NAME
Constant::Export::Lazy - Utility to write lazy exporters of constant
subroutines
SYNOPSIS
This is an example of a "My::Constants" package that you can write using
"Constant::Export::Lazy" that demonstrates most of its main features.
This is from the file t/lib/My/Constants.pm in the source distro:
package My::Constants;
use strict;
use warnings;
use Exporter 'import';
use constant {
X => -2,
Y => -1,
};
our @EXPORT_OK = qw(X Y);
use Constant::Export::Lazy (
constants => {
# This is the simplest way to go, just define plain constant
# values.
A => sub { 1 },
B => sub { 2 },
# You get a $ctx object that you can ->call() to retrieve the
# values of other constants. This is how you can make some
# constants depend on others without worrying about
# ordering. Constants are still guaranteed to only be
# fleshened once!
SUM => sub {
my ($ctx) = @_;
$ctx->call('A') + $ctx->call('B'),
},
# For convenience you can also access other constants,
# e.g. those defined with constant.pm
SUM_INTEROP => sub {
my ($ctx) = @_;
$ctx->call('X') + $ctx->call('Y'),
},
# We won't call this and die unless someone requests it when
# they import us.
DIE => sub { die },
# These subroutines are always called in scalar context, and
# thus We'll return [3..4] here.
#
# Unlike the constant.pm that ships with perl itself we don't
# support returning lists. So if you want to return lists you
# have to return a reference to one.
LIST => sub { wantarray ? (1..2) : [3..4] },
# We can also supply a HashRef with "call" with the sub, and
# "options" with options that clobber the global
# options. Actually when you supply just a plain sub instead
# of a HashRef we internally munge it to look like this more
# verbose (and more flexible) structure.
PI => {
call => sub { 3.14 },
options => {
override => sub {
my ($ctx, $name) = @_;
# You can simply "return;" here to say "I don't
# want to override", and "return undef;" if you
# want the constant to be undef.
return $ENV{PI} ? "Pi is = $ENV{PI}" : $ctx->call($name);
},
# This is an optional ref that'll be accessible via
# $ctx->stash in any subs relevant to this constant
# (call, override, after, ...)
stash => {
# This `typecheck_rx` is in no way supported by
# Constant::Export::Lazy, it's just something
# we're passing around to the 'after' sub below.
typecheck_rx => qr/\d+\.\d+/s, # such an epicly buggy typecheck...
},
},
},
},
options => {
# We're still exporting some legacy constants via Exporter.pm
wrap_existing_import => 1,
# A general override so you can override other constants in
# %ENV
override => sub {
my ($ctx, $name) = @_;
return unless exists $ENV{$name};
return $ENV{$name};
},
after => sub {
my ($ctx, $name, $value, $source) = @_;
if (defined(my $stash = $ctx->stash)) {
my $typecheck_rx = $stash->{typecheck_rx};
die "PANIC: The value <$value> for <$name> doesn't pass <$typecheck_rx>"
unless $value =~ $typecheck_rx;
}
print STDERR "Defined the constant <$name> with value <$value> from <$source>\n" if $ENV{DEBUG};
return;
},
},
);
1;
And this is an example of using it in some user code (from t/synopsis.t
in the source distro):
package My::User::Code;
use strict;
use warnings;
use Test::More qw(no_plan);
use lib 't/lib';
BEGIN {
# Supply a more accurate PI
$ENV{PI} = 3.14159;
# Override B
$ENV{B} = 3;
}
use My::Constants qw(
X
Y
A
B
SUM
SUM_INTEROP
PI
LIST
);
is(X, -2);
is(Y, -1);
is(A, 1);
is(B, 3);
is(SUM, 4);
is(SUM_INTEROP, -3);
is(PI, "Pi is = 3.14159");
is(join(",", @{LIST()}), '3,4');
And running it gives:
$ DEBUG=1 perl -Ilib t/synopsis.t
Defined the constant <A> with value <1> from <callback>
Defined the constant <B> with value <3> from <override>
Defined the constant <SUM> with value <4> from <callback>
Defined the constant <SUM_INTEROP> with value <-3> from <callback>
Defined the constant <PI> with value <Pi is = 3.14159> from <override>
Defined the constant <LIST> with value <ARRAY(0x16b8918)> from <callback>
