NAME¶
constant::defer -- constant subs with deferred value calculation
SYNOPSIS¶
use constant::defer FOO => sub { return $some + $thing; },
BAR => sub { return $an * $other; };
use constant::defer MYOBJ => sub { require My::Class;
return My::Class->new_thing; }
DESCRIPTION¶
"constant::defer" creates a subroutine which on the first call runs
given code to calculate its value, and on the second and subsequent calls just
returns that value, like a constant. The value code is discarded once run,
allowing it to be garbage collected.
Deferring a calculation is good if it might take a lot of work or produce a big
result, but is only needed sometimes or only well into a program run. If it's
never needed then the value code never runs.
A deferred constant is generally not inlined or folded (see "Constant
Folding" in perlop) like a plain "constant" since it's not a
single scalar value. In the current implementation a deferred constant becomes
a plain one after the first use, so may inline etc in code compiled after that
(see "IMPLEMENTATION" below).
Uses¶
Here are some typical uses.
- •
- A big value or slow calculation only sometimes needed,
use constant::defer SLOWVALUE => sub {
long calculation ...;
return $result;
};
if ($option) {
print "s=", SLOWVALUE, "\n";
}
- •
- A shared object instance created when needed then re-used,
use constant::defer FORMATTER =>
sub { return My::Formatter->new };
if ($something) {
FORMATTER()->format ...
}
- •
- The value code might load requisite modules too, again deferring that
until actually needed,
use constant::defer big => sub {
require Some::Big::Module;
return Some::Big::Module->create_something(...);
};
- •
- Once-only setup code can be created with no return value. The code is
garbage collected after the first run and becomes a do-nothing. Remember
to have an empty return statement so as not to keep the last expression's
value alive forever.
use constant::defer MY_INIT => sub {
many lines of setup code ...;
return;
};
sub new {
MY_INIT();
...
}
IMPORTS¶
There are no functions as such, everything works through the "use"
import.
- "use constant::defer NAME1=>SUB1, NAME2=>SUB2, ...;"
- The parameters are name/subroutine pairs. For each one a sub called
"NAME" is created, running the given "SUB" the first
time its value is needed.
"NAME" defaults to the caller's package, or a fully qualified name
can be given. Remember that the bareword stringizing of "=>"
doesn't act on a qualified name, so add quotes in that case.
use constant::defer 'Other::Package::BAR' => sub { ... };
For compatibility with the "constant" module a hash of name/sub
arguments is accepted too. But "constant::defer" doesn't need
that since there's only ever one thing (a sub) following each name.
use constant::defer { FOO => sub { ... },
BAR => sub { ... } };
# works without the hashref too
use constant::defer FOO => sub { ... },
BAR => sub { ... };
MULTIPLE VALUES¶
The value sub can return multiple values to make an array style constant sub.
use constant::defer NUMS => sub { return ('one', 'two') };
foreach (NUMS) {
print $_,"\n";
}
The value sub is always run in array context, for consistency, irrespective how
the constant is used. The return from the new constant sub is an array style
sub () { return @result }
If the value sub was a list-style return like "NUMS" shown above, then
this array-style return is slightly different. In scalar context a list return
means the last value (like a comma operator), but an array return in scalar
context means the number of elements. A multi-value constant won't normally be
used in scalar context, so the difference shouldn't arise. The array style is
easier for "constant::defer" to implement and is the same as the
plain "constant" module does.
ARGUMENTS¶
If the constant is called with arguments then they're passed on to the value
sub. This can be good for constants used as object or class methods. Passing
anything to plain constants would be unusual.
One cute use for a class method style is to make a "singleton"
instance of the class. See
examples/instance.pl in the sources for a
complete program.
package My::Class;
use constant::defer INSTANCE => sub { my ($class) = @_;
return $class->new };
package main;
$obj = My::Class->INSTANCE;
A subclass might want to be careful about letting a subclass object get into the
parent "INSTANCE", though if a program only ever used the subclass
then that might in fact be desirable.
Subs created by "constant::defer" always have prototype
"()", ensuring they always parse the same way. The prototype has no
effect when called as a method like above, but if you want a plain call with
arguments then use "&" to bypass the prototype (see perlsub).
