NAME

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

VERSION

This document describes version 0.054 of Data::Sah::Coerce (from Perl distribution Data-Sah-Coerce), released on 2023-10-24.

SYNOPSIS

 use Data::Sah::Coerce qw(gen_coercer);
 # a utility routine: gen_coercer
 my $c = gen_coercer(
     type               => 'date',
     coerce_to          => 'DateTime',
     coerce_rules       => ['From_str::natural'],  # explicitly enable a rule, etc. See Data::Sah::CoerceCommon's get_coerce_rules() for detailed syntax
     # 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::$COMPILER::To_$TARGET_TYPE::From_$SOURCE_TYPE::DESCRIPTION" module, for example: Data::Sah::Coerce::perl::To_date::From_float::epoch for converting date from integer (Unix epoch) or Data::Sah::Coerce::perl::To_date::From_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. From DefHash. Currently at 4.

History: bumped from 3 to 4 to remove "enable_by_default" property. Now the list of standard (enabled-by-default) coercion rules is maintained in Data::Sah::Coerce itself. This allows us to skip scanning all Data::Sah::Coerce::* coercion modules installed on the system. Data::Sah::Coerce still accepts version 3; it just ignores the "enable_by_default" property.

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.
summary => str
From DefHash.
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 error message 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.

This is used for coercion rules that act as a data checker.
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 From_str::natural vs From_str::flexible 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 "From_str::natural" rule, you'll find this metadata:
```
 precludes => [qr/\A(From_str::alami(_.+)?|From_str::natural)\z/]
    
```
and in "From_str::flexible" rule you'll find this metadata:
```
 precludes => [qr/\A(From_str::alami(_.+)?|From_str::flexible)\z/]
    
```
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::To_date::From_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(%args) -> 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.

Arguments ('*' denotes required arguments):

coerce_rules => array[str]
A specification of coercion rules to use (or avoid).

This setting is used to specify which coercion 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.

Some coercion modules are used by default, unless explicitly avoided using the '!NAME' rule.

To not use any rules:

To use the default rules plus R1 and R2:
```
 ['R1', 'R2']
    
```
To use the default rules but not R1 and R2:
```
 ['!R1', '!R2']
    
```
coerce_to => str
Some Sah types, like "date", can be represented in a choice of types in the target language. For example, in Perl you can store it as a floating number a.k.a. float(epoch), or as a DateTime object, or Time::Moment object. Storing in DateTime can be convenient for date manipulation but requires an overhead of loading the module and storing in a bulky format. The choice is yours to make, via this setting.
return_type => str (default: "val")
"val" means the coercer will return the input (possibly) coerced or undef if coercion fails.

"bool_coerced+val" means the coercer will return a 2-element array. The first element is a bool value set to 1 if coercion has been performed or 0 if otherwise. The second element is the (possibly) coerced input.

"bool_coerced+str_errmsg+val" means the coercer will return a 3-element array. The first element is a bool value set to 1 if coercion has been performed or 0 if otherwise. The second element is the error message string which will be set if there is a failure in coercion (or undef if coercion is successful). The third element is the (possibly) coerced input.
source => bool
If set to true, will return coercer source code string instead of compiled code.
type* => sah::type_name
(No description)

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>.

AUTHOR

perlancar <perlancar@cpan.org>

CONTRIBUTING

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.

COPYRIGHT AND LICENSE

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

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.