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Constant::Export::Lazy - Utility to write lazy exporters of constant subroutines


These increasing verbose example of a My::Constants package that you can write using Constant::Export::Lazy demonstrates most of our major features (from t/lib/My/ 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') + $ctx->call('B'),
            # For convenience you can also access other constants,
            # e.g. those defined with
            SUM_INTEROP => sub {
                my ($ctx) = @_;
                $ctx->call('X') + $ctx->call('Y'),
            # We won't call this and die unless someone requests it when
            # they import us.
            DIE => sub { die },
            # These subroutines are always called in scalar context, and
            # thus We'll return [3..4] here.
            # Unlike the that ships with perl itself we don't
            # support returning lists. So if you want to return lists you
            # have to return a reference to one.
            LIST => sub { wantarray ? (1..2) : [3..4] },
            # We can also supply a HashRef with "call" with the sub, and
            # "options" with options that clobber the global
            # options. Actually when you supply just a plain sub instead
            # of a HashRef we internally munge it to look like this more
            # verbose (and more flexible) structure.
            PI => {
                call    => sub { 3.14 },
                options => {
                    override => sub {
                        my ($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
                    # $ctx->stash in any subs relevant to this constant
                    # (call, override, after, ...)
                    stash => {
                        # This `typecheck_rx` is in no way supported by
                        # Constant::Export::Lazy, it's just something
                        # we're passing around to the 'after' sub below.
                        typecheck_rx => qr/\d+\.\d+/s, # such an epicly buggy typecheck...
        options => {
            # We're still exporting some legacy constants via
            wrap_existing_import => 1,
            # A general override so you can override other constants in
            # %ENV
            override => sub {
                my ($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};


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(

    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

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/ 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/ ],
                    my $t = $_;
                        $_ => {
                            call => sub { $t },
                            options => {
                                stash => {
                                    export_tags => [ qw/:alphabet/ ],
        options => {
            buildargs => sub {
                my ($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;

                my @gimme = map {
                    /^:/ ? @{$export_tags->{$_}} : $_
                } @$import_args;

                return \@gimme;


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(

    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


This is a utility 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 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 that you provide 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.

Constant::Import::Lazy takes away all this complexity. 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, look it up 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, you only have to be careful not to introduce circular dependencies.


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:


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 { ... } }>.


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.


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:


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 with the 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.


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 (including "list" constants), but we'll die in call if we call a foreign subroutine that returns more than one value.


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 L/<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 L/<call> subroutine multiple times. We otherwise guarantee that these subs are only called once (as discussed in "It's lazy" and "call").


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".


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 (e.g. <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.


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.


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.


Ævar Arnfjörð Bjarmason <>