Contextual::Return(3) User Contributed Perl DocumentationContextual::Return(3)NAMEContextual::Return - Create context-senstive return values
VERSION
This document describes Contextual::Return version 0.2.1
SYNOPSIS
use Contextual::Return;
use Carp;
sub foo {
return
SCALAR { 'thirty-twelve' }
BOOL { 1 }
NUM { 7*6 }
STR { 'forty-two' }
LIST { 1,2,3 }
HASHREF { {name => 'foo', value => 99} }
ARRAYREF { [3,2,1] }
GLOBREF { \*STDOUT }
CODEREF { croak "Don't use this result as code!"; }
;
}
# and later...
if (my $foo = foo()) {
for my $count (1..$foo) {
print "$count: $foo is:\n"
. " array: @{$foo}\n"
. " hash: $foo->{name} => $foo->{value}\n"
;
}
print {$foo} $foo->();
}
DESCRIPTION
Usually, when you need to create a subroutine that returns different
values in different contexts (list, scalar, or void), you write
something like:
sub get_server_status {
my ($server_ID) = @_;
# Acquire server data somehow...
my %server_data = _ascertain_server_status($server_ID);
# Return different components of that data,
# depending on call context...
if (wantarray()) {
return @server_data{ qw(name uptime load users) };
}
if (defined wantarray()) {
return $server_data{load};
}
if (!defined wantarray()) {
carp 'Useless use of get_server_status() in void context';
return;
}
else {
croak q{Bad context! No biscuit!};
}
}
That works okay, but the code could certainly be more readable. In its
simplest usage, this module makes that code more readable by providing
three subroutines--"LIST()", "SCALAR()", "VOID()"--that are true only
when the current subroutine is called in the corresponding context:
use Contextual::Return;
sub get_server_status {
my ($server_ID) = @_;
# Acquire server data somehow...
my %server_data = _ascertain_server_status($server_ID);
# Return different components of that data
# depending on call context...
if (LIST) { return @server_data{ qw(name uptime load users) } }
if (SCALAR) { return $server_data{load} }
if (VOID) { print "$server_data{load}\n" }
else { croak q{Bad context! No biscuit!} }
}
Contextual returns
Those three subroutines can also be used in another way: as labels on a
series of contextual return blocks (collectively known as a contextual
return sequence). When a context sequence is returned, it automatically
selects the appropriate contextual return block for the calling
context. So the previous example could be written even more cleanly
as:
use Contextual::Return;
sub get_server_status {
my ($server_ID) = @_;
# Acquire server data somehow...
my %server_data = _ascertain_server_status($server_ID);
# Return different components of that data
# depending on call context...
return (
LIST { return @server_data{ qw(name uptime load users) } }
SCALAR { return $server_data{load} }
VOID { print "$server_data{load}\n" }
DEFAULT { croak q{Bad context! No biscuit!} }
);
}
The context sequence automatically selects the appropriate block for
each call context.
Lazy contextual return values
"LIST" and "VOID" blocks are always executed during the "return"
statement. However, scalar return blocks ("SCALAR", "STR", "NUM",
"BOOL", etc.) blocks are not. Instead, returning any of scalar block
types causes the subroutine to return an object that lazily evaluates
that block only when the return value is used.
This means that returning a "SCALAR" block is a convenient way to
implement a subroutine with a lazy return value. For example:
sub digest {
return SCALAR {
my ($text) = @_;
md5($text);
}
}
my $digest = digest($text);
print $digest; # md5() called only when $digest used as string
To better document this usage, the "SCALAR" block has a synonym:
"LAZY".
sub digest {
return LAZY {
my ($text) = @_;
md5($text);
}
}
Active contextual return values
Once a return value has been lazily evaluated in a given context, the
resulting value is cached, and thereafter reused in that same context.
However, you can specify that, rather than being cached, the value
should be re-evaluated every time the value is used:
sub make_counter {
my $counter = 0;
return ACTIVE
SCALAR { ++$counter }
ARRAYREF { [1..$counter] }
}
my $idx = make_counter();
print "$idx\n"; # 1
print "$idx\n"; # 2
print "[@$idx]\n"; # [1 2]
print "$idx\n"; # 3
print "[@$idx]\n"; # [1 2 3]
Semi-lazy contextual return values
Sometimes, single or repeated lazy evaluation of a scalar return value
in different contexts isn't what you really want. Sometimes what you
really want is for the return value to be lazily evaluated once only
(the first time it's used in any context), and then for that first
value to be reused whenever the return value is subsequently
reevaluated in any other context.