ok 1
ok 2
ok 3
ok 4
ok 5
ok 6
ok 7
ok 8
1..8
By default we only support importing constants explicitly by their own
names and not something like Exporter's @EXPORT, @EXPORT_OK or
%EXPORT_TAGS, but you can trivially add support for that (or any other
custom import munging) using the "buildargs" callback. This example is
from t/lib/My/Constants/Tags.pm in the source distro:
package My::Constants::Tags;
use v5.10;
use strict;
use warnings;
use Constant::Export::Lazy (
constants => {
KG_TO_MG => sub { 10**6 },
SQRT_2 => {
call => sub { sqrt(2) },
options => {
stash => {
export_tags => [ qw/:math/ ],
},
},
},
PI => {
call => sub { atan2(1,1) * 4 },
options => {
stash => {
export_tags => [ qw/:math/ ],
},
},
},
map(
{
my $t = $_;
+(
$_ => {
call => sub { $t },
options => {
stash => {
export_tags => [ qw/:alphabet/ ],
},
}
}
)
}
"A".."Z"
),
},
options => {
buildargs => sub {
my ($import_args, $constants) = @_;
state $export_tags = do {
my %export_tags;
for my $constant (keys %$constants) {
my @export_tags = @{$constants->{$constant}->{options}->{stash}->{export_tags} || []};
push @{$export_tags{$_}} => $constant for @export_tags;
}
\%export_tags;
};
my @gimme = map {
/^:/ ? @{$export_tags->{$_}} : $_
} @$import_args;
return \@gimme;
},
},
);
1;
And this is an example of using it in some user code (from
t/synopsis_tags.t in the source distro):
package My::More::User::Code;
use strict;
use warnings;
use Test::More qw(no_plan);
use lib 't/lib';
use My::Constants::Tags qw(
KG_TO_MG
:math
:alphabet
);
is(KG_TO_MG, 10**6);
is(A, "A");
is(B, "B");
is(C, "C");
like(PI, qr/^3\.14/);
And running it gives:
$ perl -Ilib t/synopsis_tags.t
ok 1
ok 2
ok 3
ok 4
ok 5
1..5
DESCRIPTION
This is a library to write lazy exporters of constant subroutines. It's
not meant to be a user-facing constant exporting API, it's something you
use to write user-facing constant exporting APIs.
There's dozens of modules on the CPAN that define constants in one way
or another, why did I need to write this one?
It's lazy
Our constants are fleshened via callbacks that are guaranteed to be
called only once for the lifetime of the process (not once per importer
or whatever), and we only call the callbacks lazily if someone actually
requests that a constant of ours be defined.
This makes it easy to have one constant exporting module that runs in
different environments, and generates some subset of its constants
depending on what the program that's using it actually needs.
Some data that you may want to turn into constants may require modules
that aren't available everywhere, queries to databases that aren't
available everywhere, or make certain assumptions about the environment
they're running under that may not be true across all your environments.
By only defining those constants you actually need via callbacks
managing all these special-cases becomes a lot easier.
It makes it easier to manage creating constants that require other constants
Maybe you have one constant indicating whether you're running in a dev
environment, and a bunch of other constants that are defined differently
if the dev environment constant is true.
Now say you have several hundred constants like that, managing the
inter-dependencies and ensuring that they're all defined in the right
order with dependencies before dependents quickly gets messy.
All this complexity becomes a non-issue when you use this module. When
you define a constant you get a callback object that can give you the
value of other constants.
When you look up another constant we'll either generate it if it hasn't
been materialized yet, or look up the materialized value in the symbol
table if it has.
Thus we end up with a Makefile-like system where you can freely use
whatever other constants you like when defining your constants, and
we'll lazily define the entire tree of constants on-demand.
