless [ @{$_[0]} ] }, # …
• If fallback is TRUE and no copy constructor is defined then, for objects not based on scalars, Perl may silently fall back on simple assignment - that is, assignment of the object reference. In effect, this disables the copy constructor mechanism since no new copy of the object data is created. This is almost certainly not what you want. (It is, however, consistent: for example, Perl’s fallback for the ++ operator is to increment the reference itself.)

Which is checked first, nomethod or fallback? If the two operands of an operator are of different types and both overload the operator, which implementation is used? The following are the precedence rules:

1. If the first operand has declared a subroutine to overload the operator then use that implementation.
2. Otherwise, if fallback is TRUE or undefined for the first operand then see if the rules for autogeneration allows another of its operators to be used instead.
3. Unless the operator is an assignment (+=, -=, etc.), repeat step (1) in respect of the second operand.
4. Repeat Step (2) in respect of the second operand.
5. If the first operand has a nomethod method then use that.
6. If the second operand has a nomethod method then use that.
7. If fallback is TRUE for both operands then perform the usual operation for the operator, treating the operands as numbers, strings, or booleans as appropriate for the operator (see note).
8. Nothing worked - die.

Where there is only one operand (or only one operand with overloading) the checks in respect of the other operand above are skipped.

There are exceptions to the above rules for dereference operations (which, if Step 1 fails, always fall back to the normal, built-in implementations - see Dereferencing), and for ~~ (which has its own set of rules - see Matching under Overloadable Operations above).

Note on Step 7: some operators have a different semantic depending on the type of their operands. As there is no way to instruct Perl to treat the operands as, e.g., numbers instead of strings, the result here may not be what you expect. See BUGS AND PITFALLS.

The restriction for the comparison operation is that even if, for example, cmp should return a blessed reference, the autogenerated lt function will produce only a standard logical value based on the numerical value of the result of cmp. In particular, a working numeric conversion is needed in this case (possibly expressed in terms of other conversions).

Similarly, .= and x= operators lose their mathemagical properties if the string conversion substitution is applied.

When you chop() a mathemagical object it is promoted to a string and its mathemagical properties are lost. The same can happen with other operations as well.

Overloading respects inheritance via the @ISA hierarchy. Inheritance interacts with overloading in two ways.

Method names in the “use overload” directive

If value in use overload key => value; is a string, it is interpreted as a method name - which may (in the usual way) be inherited from another class.

Overloading of an operation is inherited by derived classes

Any class derived from an overloaded class is also overloaded and inherits its operator implementations. If the same operator is overloaded in more than one ancestor then the implementation is determined by the usual inheritance rules. For example, if A inherits from B and C (in that order), B overloads + with \&D::plus_sub, and C overloads + by "plus_meth", then the subroutine D::plus_sub will be called to implement operation + for an object in package A.

Note that in Perl version prior to 5.18 inheritance of the fallback key was not governed by the above rules. The value of fallback in the first overloaded ancestor was used. This was fixed in 5.18 to follow the usual rules of inheritance.

Since all use directives are executed at compile-time, the only way to change overloading during run-time is to

eval use overload “+” => \&addmethod;

You can also use

eval no overload “+”, “–”, “<=”;

though the use of these constructs during run-time is questionable.

Package overload.pm provides the following public functions:

overload::StrVal(arg)

Gives the string value of arg as in the absence of stringify overloading. If you are using this to get the address of a reference (useful for checking if two references point to the same thing) then you may be better off using Scalar::Util::refaddr(), which is faster.

overload::Overloaded(arg)

Returns true if arg is subject to overloading of some operations.

overload::Method(obj,op)

Returns undef or a reference to the method that implements op.

For some applications, the 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:

integer

to overload integer constants,

float

to overload floating point constants,

binary

to overload octal and hexadecimal constants,

(no term)

to overload q-quoted strings, constant pieces of qq- and qx-quoted strings and here-documents,

qr

to overload constant pieces of regular expressions.

The corresponding values are references to functions which take three arguments: the first one is the initial 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 q- and qr- constants, it is q in single-quote context (comes from strings, regular expressions, and single-quote HERE documents), it is tr for arguments of tr=/=y operators, it is s for right-hand side of s-operator, and it is qq otherwise.

Since an expression "ab$cd,," is just a shortcut for 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)}; }

Put this in two_face.pm 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 $seven = two_face->new(“vii”, 7); printf “seven=$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

Suppose you want to create an object which is accessible as both an array reference and a hash reference.

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 = two_refs->new(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 actual type of $bar is a scalar reference, and we do not overload the scalar dereference. Thus we can get the actual non-overloaded contents of $bar by just using $$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 implemented as a hash itself, the only problem one has to circumvent is how to access this actual hash (as opposed to the virtual hash exhibited by the overloaded dereference operator). Here is one possible fetching routine:

sub access_hash { my ($self, $key) = (shift, shift); my $class = ref $self; bless $self, overload::dummy; # Disable overloading of %{} 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 $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

• There are no loops of references.
• Both objects which are blessed into the class two_refs1 are references to a reference to an array, thus references to a scalar. Thus the accessor expression $$foo->[$ind] involves no overloaded operations.

Put this in symbolic.pm 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 an implementation for "nomethod". In this example the nomethod subroutine returns an object which encapsulates operations done over the objects: symbolic->new(3) contains [n, 3], 2 + symbolic->new(3) contains [+, 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 = symbolic->new(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 use this value. In fact this value may be inspected in debugger (see perldebug), but only if bareStringify *O*ption is set, and not via p command.

If one attempts to print this value, then the overloaded operator "" will be called, which will call nomethod operator. The result of this operator will be stringified again, but this result is again of type symbolic, which will lead to an infinite loop.

Add a pretty-printer method to the module symbolic.pm:

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 pretty is doing object-to-string conversion, so it is natural to overload the operator "" using this method. However, inside such a method it is not necessary to pretty-print the components $a and $b of an object. In the above subroutine "[$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 .; if not present, it will look for an overloaded operator "". 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 symbolic, since then the loop will not terminate.

Indeed, to terminate the cycle, the $cnt should become false. However, the operator bool for checking falsity is overloaded (this time via overloaded ""), and returns a long string, thus any object of type symbolic 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 symbolic.pm with such a routine added (and slightly modified str()):

package symbolic; # Primitive symbolic calculator use overload nomethod => \&wrap, “” => \&str, 0+ => \&num; 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()

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 used.

To see it in action, add a method

sub STORE { my $obj = shift; $#$obj = 1; @$obj->[0,1] = (=, shift); }

to the package symbolic. After this change one can do

my $a = symbolic->new(3); my $b = symbolic->new(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 symbolic calculator indeed.

To hide the rough edges under the hood, provide a *tie()*d interface to the package symbolic. Add methods

sub TIESCALAR { my $pack = shift; $pack->new(@_) } sub FETCH { shift } sub nop { } # Around a bug

(the bug, fixed in Perl 5.14, is described in 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 $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.