To get that behaviour, you can use the "FIXED" modifier, which causes
the return value to morph itself into the actual value the first time
it is used. For example:
sub lazy {
return
SCALAR { 42 }
ARRAYREF { [ 1, 2, 3 ] }
;
}
my $lazy = lazy();
print $lazy + 1; # 43
print "@{$lazy}"; # 1 2 3
sub semilazy {
return FIXED
SCALAR { 42 }
ARRAYREF { [ 1, 2, 3 ] }
;
}
my $semi = semilazy();
print $semi + 1; # 43
print "@{$semi}"; # die q{Can't use string ("42") as an ARRAY ref}
Finer distinctions of scalar context
Because the scalar values returned from a context sequence are lazily
evaluated, it becomes possible to be more specific about what kind of
scalar value should be returned: a boolean, a number, or a string. To
support those distinctions, Contextual::Return provides three extra
context blocks: "BOOL", "NUM", and "STR":
sub get_server_status {
my ($server_ID) = @_;
# Acquire server data somehow...
my %server_data = _ascertain_server_status($server_ID);
# Return different components of that data
# depending on call context...
return (
LIST { @server_data{ qw(name uptime load users) } }
BOOL { $server_data{uptime} > 0 }
NUM { $server_data{load} }
STR { "$server_data{name}: $server_data{uptime}" }
VOID { print "$server_data{load}\n" }
DEFAULT { croak q{Bad context! No biscuit!} }
);
}
With these in place, the object returned from a scalar-context call to
"get_server_status()" now behaves differently, depending on how it's
used. For example:
if ( my $status = get_server_status() ) { # True if uptime > 0
$load_distribution[$status]++; # Evaluates to load value
print "$status\n"; # Prints name: uptime
}
Referential contexts
The other major kind of scalar return value is a reference.
Contextual::Return provides contextual return blocks that allow you to
specify what to (lazily) return when the return value of a subroutine
is used as a reference to a scalar ("SCALARREF {...}"), to an array
("ARRAYREF {...}"), to a hash ("HASHREF {...}"), to a subroutine
("CODEREF {...}"), or to a typeglob ("GLOBREF {...}").
For example, the server status subroutine shown earlier could be
extended to allow it to return a hash reference, thereby supporting
"named return values":
sub get_server_status {
my ($server_ID) = @_;
# Acquire server data somehow...
my %server_data = _ascertain_server_status($server_ID);
# Return different components of that data
# depending on call context...
return (
LIST { @server_data{ qw(name uptime load users) } }
BOOL { $server_data{uptime} > 0 }
NUM { $server_data{load} }
STR { "$server_data{name}: $server_data{uptime}" }
VOID { print "$server_data{load}\n" }
HASHREF { return \%server_data }
DEFAULT { croak q{Bad context! No biscuit!} }
);
}
# and later...
my $users = get_server_status->{users};
# or, lazily...
my $server = get_server_status();
print "$server->{name} load = $server->{load}\n";
Interpolative referential contexts
The "SCALARREF {...}" and "ARRAYREF {...}" context blocks are
especially useful when you need to interpolate a subroutine into
strings. For example, if you have a subroutine like:
sub get_todo_tasks {
return (
SCALAR { scalar @todo_list } # How many?
LIST { @todo_list } # What are they?
);
}
# and later...
print "There are ", scalar(get_todo_tasks()), " tasks:\n",
get_todo_tasks();
then you could make it much easier to interpolate calls to that
subroutine by adding:
sub get_todo_tasks {
return (
SCALAR { scalar @todo_list } # How many?
LIST { @todo_list } # What are they?
SCALARREF { \scalar @todo_list } # Ref to how many
ARRAYREF { \@todo_list } # Ref to them
);
}
# and then...
print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";
In fact, this behaviour is so useful that it's the default. If you
don't provide an explicit "SCALARREF {...}" block, Contextual::Return
automatically provides an implicit one that simply returns a reference
to whatever would have been returned in scalar context. Likewise, if
no "ARRAYREF {...}" block is specified, the module supplies one that
returns the list-context return value wrapped up in an array reference.
So you could just write:
sub get_todo_tasks {
return (
SCALAR { scalar @todo_list } # How many?
LIST { @todo_list } # What are they?
);
}
# and still do this...
print "There are ${get_todo_tasks()} tasks:\n@{get_todo_tasks()}";
Fallback contexts
As the previous sections imply, the "BOOL {...}", "NUM {...}", "STR
{...}", and various "*REF {...}" blocks, are special cases of the
general "SCALAR {...}" context block. If a subroutine is called in one
of these specialized contexts but does not use the corresponding
context block, then the more general "SCALAR {...}" block is used
instead (if it has been specified).