You only have to be careful not to introduce circular dependencies.
API
Our API is exposed via a nested key-value pair list passed to "use", see
the "SYNOPSIS" for an example. Here's description of the data structure
you can pass in:
constants
This is a key-value pair list of constant names to either a subroutine
or a hash with "call" and optional options. Internally we just convert
the former type of call into the latter, i.e. "CONST => sub {...}"
becomes "CONST => { call => sub { ... } }".
call
The subroutine we'll call with a context object to fleshen the constant.
It's guaranteed that this sub will only ever be called once for the
lifetime of the process, except if you manually call it multiple times
during an "override".
options (local)
Our options hash to override the global "options". The semantics are
exactly the same as for the global hash.
options
We support various options, most of these can be defined either globally
if you want to use them for all the constants, or locally to one
constant at a time with the more verbose hash invocation to "constants".
The following options are supported:
buildargs
A callback that can only be supplied as a global option. If you provide
this the callback we'll call it to munge any parameters to import we
might get. This can be used (as shown in the synopsis) to strip or map
parameters to e.g. implement support for %EXPORT_TAGS, or to do any
other arbitrary mapping.
This callback will be called with a reference to the parameters passed
to import, and for convenience with the constants hash you provided
(e.g. for introspecting the stashes of constants, see the synopsis
example.
This is expected to return an array with a list of constants to import,
or the empty list if we should discard the return value of this callback
and act is if it wasn't present at all.
This plays nice with the "wrap_existing_import" parameter. When it's in
force any constant names (or tag names, or whatever) you return that we
don't know about ourselves we'll pass to the fallback import subroutine
we're wrapping as we would if buildargs hadn't been defined.
wrap_existing_import
A boolean that can only be supplied as a global option. If you provide
this the package you're importing us into has to already have a defined
"import" subroutine.
We'll clobber it with something that uses us to export all the constants
we know about (i.e. the ones passed to "constants"), but anything we
don't know about will be passed to the "import" subroutine we clobbered.
This is handy for converting existing packages that use e.g. a
combination of Exporter to export a bunch of constant constants without
having to port them all over to "Constant::Export::Lazy" at the same
time. This allows you to do so incrementally.
For convenience we also support calling these foreign subroutines with
"$ctx->call($name)". This is handy because when migrating an existing
package you can already start calling existing constants with our
interface, and then when you migrate those constants over you won't have
to change any of the old code.
We'll handle calling subroutines generated with perl's own constant.pm
(including "list" constants), but we'll die in "call" if we call a
foreign subroutine that returns more than one value, i.e. constants
defined as "use constant FOO =" (1, 2, 3)> instead of "use constant FOO
=" [1, 2, 3]>.
If this isn't set and the class we're being imported into already has an
"import" subroutine we'll die.
There's a caveat with this related to how we check for an existing
"import" subroutine. We don't use "UNIVERSAL::can", instead we manually
check the symbol table for the package we're being imported into.
So this won't do the "right" thing if we're being imported into a
package that doesn't have its own "import" subroutine, but gets it via a
base class, we'll just silently shadow that "import" routine.
The reason for this caveat is that if someone's ruined your day and
imported the UNIVERSAL package "UNIVERSAL::can("import")" will return
true for every single package, our check for an existing "import" will
always return true, so those packages that are using us without setting
"wrap_existing_import" will all fail.
Maybe we could deal with this in some better way, e.g. do the "right"
thing and e.g. not die if we detect an "import" routine and UNIVERSAL is
loaded, but this edge case is really obscure, and I doubt anyone
actually needs to use this package to export constants from a package
that has a base class with an existing import routine.
override
This callback can be defined either globally or locally and will be
called instead of your "call". In addition to the context object this
will also get an argument to the $name of the constant that we're
requesting an override for.
This can be used for things like overriding default values based on
entries in %ENV (see the "SYNOPSIS"), or anything else you can think of.
