Provided by: librinci-perl_1.1.43-1_all bug

NAME

       Rinci::function - Metadata for your functions/methods

VERSION

       version 1.1.43

SPECIFICATION VERSION

        1.1

INTRODUCTION

       This document describes metadata for functions/methods. Since the metadata properties
       describe features and the way a function works, this document also describes how a
       function should support those properties.

       This specification is part of Rinci. Please do a read up on it first, if you have not
       already done so.

SPECIFICATION

       Result envelope. Function should return an enveloped result to express error code/message
       as well as actual result. The envelope can be produced by the function itself, or added by
       a wrapper tool. Result envelope is modeled after HTTP or PSGI response; it is an array in
       the following format:

        [STATUS, MESSAGE, RESULT, META]

       STATUS is a 3-digit integer, much like HTTP response status code and is explained further
       in "Envelope status codes". MESSAGE is a string containing error message. RESULT (or
       PAYLOAD) is the actual content to be returned and can be omitted or set to undef if the
       function does not need to return anything.  META is called result metadata, a defhash
       containing extra data, analogous to HTTP response headers. Result metadata is specified
       further in Rinci::result.

       Some example of an enveloped results:

        [200, "OK", 42]
        [404, "Not found"]
        [500, "Can't delete foo: permission denied", {errno=>51}]
        [200, "Account created", {id=>9323},
         {undo_calls=>[["delete_account", {id=>9323}]]}]

       As mentioned, an enveloped result can contain error code/message as well as the actual
       result. It can also be easily converted to HTTP response message. And it can also contain
       extra data, useful for things like the transaction protocol (explained in
       Rinci::Transaction).

       Special arguments. Special arguments are some known arguments that start with dash ("-")
       and serve special purposes. You need not specify them in the "args" metadata property.
       Examples of special arguments include "-dry_run", "-tx_action", and they will be explained
       in other related sections/documents.

       Functions vs methods. Since in many programming languages (like Perl 5, Python, Ruby, PHP)
       static functions are not that differentiated from methods, functions and methods share the
       same Rinci spec. But there are certain properties that can be used to declare if a
       function is (also) a method or not.  See "is_func", "is_meth", "is_class_meth" properties
       below for details.

       Multiple dispatch. This specification also does not (yet) have any recommendation on how
       to best handle functions in languages that support multiple dispatch, like Perl 6: whether
       we should create multiple metadata or just one. It is more up to the tool and what you
       want to do with the metadata.

   Envelope status codes
       In general, status codes map directly to HTTP response status codes. Below are the
       suggestion on which codes to use (or avoid). An asterisk ("*") marks which codes are not
       defined in HTTP specification and introduced by this specification.

       •   1xx code

           Currently not used.

       •   2xx code - success

           200 should be used to mean success.

           206 can be used to signal partial content, for example: a "read_file()" function which
           accepts "byte_start" and "byte_end" arguments should return 206 when only partial file
           content is returned. But in general, use 200 as some callers will simply check for
           this exact code (instead of checking for range 200-299).

       •   3xx code - further actions needs to be taken by user agent (caller)

           301 (moved) can be used to redirect callers to alternate location, although this is
           very rare.

           304 (not modified, nothing done). Used for example by setup functions to indicate that
           nothing is being modified or no modifying action has been performed (see Setup::*
           modules in CPAN).

           331* (confirmation required). Function requires confirmation, for example if action to
           be taken is dangerous or requires user's attention. Confirmation message/prompt from
           function can be returned in the message, or in the "confirm_prompt" property (e.g. to
           provide translations). Confirmation from caller is in the form of special argument
           "-confirm" with boolean value of true (TODO: A more detailed confirmation for
           different actions can be specified later in the form of "-confirm_XXX" special
           arguments.) For an example of application of this, see Perinci::Tx::Manager.

       •   4xx code - client (caller) side error

           400 (bad request, bad arguments) should be returned when the function encounters
           invalid input. A function wrapper can return this code when the function arguments
           fail the argument schema validation (specified in the "args" property).

           401 (authentication required).

           403 (forbidden, access denied, authorization failed).

           404 (not found). Can be used for example by an object-retrieval functions (like
           "get_user()") and the object is not found.

           For object-listing functions (like "list_users()"), when there are no users found
           matching the requested criteria, 200 code should still be returned with an empty
           result (like an empty array or hash).

           Also in general, an object-deletion function (like "delete_user()") should also return
           200 (or perhaps 304, but 200 is preferred) instead of 404 when the object specified to
           be deleted is not found, since the goal of the delete function is reached anyway.

           408 (request timeout).

           409 (conflict). Can be used for example by a "create_user()" function when receiving
           an already existing username.

           412 (precondition failed). Similar to 409, but can be used to indicate lack of
           resources, like disk space or bandwidth. For lacking authentication and authorization,
           use 401 and 403 respectively.

           429 (too many requests).

           (EXPERIMENTAL) 44x codes are reserved for function-specific codes. Each function is
           free to define what each number means. However, this is not really encouraged and
           should only be used if necessary. Function should perhaps stick to predefined codes
           here. To return more detailed status, result metadata can be used.

           480* is general transaction error, e.g. transaction status is aborted so further
           requests for this transaction is ignored until transaction is aborted.

           484* (no such transaction).

       •   5xx code - server (callee) side error

           500 is the general code to use when a failure occurs during the execution of a
           function. for example when a "delete_file()" function fails to delete specified file
           (though in this case it can also choose to return 403 instead, which is more
           specific).