So, for example:
sub read_value_from {
my ($fh) = @_;
my $value = <$fh>;
chomp $value;
return (
BOOL { defined $value }
SCALAR { $value }
);
}
ensures that the "read_value_from()" subroutine returns true in boolean
contexts if the read was successful. But, because no specific "NUM
{...}" or "STR {...}" return behaviours were specified, the subroutine
falls back on using its generic "SCALAR {...}" block in all other
scalar contexts.
Another way to think about this behaviour is that the various kinds of
scalar context blocks form a hierarchy:
SCALAR
^
|
|--< BOOL
|
|--< NUM
|
`--< STR
Contextual::Return uses this hierarchical relationship to choose the
most specific context block available to handle any particular return
context, working its way up the tree from the specific type it needs,
to the more general type, if that's all that is available.
There are two slight complications to this picture. The first is that
Perl treats strings and numbers as interconvertable so the diagram (and
the Contextual::Return module) also has to allow these interconversions
as a fallback strategy:
SCALAR
^
|
|--< BOOL
|
|--< NUM
| : ^
| v :
`--< STR
The dotted lines are meant to indicate that this intraconversion is
secondary to the main hierarchical fallback. That is, in a numeric
context, a "STR {...}" block will only be used if there is no "NUM
{...}" block and no "SCALAR {...}" block. In other words, the generic
context type is always used in preference to string<->number
conversion.
The second slight complication is that the above diagram only shows a
small part of the complete hierarchy of contexts supported by
Contextual::Return. The full fallback hierarchy (including dotted
interconversions) is:
DEFAULT
^
|
|--< VOID
|
`--< NONVOID
^
|
|--< VALUE <..............
| ^ :
| | :
| |--< SCALAR <.....:..
| | ^ :
| | | :
| | |--< BOOL :
| | | :
| | |--< NUM <..:..
| | | : ^ :
| | | v : :
| | `--< STR <....:..
| | :
| | .:
| `--< LIST ............. :
| : ^ :
| : : :
`--- REF : : :
^ : : :
| v : :
|--< ARRAYREF :
| .
|--< SCALARREF .........
|
|--< HASHREF
|
|--< CODEREF
|
|--< GLOBREF
|
`--< OBJREF
As before, each dashed arrow represents a fallback relationship. That
is, if the required context specifier isn't available, the arrows are
followed until a more generic one is found. The dotted arrows again
represent the interconversion of return values, which is attempted only
after the normal hierarchical fallback fails.
For example, if a subroutine is called in a context that expects a
scalar reference, but no "SCALARREF {...}" block is provided, then
Contextual::Return tries the following blocks in order:
REF {...}
NONVOID {...}
DEFAULT {...}
STR {...} (automatically taking a reference to the result)
NUM {...} (automatically taking a reference to the result)
SCALAR {...} (automatically taking a reference to the result)
VALUE {...} (automatically taking a reference to the result)
Likewise, in a list context, if there is no "LIST {...}" context block,
the module tries:
VALUE {...}
NONVOID {...}
DEFAULT {...}
ARRAYREF {...} (automatically dereferencing the result)
STR {...} (treating it as a list of one element)
NUM {...} (treating it as a list of one element)
SCALAR {...} (treating it as a list of one element)
VALUE {...} (treating it as a list of one element)
The more generic context blocks are especially useful for intercepting
unexpected and undesirable call contexts. For example, to turn off the
automatic scalar-ref and array-ref interpolative behaviour described in
"Interpolative referential contexts", you could intercept all
referential contexts using a generic "REF {...}" context block:
sub get_todo_tasks {
return (
SCALAR { scalar @todo_list } # How many?
LIST { @todo_list } # What are they?
REF { croak q{get_todo_task() can't be used as a reference} }
);
}
print 'There are ', get_todo_tasks(), '...'; # Still okay
print "There are ${get_todo_tasks()}..."; # Throws an exception
Failure contexts
Two of the most common ways to specify that a subroutine has failed are
to return a false value, or to throw an exception. The
Contextual::Return module provides a mechanism that allows the
subroutine writer to support both of these mechanisms at the same time,
by using the "FAIL" specifier.