In an override subroutine "return $value" will return a value to be used
instead of the value we'd have retrieved from "call", doing a "return;"
on the other hand means you don't want to use the subroutine to override
this constant, and we'll stop trying to do so and just call "call" to
fleshen it.
You can also get the value of "call" by doing "$ctx->call($name)". We
have some magic around override ensuring that we only get the value, we
don't actually intern it in the symbol table.
This means that calling "$ctx->call($name)" multiple times in the scope
of an override subroutine is the only way to get
"Constant::Export::Lazy" to call a "call" subroutine multiple times. We
otherwise guarantee that these subs are only called once (as discussed
in "It's lazy" and "call").
after
This callback will be called after we've just interned a new constant
into the symbol table. In addition to the context object this will also
get $name, $value and $source arguments. The $name argument is the name
of the constant we just defined, $value is its value, and $source is
either "override" or "callback" depending on how the constant was
defined. I.e. via "override" or directly via "call".
This was added to support replacing modules that in addition to just
defining constants might also want to check them for well-formedness
after they're defined, or push known constants to a hash somewhere so
they can all be retrieved by some complimentary API that e.g. spews out
"all known settings".
You must "return:" from this subroutine, if anything's returned from it
we'll die, this is to reserve any returning of values for future use.
stash
This is a reference that you can provide for your own use, we don't care
what's in it. It'll be accessible via the context object's "stash"
method (i.e. "my $stash = $ctx->stash") for "call", "override" and
"after" calls relevant to its scope, i.e. global if you define it
globally, otherwise local if it's defined locally.
private_name_munger
This callback can be defined either globally or locally. When it's
provided it'll be used to munge the internal name of the subroutine we
define in the exporting package.
This allows for preventing the anti-pattern of user code not importing
constants before using them. To take the example in the synopsis it's
for preventing "My::Constants::PI" and "My::User::Code::PI"
interchangeably, using this facility we can change "My::Constants::PI"
to e.g. "My::Constants::SOME_OPAQUE_VALUE_PI".
This is useful because users used to using other constant modules might
be in the habit of using non-imported and imported names
interchangeably.
This is fine when the constant exporting module isn't lazy, however with
Constant::Export::Lazy this relies on someone else having previously
defined the constant at a distance, and if that someone goes away
this'll silently turn into an error at a distance.
By using the "private_name_munger" option you can avoid this happening
in the first place by specifying a subroutine like:
private_name_munger => sub {
my ($gimme) = @_;
# We guarantee that these constants are always defined by us,
# and we don't want to munge them because legacy code calls
# them directly for historical reasons.
return if $gimme =~ /^ALWAYS_DEFINED_/;
state $now = time();
return $gimme . '_TIME_' . $now;
},
Anyone trying to call that directly from your exporting package as
opposed to importing into their package will very quickly discover that
it doesn't work.
Because this is called really early on this routine doesn't get passed a
$ctx object, just the name of the constant you might want to munge. To
skip munging it return the empty list, otherwise return a munged name to
be used in the private symbol table.
We consider this a purely functional subroutine and you MUST return the
same munged name for the same $gimme because we might resolve that
$gimme multiple times. Failure to do so will result your callbacks being
redundantly re-defined.
CONTEXT OBJECT
As discussed above we pass around a context object to all callbacks that
you can define. See $ctx in the "SYNOPSIS" for examples.
This objects has only two methods:
* "call"
This method will do all the work of fleshening constants via the sub
provided in the "call" option, taking the "override" callback into
account if provided, and if applicable calling the "after" callback
after the constant is defined.
If you call a subroutine you haven't defined yet (or isn't being
imported directly) we'll fleshen it if needed, making sure to only
export it to a user's namespace if explicitly requested.
See "override" for caveats with calling this inside the scope of an
override callback.
* "stash"
An accessor for the "stash" reference, will return the empty list if
there's no stash reference defined.
AUTHOR
Ævar Arnfjörð Bjarmason <avar@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 by Ævar Arnfjörð Bjarmason
<avar@cpan.org>
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.