Data::Sah::DefaultValue - Default-value rules for Data::Sah


This document describes version 0.003 of Data::Sah::DefaultValue (from Perl distribution Data-Sah-DefaultValue), released on 2023-03-30.


 use Data::Sah::DefaultValue  qw(gen_default_value_code);

 # a utility routine: gen_default_value_code
 my $dv = gen_default_value_code(
     default_value_rules => ['Perl::this_mod'],

 my $val = $c->(123);          # unchanged, 123
 my $val = $c->(undef);        # becomes "Some::Module"


This module generates code to set default value using value rules.

A value rule is put in Data::Sah::Value::$COMPILER::$TOPIC::$DESCRIPTION module, for example: Data::Sah::Value::perl::Perl::this_mod contains the value "this module" (see App::ThisDist for more details on the meaning of "this module").

Basically, a value rule will provide an expression (expr_value) that return some value.

The module must contain meta subroutine which must return a hashref that has the following keys (* marks that the key is required):

  • v* => int (default: 1)

    Metadata specification version. From DefHash. Currently at 4.

  • summary => str

    From DefHash.

The module must also contain default_value subroutine which must generate the code for default value. The subroutine must accept a hash of arguments (* indicates required arguments):

The value subroutine must return a hashref with the following keys (* indicates required keys):

  • expr_value => str

    Expression in the target language that produces the value.

  • modules => hash

    A list of modules required by the expressions.


$Log_Default_Value_Code => bool (default: from ENV or 0)

If set to true, will log the generated default-value code (currently using Log::ger at trace level). To see the log message, e.g. to the screen, you can use something like:

 % TRACE=1 perl -MLog::ger::LevelFromEnv -MLog::ger::Output=Screen \
     -MData::Sah::DefaultValue=gen_default_value_code -E'my $c = gen_default_value_code(...)'




 gen_default_value_code(%args) -> any

Generate code to set default value.

This is mostly for testing. Normally the default value rules will be used from Data::Sah via the x.perl.default_value_rules or x.js.default_value_rules or x.default_value_rules property.

This function is not exported by default, but exportable.

Arguments ('*' denotes required arguments):

  • default_value_rules => array[str]

    A specification of default-value rules to use (or avoid).

    This setting is used to specify which default-value rules to use (or avoid) in a flexible way. Each element is a string, in the form of either NAME to mean specifically include a rule, or !NAME to exclude a rule.

    To use the default-value rules R1 and R2:

     ['R1', 'R2']
  • source => bool

    If set to true, will return coercer source code string instead of compiled code.

Return value: (any)



Set default for $Log_Default_Value_Code.


Please visit the project's homepage at


Source repository is at


Data::Sah::DefaultValueCommon for detailed syntax of default-value rules (explicitly including/excluding rules etc).





2021-11-28: Created modelled from Data::Sah::Coerce and Data::Sah::Filter to be able to express dynamic default value into schema.


perlancar <>


To contribute, you can send patches by email/via RT, or send pull requests on GitHub.

Most of the time, you don't need to build the distribution yourself. You can simply modify the code, then test via:

 % prove -l

If you want to build the distribution (e.g. to try to install it locally on your system), you can install Dist::Zilla, Dist::Zilla::PluginBundle::Author::PERLANCAR, Pod::Weaver::PluginBundle::Author::PERLANCAR, and sometimes one or two other Dist::Zilla- and/or Pod::Weaver plugins. Any additional steps required beyond that are considered a bug and can be reported to me.


This software is copyright (c) 2023, 2021 by perlancar <>.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.


Please report any bugs or feature requests on the bugtracker website

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.