A return statement of the form:
return FAIL;
causes the surrounding subroutine to return "undef" (i.e. false) in
boolean contexts, and to throw an exception in any other context. For
example:
use Contextual::Return;
sub get_next_val {
my $next_val = <>;
return FAIL if !defined $next_val;
chomp $next_val;
return $next_val;
}
If the "return FAIL" statement is executed, it will either return false
in a boolean context:
if (my $val = get_next_val()) { # returns undef if no next val
print "[$val]\n";
}
or else throw an exception if the return value is used in any other
context:
print get_next_val(); # throws exception if no next val
my $next_val = get_next_val();
print "[$next_val]\n"; # throws exception if no next val
The exception that is thrown is of the form:
Call to main::get_next_val() failed at demo.pl line 42
but you can change that message by providing a block to the "FAIL",
like so:
return FAIL { "No more data" } if !defined $next_val;
in which case, the final value of the block becomes the exception
message:
No more data at demo.pl line 42
Configurable failure contexts
The default "FAIL" behaviour--false in boolean context, fatal in all
others--works well in most situations, but violates the Platinum Rule
("Do unto others as they would have done unto them").
So it may be user-friendlier if the user of a module is allowed decide
how the module's subroutines should behave on failure. For example, one
user might prefer that failing subs always return undef; another might
prefer that they always throw an exception; a third might prefer that
they always log the problem and return a special Failure object; whilst
a fourth user might want to get back 0 in scalar contexts, an empty
list in list contexts, and an exception everywhere else.
You could create a module that allows the user to specify all these
alternatives, like so:
package MyModule;
use Contextual::Return;
use Log::StdLog;
sub import {
my ($package, @args) = @_;
Contextual::Return::FAIL_WITH {
':false' => sub { return undef },
':fatal' => sub { croak @_ },
':filed' => sub {
print STDLOG 'Sub ', (caller 1)[3], ' failed';
return Failure->new();
},
':fussy' => sub {
SCALAR { undef }
LIST { () }
DEFAULT { croak @_ }
},
}, @args;
}
This configures Contextual::Return so that, instead of the usual false-
or-fatal semantics, every "return FAIL" within MyModule's namespace is
implemented by one of the four subroutines specified in the hash that
was passed to "FAIL_WITH".
Which of those four subs implements the "FAIL" is determined by the
arguments passed after the hash (i.e. by the contents of @args).
"FAIL_WITH" walks through that list of arguments and compares them
against the keys of the hash. If a key matches an argument, the
corresponding value is used as the implementation of "FAIL". Note that,
if subsequent arguments also match a key, their subroutine overrides
the previously installed implementation, so only the final override has
any effect. Contextual::Return generates warnings when multiple
overrides are specified.
All of which mean that, if a user loaded the MyModule module like this:
use MyModule qw( :fatal other args here );
then every "FAIL" within MyModule would be reconfigured to throw an
exception in all circumstances, since the presence of the ':fatal' in
the argument list will cause "FAIL_WITH" to select the hash entry whose
key is ':fatal'.
On the other hand, if they loaded the module:
use MyModule qw( :fussy other args here );
then each "FAIL" within MyModule would return undef or empty list or
throw an exception, depending on context, since that's what the
subroutine whose key is ':fussy' does.
Many people prefer module interfaces with a "flag =" value> format, and
"FAIL_WITH" supports this too. For example, if you wanted your module
to take a "-fail" flag, whose associated value could be any of
"undefined", "exception", "logged", or "context", then you could
implement that simply by specifying the flag as the first argument
(i.e. before the hash) like so:
sub import {
my $package = shift;
Contextual::Return::FAIL_WITH -fail => {
'undefined' => sub { return undef },
'exception' => sub { croak @_ },
'logged' => sub {
print STDLOG 'Sub ', (caller 1)[3], ' failed';
return Failure->new();
},
'context' => sub {
SCALAR { undef }
LIST { () }
DEFAULT { croak @_ }
},
}, @_;
and then load the module:
use MyModule qw( other args here ), -fail=>'undefined';
or:
use MyModule qw( other args here ), -fail=>'exception';
In this case, "FAIL_WITH" scans the argument list for a pair of values:
its flag string, followed by some other selector value. Then it looks
up the selector value in the hash, and installs the corresponding
subroutine as its local "FAIL" handler.
If this "flagged" interface is used, the user of the module can also
specify their own handler directly, by passing a subroutine reference
as the selector value instead of a string:
use MyModule qw( other args here ), -fail=>sub{ die 'horribly'};
If this last example were used, any call to "FAIL" within MyModule
would invoke the specified anonymous subroutine (and hence throw a
'horribly' exception).
