NAME
    Data::Sah::Coerce - Coercion rules for Data::Sah

VERSION
    This document describes version 0.028 of Data::Sah::Coerce (from Perl
    distribution Data-Sah-Coerce), released on 2018-12-15.

SYNOPSIS
     use Data::Sah::Coerce qw(gen_coercer);

     # a utility routine: gen_coercer
     my $c = gen_coercer(
         type               => 'date',
         coerce_to          => 'DateTime',
         coerce_rules       => ['str_alami'],  # explicitly enable a rule, etc
         # return_type      => 'str+val',      # default is 'val'
     );

     my $val = $c->(123);          # unchanged, 123
     my $val = $c->(1463307881);   # becomes a DateTime object
     my $val = $c->("2016-05-15"); # becomes a DateTime object
     my $val = $c->("2016foo");    # unchanged, "2016foo"

DESCRIPTION
    This distribution contains a standard set of coercion rules for
    Data::Sah. It is separated from the "Data-Sah" distribution and can be
    used independently.

    A coercion rule is put in
    "Data::Sah::Coerce::$COMPILER::$TARGET_TYPE::$SOURCE_TYPE_AND_EXTRA_DESC
    RIPTION" module, for example: Data::Sah::Coerce::perl::date::float_epoch
    for converting date from integer (Unix epoch) or
    Data::Sah::Coerce::perl::date::str_iso8601 for converting date from
    ISO8601 strings like "2016-05-15".

    Basically, a coercion rule will provide an expression ("expr_match")
    that evaluates to true when data can be coerced, and an expression
    ("expr_coerce") to actually coerce/convert data to the target type. This
    rule can be combined with other rules to form the final coercion code.

    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. Currently at 3.

        History: bumped from 2 to 3 to allow coercion expression to return
        error message explaining why coercion fails. The "might_die"
        metadata property is replaced with "might_fail". When "might_fail"
        is set to true, "expr_coerce" must return array containing error
        message and coerced data, instead of just coerced data.

        History: Bumped from 1 to 2 to exclude old module names.

    *   enable_by_default* => bool

        Whether the rule should be used by default. Some rules might be
        useful in certain situations only and can set this key's value to 0.

        To explicitly enable a disabled-by-default rule or explicitly
        disable an enabled-by-default rule, a Sah schema can set the
        attribute "x.coerce_rules" or "x.perl.coerce_rules" to something
        like "["!str_iso8601", "str_alami"]" (this means to exclude the
        "str_iso8601" rule but enable the "str_alami" rule).

    *   might_fail => bool (default: 0)

        Whether coercion might fail, e.g. because of invalid input. If set
        to 1, "expr_coerce" key that the "coerce()" routine returns must be
        an expression that returns an array (envelope) of "(error_msg,
        data)" instead of just coerced data. Error message should be a
        string that is set when coercion fails and explains why. Otherwise,
        if coercion succeeds, the string should be set to undefined value.

        An example of a rule like this is coercing from string in the form
        of "YYYY-MM-DD" to a DateTime object. The rule might match any
        string in the form of "/\A(\d{4})-(\d{2})-(\d{2})\z/" while it might
        not be a valid date.

    *   prio => int (0-100, default: 50)

        This is to regulate the ordering of rules. The higher the number,
        the lower the priority (meaning the rule will be put further back).
        Rules that are computationally more expensive and/or match more
        broadly in general should be put further back (lower priority,
        higher number).

    *   precludes => array of (str|re)

        List the other rules or rule patterns that are precluded by this
        rule. Rules that are mutually exclusive or pure alternatives to one
        another (e.g. date coercien rules str_alami vs str_alami both parse
        natural language date string; there is usually little to none of
        usefulness in using both; besides, both rules match all string and
        dies when failing to parse the string. So in "str_natural" rule,
        you'll find this metadata:

         precludes => [qr/\Astr_alami(_.+)?\z/]

        and in "str_alami" rule you'll find this metadata:

         precludes => [qr/\Astr_alami(_.+)?\z/, 'str_natural']

        Note that the "str_alami" rule also precludes other "str_alami_*"
        rules (like "str_alami_en" and "str_alami_id").

        Also note that rules which are specifically requested to be used
        (e.g. using "x.perl.coerce_rules" attribute in Sah schema) will
        still be precluded.

    The module must also contain "coerce" subroutine which must generate the
    code for coercion. The subroutine must accept a hash of arguments ("*"
    indicates required arguments):

    *   data_term => str

    *   coerce_to => str

        Some Sah types are "abstract" and can be represented using a choice
        of several actual types in the target programming language. For
        example, "date" can be represented in Perl as an integer (Unix epoch
        value), or a DateTime object, or a Time::Moment object.

        Not all target Sah types will need this argument.

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

    *   expr_match => str

        Expression in the target language to test whether the data can be
        coerced. For example, in
        "Data::Sah::Coerce::perl::date::float_epoch", only integers ranging
        from 10^8 to 2^31 are converted into date. Non-integers or integers
        outside this range are not coerced.

    *   expr_coerce => str

        Expression in the target language to actually convert data to the
        target type.

    *   modules => hash

        A list of modules required by the expressions.

    Basically, the "coerce" subroutine must generates a code that accepts a
    non-undef data and must convert this data to the desired type/format
    under the right condition. The code to match the right condition must be
    put in "expr_match" and the code to convert data must be put in
    "expr_coerce".

    Program/library that uses Data::Sah::Coerce can collect rules from the
    rule modules then compose them into the final code, something like (in
    pseudocode):

     if (data is undef) {
       return undef;
     } elsif (data matches expr-match-from-rule1) {
       return expr-coerce-from-rule1;
     } elsif (data matches expr-match-from-rule2) {
       return expr-coerce-from-rule1;
     ...
     } else {
       # does not match any expr-match
       return original data;
     }

VARIABLES
  $Log_Coercer_Code => bool (default: from ENV or 0)
    If set to true, will log the generated coercer 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::Coerce=gen_coercer -E'my $c = gen_coercer(...)'

FUNCTIONS
  gen_coercer
    Usage:

     gen_coercer() -> any

    Generate coercer code.

    This is mostly for testing. Normally the coercion rules will be used
    from Data::Sah.

    This function is not exported by default, but exportable.

    No arguments.

    Return value: (any)

ENVIRONMENT
  LOG_SAH_COERCER_CODE => bool
    Set default for $Log_Coercer_Code.

HOMEPAGE
    Please visit the project's homepage at
    <https://metacpan.org/release/Data-Sah-Coerce>.

SOURCE
    Source repository is at
    <https://github.com/perlancar/perl-Data-Sah-Coerce>.

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://rt.cpan.org/Public/Dist/Display.html?Name=Data-Sah-Coerce>

    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.

SEE ALSO
    Data::Sah

    Data::Sah::CoerceJS

    App::SahUtils, including coerce-with-sah to conveniently test coercion
    from the command-line.

AUTHOR
    perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2018, 2017, 2016 by perlancar@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.