&MYCONST ('Some value');
IMPLEMENTATION¶
Currently "constant::defer" creates a sub under the requested name and
when called it replaces that with a new constant sub the same as "use
constant" would make. This is compact and means that later loaded code
might be able to inline it.
It's fine to keep a reference to the initial sub and in fact that happens quite
normally if importing into another module (with the usual
"Exporter"), or an explicit "\&foo", or a
"$package->can('foo')". The initial sub changes itself to jump to
the new constant, it doesn't re-run the value code.
The jump is currently done by a "goto" to the new coderef, so it's a
touch slower than the new constant sub directly. A spot of XS would no doubt
make the difference negligible, in fact perhaps to the point where there'd be
no need for a new sub, just have the initial transform itself. If the new form
looked enough like a plain constant it might inline in later loaded code.
For reference, "Package::Constants" (as of version 0.02) considers
"constant::defer" subrs as constants, both before and after the
first call that runs the value code. "Package::Constants" just looks
for prototyped "sub foo () { }"
functions, so any such subr rates as a constant.
OTHER WAYS TO DO IT¶
There's many ways to do "deferred" or "lazy" calculations.
- •
- "Memoize" makes a function repeat its return. Results are cached
against the arguments, so it keeps the original code whereas
"constant::defer" discards after the first run.
- •
- "Class::Singleton" and friends make a create-once
"My::Class->instance" method. "constant::defer" can
get close with the fakery shown under "ARGUMENTS" above, though
without a "has_instance" to query.
- •
- "Sub::Become" offers some syntactic sugar for redefining the
running subroutine, including to a constant.
- •
- "Sub::SingletonBuilder" can create an instance function for a
class. It's geared towards objects and so won't allow 0 or
"undef" as the return value.
- •
- A scalar can be rigged up to run code on its first access.
"Data::Lazy" uses a "tie". "Scalar::Defer"
and "Scalar::Lazy" use "overload" on an object.
"Data::Thunk" optimizes out the object from
"Scalar::Defer" after the first run. "Variable::Lazy"
uses XS magic removed after the first fetch and some parsing for syntactic
sugar.
The advantage of a variable is that it interpolates in strings, but it won't
inline in later loaded code; sloppy XS code might bypass the magic; and
package variables aren't very friendly when subclassing.
- •
- "Object::Lazy" and "Object::Trampoline" rig up an
object wrapper to load and create an actual object only when a method is
called, dispatching to it and replacing the callers $_[0]. The advantage
is you can pass the wrapper object around, etc, deferring creation to an
even later point than a sub or scalar can.
- •
- "Object::Realize::Later", "Class::LazyObject" and
"Class::LazyFactory" help make a defer class which transforms
lazy stub objects to real ones when a method call is made. A separate
defer class is required for each real class.
- •
- "once.pm" sets up a run-once code block, but with no particular
return value and not discarding the code after run.
- •
- "Class::LazyLoad" and "deferred" load code on a class
method call such as object creation. They're more about module loading
than a defer of a particular value.
- •
- "Tie::LazyList" and "Tie::Array::Lazy" makes an array
calculate values on-demand from a generator function.
"Hash::Lazy" does a similar thing for hashes.
"Tie::LazyFunction" hides a function behind a scalar; its
laziness is in the argument evaluation, the function is called every
time.
SEE ALSO¶
constant, perlsub, constant::lexical
Memoize, Attribute::Memoize, Memoize::Attrs, Class::Singleton, Data::Lazy,
Scalar::Defer, Scalar::Lazy, Data::Thunk, Variable::Lazy, Sub::Become,
Sub::SingletonBuilder, Object::Lazy, Object::Trampoline,
Object::Realize::Later, once, Class::LazyLoad, deferred
HOME PAGE¶
http://user42.tuxfamily.org/constant-defer/index.html
COPYRIGHT¶
Copyright 2009, 2010, 2011 Kevin Ryde
constant-defer is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 3, or (at your option) any later version.
constant-defer is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with
constant-defer. If not, see <
http://www.gnu.org/licenses/>.