Note that, any overriding of a "FAIL" handler is specific to the
namespace and file from which the subroutine that calls "FAIL_WITH" is
itself called. Since "FAIL_WITH" is designed to be called from within a
module's "import()" subroutine, that generally means that the "FAIL"s
within a given module X are only overridden for the current namespace
within the particular file from module X is loaded. This means that two
separate pieces of code (in separate files or separate namespaces) can
each independently overide a module's "FAIL" behaviour, without
interfering with each other.
Lvalue contexts
Recent versions of Perl offer (limited) support for lvalue subroutines:
subroutines that return a modifiable variable, rather than a simple
constant value.
Contextual::Return can make it easier to create such subroutines,
within the limitations imposed by Perl itself. The limitations that
Perl places on lvalue subs are:
1. The subroutine must be declared with an ":lvalue" attribute:
sub foo :lvalue {...}
2. The subroutine must not return via an explicit "return". Instead,
the last statement must evaluate to a variable, or must be a call
to another lvalue subroutine call.
my ($foo, $baz);
sub foo :lvalue {
$foo; # last statement evals to a var
}
sub bar :lvalue {
foo(); # last statement is lvalue sub call
}
sub baz :lvalue {
my ($arg) = @_;
$arg > 0 # last statement evals...
? $baz # ...to a var
: bar(); # ...or to an lvalue sub call
}
Thereafter, any call to the lvalue subroutine produces a result that
can be assigned to:
baz(0) = 42; # same as: $baz = 42
baz(1) = 84; # same as: bar() = 84
# which is the same as: foo() = 84
# which is the same as: $foo = 84
Ultimately, every lvalue subroutine must return a scalar variable,
which is then used as the lvalue of the assignment (or whatever other
lvalue operation is applied to the subroutine call). Unfortunately,
because the subroutine has to return this variable before the
assignment can take place, there is no way that a normal lvalue
subroutine can get access to the value that will eventually be assigned
to its return value.
This is occasionally annoying, so the Contextual::Return module offers
a solution: in addition to all the context blocks described above, it
provides three special contextual return blocks specifically for use in
lvalue subroutines: "LVALUE", "RVALUE", and "NVALUE".
Using these blocks you can specify what happens when an lvalue
subroutine is used in lvalue and non-lvalue (rvalue) context. For
example:
my $verbosity_level = 1;
# Verbosity values must be between 0 and 5...
sub verbosity :lvalue {
LVALUE { $verbosity_level = max(0, min($_, 5)) }
RVALUE { $verbosity_level }
}
The "LVALUE" block is executed whenever "verbosity" is called as an
lvalue:
verbosity() = 7;
The block has access to the value being assigned, which is passed to it
as $_. So, in the above example, the assigned value of 7 would be
aliased to $_ within the "LVALUE" block, would be reduced to 5 by the
"min-of-max" expression, and then assigned to $verbosity_level.
(If you need to access the caller's $_, it's also still available: as
$CALLER::_.)
When the subroutine isn't used as an lvalue:
print verbosity();
the "RVALUE" block is executed instead and its final value returned.
Within an "RVALUE" block you can use any of the other features of
Contextual::Return. For example:
sub verbosity :lvalue {
LVALUE { $verbosity_level = int max(0, min($_, 5)) }
RVALUE {
NUM { $verbosity_level }
STR { $description[$verbosity_level] }
BOOL { $verbosity_level > 2 }
}
}
but the context sequence must be nested inside an "RVALUE" block.
You can also specify what an lvalue subroutine should do when it is
used neither as an lvalue nor as an rvalue (i.e. in void context), by
using an "NVALUE" block:
sub verbosity :lvalue {
my ($level) = @_;
NVALUE { $verbosity_level = int max(0, min($level, 5)) }
LVALUE { $verbosity_level = int max(0, min($_, 5)) }
RVALUE {
NUM { $verbosity_level }
STR { $description[$verbosity_level] }
BOOL { $verbosity_level > 2 }
}
}
In this example, a call to "verbosity()" in void context sets the
verbosity level to whatever argument is passed to the subroutine:
verbosity(1);
Note that you cannot get the same effect by nesting a "VOID" block
within an "RVALUE" block:
LVALUE { $verbosity_level = int max(0, min($_, 5)) }
RVALUE {
NUM { $verbosity_level }
STR { $description[$verbosity_level] }
BOOL { $verbosity_level > 2 }
VOID { $verbosity_level = $level } # Wrong!
}
That's because, in a void context the return value is never evaluated,
so it is never treated as an rvalue, which means the "RVALUE" block
never executes.
Result blocks
Occasionally, it's convenient to calculate a return value before the
end of a contextual return block. For example, you may need to clean up
external resources involved in the calculation after it's complete.
Typically, this requirement produces a slightly awkward code sequence
like this:
return
VALUE {
$db->start_work();
my $result = $db->retrieve_query($query);
$db->commit();
$result;
}
Such code sequences become considerably more awkward when you want the
return value to be context sensitive, in which case you have to write
either:
return
LIST {
$db->start_work();
my @result = $db->retrieve_query($query);
$db->commit();
@result;
}
SCALAR {
$db->start_work();
my $result = $db->retrieve_query($query);
$db->commit();
$result;
}
or, worse:
return
VALUE {
$db->start_work();
my $result = LIST ? [$db->retrieve_query($query)]
: $db->retrieve_query($query);
$db->commit();
LIST ? @{$result} : $result;
}
To avoid these infelicities, Contextual::Return provides a second way
of setting the result of a context block; a way that doesn't require
that the result be the last statement in the block:
return
LIST {
$db->start_work();
RESULT { $db->retrieve_query($query) };
$db->commit();
}
SCALAR {
$db->start_work();
RESULT { $db->retrieve_query($query) };
$db->commit();
}
The presence of a "RESULT" block inside a contextual return block
causes that block to return the value of the final statement of the
"RESULT" block as the handler's return value, rather than returning the
value of the handler's own final statement. In other words, the
presence of a "RESULT" block overrides the normal return value of a
context handler.
Better still, the "RESULT" block always evaluates its final statement
in the same context as the surrounding "return", so you can just write:
return
VALUE {
$db->start_work();
RESULT { $db->retrieve_query($query) };
$db->commit();
}
and the "retrieve_query()" method will be called in the appropriate
context in all cases.
A "RESULT" block can appear anywhere inside any contextual return
block, but may not be used outside a context block. That is, this is an
error:
if ($db->closed) {
RESULT { undef }; # Error: not in a context block
}
return
VALUE {
$db->start_work();
RESULT { $db->retrieve_query($query) };
$db->commit();
}
Post-handler clean-up
If a subroutine uses an external resource, it's often necessary to
close or clean-up that resource after the subroutine ends...regardless
of whether the subroutine exits normally or via an exception.
Typically, this is done by encapsulating the resource in a lexically
scoped object whose constructor does the clean-up. However, if the
clean-up doesn't involve deallocation of an object (as in the
"$db->commit()" example in the previous section), it can be annoying to
have to create a class and allocate a container object, merely to
mediate the clean-up.
To make it easier to manage such resources, Contextual::Return supplies
a special labelled block: the "RECOVER" block. If a "RECOVER" block is
specified as part of a contextual return sequence, that block is
executed after any context handler, even if the context handler exits
via an exception.
So, for example, you could implement a simple commit-or-revert policy
like so:
return
LIST { $db->retrieve_all($query) }
SCALAR { $db->retrieve_next($query) }
RECOVER {
if ($@) {
$db->revert();
}
else {
$db->commit();
}
}
The presence of a "RECOVER" block also intercepts all exceptions thrown
in any other context block in the same contextual return sequence. Any
such exception is passed into the "RECOVER" block in the usual manner:
via the $@ variable. The exception may be rethrown out of the "RECOVER"
block by calling "die":
return
LIST { $db->retrieve_all($query) }
DEFAULT { croak "Invalid call (not in list context)" }
RECOVER {
die $@ if $@; # Propagate any exception
$db->commit(); # Otherwise commit the changes
}
A "RECOVER" block can also access or replace the returned value, by
invoking a "RESULT" block. For example:
return
LIST { attempt_to_generate_list_for(@_) }
SCALAR { attempt_to_generate_count_for(@_) }
RECOVER {
if ($@) { # On any exception...
RESULT { undef } # ...return undef
}
}
INTERFACE
Context tests
"LIST()"
Returns true if the current subroutine was called in list context.
A cleaner way of writing: "wantarray()"
"SCALAR()"
Returns true if the current subroutine was called in scalar
context. A cleaner way of writing: "defined wantarray() && !
wantarray()"
"VOID()"
Returns true if the current subroutine was called in void context.
A cleaner way of writing: "!defined wantarray()"
"NONVOID()"
Returns true if the current subroutine was called in list or scalar
context. A cleaner way of writing: "defined wantarray()"
Standard contexts
"LIST {...}"
The block specifies what the context sequence should evaluate to
when called in list context.
"SCALAR {...}"
The block specifies what the context sequence should evaluate to in
scalar contexts, unless some more-specific specifier scalar context
specifier (see below) also occurs in the same context sequence.
"VOID {...}"
The block specifies what the context sequence should do when called
in void context.
Scalar value contexts
"BOOL {...}"
The block specifies what the context sequence should evaluate to
when treated as a boolean value.
"NUM {...}"
The block specifies what the context sequence should evaluate to
when treated as a numeric value.
"STR {...}"
The block specifies what the context sequence should evaluate to
when treated as a string value.
"LAZY {...}"
Another name for "SCALAR {...}". Usefully self-documenting when the
primary purpose of the contextual return is to defer evaluation of
the return value until it's actually required.
Scalar reference contexts
"SCALARREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to a scalar.
"ARRAYREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to an array.
"HASHREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to a hash.
Note that a common error here is to write:
HASHREF { a=>1, b=>2, c=>3 }
The curly braces there are a block, not a hash constructor, so the
block doesn't return a hash reference and the interpreter throws an
exception. What's needed is:
HASHREF { {a=>1, b=>2, c=>3} }
in which the inner braces are a hash constructor.
"CODEREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to a subroutine.
"GLOBREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to a typeglob.
"OBJREF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference to an object.
Generic contexts
"VALUE {...}"
The block specifies what the context sequence should evaluate to
when treated as a non-referential value (as a boolean, numeric,
string, scalar, or list). Only used if there is no more-specific
value context specifier in the context sequence.
"REF {...}"
The block specifies what the context sequence should evaluate to
when treated as a reference of any kind. Only used if there is no
more-specific referential context specifier in the context
sequence.
"NONVOID {...}"
The block specifies what the context sequence should evaluate to
when used in a non-void context of any kind. Only used if there is
no more-specific context specifier in the context sequence.
"DEFAULT {...}"
The block specifies what the context sequence should evaluate to
when used in a void or non-void context of any kind. Only used if
there is no more-specific context specifier in the context
sequence.
Failure context
"FAIL"
This block is executed unconditionally and is used to indicate
failure. In a Boolean context it return false. In all other
contexts it throws an exception consisting of the final evaluated
value of the block.
That is, using "FAIL":
return
FAIL { "Could not defenestrate the widget" }
is exactly equivalent to writing:
return
BOOL { 0 }
DEFAULT { croak "Could not defenestrate the widget" }
except that the reporting of errors is a little smarter under
"FAIL".
If "FAIL" is called without specifying a block:
return FAIL;
it is equivalent to:
return FAIL { croak "Call to <subname> failed" }
(where "<subname>" is replaced with the name of the surrounding
subroutine).
Note that, because "FAIL" implicitly covers every possible return
context, it cannot be chained with other context specifiers.
"Contextual::Return::FAIL_WITH"
This subroutine is not exported, but may be called directly to
reconfigure "FAIL" behaviour in the caller's namespace.
The subroutine is called with an optional string (the flag),
followed by a mandatory hash reference (the configurations hash),
followed by a list of zero-or-more strings (the selector list). The
values of the configurations hash must all be subroutine
references.
If the optional flag is specified, "FAIL_WITH" searches the
selector list looking for that string, then uses the following item
in the selector list as its selector value. If that selector value
is a string, "FAIL_WITH" looks up that key in the hash, and
installs the corresponding subroutine as the namespace's "FAIL"
handler (an exception is thrown if the selector string is not a
valid key of the configurations hash). If the selector value is a
subroutine reference, "FAIL_WITH" installs that subroutine as the
"FAIL" handler.
If the optional flag is not specified, "FAIL_WITH" searches the
entire selector list looking for the last element that matches any
key in the configurations hash. It then looks up that key in the
hash, and installs the corresponding subroutine as the namespace's
"FAIL" handler.
See "Configurable failure contexts" for examples of using this
feature.
Lvalue contexts
"LVALUE"
This block is executed when the result of an ":lvalue" subroutine
is assigned to. The assigned value is passed to the block as $_. To
access the caller's $_ value, use $CALLER::_.
"RVALUE"
This block is executed when the result of an ":lvalue" subroutine
is used as an rvalue. The final value that is evaluated in the
block becomes the rvalue.
"NVALUE"
This block is executed when an ":lvalue" subroutine is evaluated in
void context.
Explicit result blocks
"RESULT"
This block may only appear inside a context handler block. It
causes the surrounding handler to return the final value of the
"RESULT"'s block, rather than the final value of the handler's own
block. This override occurs regardless of the location to the
"RESULT" block within the handler.
Recovery blocks
"RECOVER"
If present in a context return sequence, this block grabs control
after any context handler returns or exits via an exception. If an
exception was thrown it is passed to the "RECOVER" block via the $@
variable.
Modifiers
"FIXED"
This specifies that the scalar value will only be evaluated once,
the first time it is used, and that the value will then morph into
that evaluated value.
"ACTIVE"
This specifies that the scalar value's originating block will be
re- evaluated every time the return value is used.
DIAGNOSTICS
Can't call %s in %s context";
The subroutine you called uses a contextual return, but doesn't
specify what to return in the particular context in which you
called it. You either need to change the context in which you're
calling the subroutine, or else add a context block corresponding
to the offending context (or perhaps a "DEFAULT {...}" block).
%s can't return a %s reference";
You called the subroutine in a context that expected to get back a
reference of some kind but the subroutine didn't specify the
corresponding "SCALARREF", "ARRAYREF", "HASHREF", "CODEREF",
"GLOBREF", or generic "REF", "NONVOID", or "DEFAULT" handlers. You
need to specify the appropriate one of these handlers in the
subroutine.
Can't call method '%s' on %s value returned by %s";
You called the subroutine and then tried to call a method on the
return value, but the subroutine returned a classname or object
that doesn't have that method. This probably means that the
subroutine didn't return the classname or object you expected. Or
perhaps you need to specify an "OBJREF {...}" context block.
Can't install two %s handlers
You attempted to specify two context blocks of the same name in the
same return context, which is ambiguous. For example:
sub foo: lvalue {
LVALUE { $foo = $_ }
RVALUE { $foo }
LVALUE { $foo = substr($_,1,10) }
}
or:
sub bar {
return
BOOL { 0 }
NUM { 1 }
STR { "two" }
BOOL { 1 };
}
Did you cut-and-paste wrongly, or mislabel one of the blocks?
Expected a %s block after the %s block but found instead: %s
If you specify any of "LVALUE", "RVALUE", or "NVALUE", then you can
only specify "LVALUE", "RVALUE", or "NVALUE" blocks in the same
return context. If you need to specify other contexts (like
"BOOL", or "STR", or "REF", etc.), put them inside an "RVALUE"
block. See "Lvalue contexts" for an example.
Call to %s failed at %s.
This is the default exception that a "FAIL" throws in a non-scalar
context. Which means that the subroutine you called has signalled
failure by throwing an exception, and you didn't catch that
exception. You should either put the call in an "eval {...}" block
or else call the subroutine in boolean context instead.
Call to %s failed at %s. Failure value used at %s
This is the default exception that a "FAIL" throws when a failure
value is captured in a scalar variable and later used in a non-
boolean context. That means that the subroutine you called must
have failed, and you didn't check the return value for that
failure, so when you tried to use that invalid value it killed your
program. You should either put the original call in an "eval {...}"
or else test the return value in a boolean context and avoid using
it if it's false.
Usage: FAIL_WITH $flag_opt, \%selector, @args
The "FAIL_WITH" subroutine expects an optional flag, followed by a
reference to a configuration hash, followed by a list or selector
arguments. You gave it something else. See "Configurable Failure
Contexts".
Selector values must be sub refs
You passed a configuration hash to "FAIL_WITH" that specified non-
subroutines as possible "FAIL" handlers. Since non-subroutines
can't possibly be handlers, maybe you forgot the "sub" keyword
somewhere?
Invalid option: %s => %s
The "FAIL_WITH" subroutine was passed a flag/selector pair, but the
selector was not one of those allowed by the configuration hash.
FAIL handler for package %s redefined
A warning that the "FAIL" handler for a particular package was
reconfigured more than once. Typically that's because the module
was loaded in two places with difference configurations specified.
You can't reasonably expect two different sets of behaviours from
the one module within the one namespace.
CONFIGURATION AND ENVIRONMENTContextual::Return requires no configuration files or environment
variables.
DEPENDENCIES
Requires version.pm and Want.pm.
INCOMPATIBILITIES
None reported.
BUGS AND LIMITATIONS
No bugs have been reported.
AUTHOR
Damian Conway "<DCONWAY@cpan.org>"
LICENCE AND COPYRIGHT
Copyright (c) 2005-2006, Damian Conway "<DCONWAY@cpan.org>". All rights
reserved.
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
DISCLAIMER OF WARRANTY
BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT
WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND,
EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR, OR CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
DAMAGES.
perl v5.14.1 2007-03-29 Contextual::Return(3)