           501 (not implemented)

           503 (service unavailable). You can use this when service is temporarily unavailable,
           e.g. when system load is too high, a required service is down, etc.  Users should try
           again at a later time.

           507 (insufficient storage)

           521 (maximum retries reached)

           531* (bad metadata) is used when there is something wrong with the metadata.

           532* (failure in recording transaction) when there is a failure in updating
           transaction status or in preparing/committing/rolling back the transaction.

           (EXPERIMENTAL) 54x codes are reserved for function-specific codes. Each function is
           free to define what each number means. However, this is not really encouraged and
           should only be used if necessary. Function should perhaps stick to predefined codes
           here. To return more detailed status, result metadata can be used.

           Try not to use code greater than 555, as some tools use (CODE-300) for error codes
           that must fit in one unsigned byte (like Perinci::CmdLine).

   Property: is_func => BOOL
       Specify that the function can be called as a static function (i.e. procedural, not as a
       method). Default is true if unspecified, but becomes false if is_meth or is_class_meth is
       set to true.

       Example:

        # specify that function can be called a method *as well as* a static function
        is_meth => 1
        is_func => 1 # if not specified, will default to false after is_meth set to 1

   Property: is_meth => BOOL
       Specify that the function can be called as an instance (object) method. Default is false.

       Example:

        # specify that function is a method
        is_meth => 1

   Property: is_class_meth => BOOL
       Specify that the function can be called as a class method. Examples of class methods
       include the constructor, but there are others. Default is false.

       Example:

        # specify that function is a class method
        is_class_meth => 1

   Property: args => HASH
       Specify arguments. Property value is defhash of argument names and argument specification.
       Argument name must only contain letters, numbers, and underscores (and do not start with a
       number).

       Argument specification is a hash containing these keys:

       •   schema => SCHEMA

           Data::Sah schema for argument value.

       •   default => ANY

           Give default value for argument. This takes precedence over schema, which can also
           specify default value. This is useful if you want to share a common schema over
           several arguments but want to have different default for each argument. For example,
           you have a "ticket_status" schema. In "create_ticket" function you want the default
           "status" argument to be "new", while in "reply_ticket" you want the default "status"
           to be "answered".

       •   summary => STR

           From DefHash. A one-line plaintext summary, much like the "summary" property in
           variable metadata.

       •   req => BOOL

           Specify that argument is required (although its value can be undef/null).  Default is
           false.

       •   description => STR

           From DefHash. A longer description of marked up text, much like the "description"
           property. It is suggested to format the text to 74 columns.

       •   tags => ARRAY OF (STR|HASH)

           From DefHash. An array of tags, can be used by tools to categorize arguments.  Not
           unlike the "tags" property.

       •   pos => INT

           Argument position when specified in an ordered fashion, e.g. in an array. Starts from
           zero.

       •   greedy => BOOL

           Only relevant if pos is specified, specify whether argument should gobble up all
           remaining values in an ordered argument list into an array.

       •   cmdline_aliases => HASH

           Specify aliases for use in command-line options (or other possibly suitable situation
           where arguments are parsed from command-line-like options). Keys are alias names,
           values are itself hashes (alias specification). Valid alias specification keys:
           "summary" (a string, optional), "schema" (optional, defaults to argument's schema),
           "code" (a code to set argument value, optional, will be given "(\%args, $val)"); if
           not set, the default behavior is simply to set the argument value).

       •   cmdline_on_getopt => CODE

           A hook that will be called when argument is specified as a command-line option.  In
           Perl, hook will be called with a hash argument containing this key: "arg" (str,
           argument name), "value" (str, option value), "args" (hash, the argument hash defined
           so far).

           This can be useful if you want to process a command-line option directly on a per-
           option basis instead of getting the final resulting argument value. For example (in
           Perl):

            args => {
                library => {
                    schema            => ['array*' => of => 'str*'],
                    cmdline_aliases   => { I => {} },
                    cmdline_on_getopt => sub {
                        my %args = @_;
                        require lib;
                        lib->import($args{value});
                    },
                },
                module => {
                    schema            => ['array*' => of => 'str*'],
                    cmdline_aliases   => { M => {} },
                    cmdline_on_getopt => sub {
                        my %args = @_;
                        require Module::Load;
                        Module::Load::load($args{value});
                    },
                },
            }

           With command-line argument like this:

            -I dir1 -M mod1 -I dir2 -M mod2

           Without any "cmdline_on_getopt" hooks, the function will receive this argument hash:

            { library => ['dir1', 'dir2'], module => ['mod1', 'mod2'] }

           but there is no way to know the order of options being specified in the command-line.
           With the hooks, the function can load modules correctly (e.g.  loading "mod1" won't
           search in "dir2" as that directory has not been added by -I).

       •   cmdline_on_getarg => CODE

           Like "cmdline_on_getopt" but will be called when argument is specified as a command-
           line argument. For example:

            args => {
                foo => {
                    schema            => ['array*' => of => 'str*'],
                    pos               => 0,
                    greedy            => 1,
                    cmdline_on_getopt => sub { ... },
                    cmdline_on_getarg => sub { ... },
                },
                bar => { ... },
            },

           and the command-line argument:

            --foo o1 --bar o2 --foo o3 a1 a2

           The "cmdline_on_getopt" hook will be called twice for "o1" and "o3", while the
           "cmdline_on_getarg" hook will be called twice with "a1" and "a2".

       •   completion => CODE

           A code to supply argument value completion. Will be explained in the examples.

       •   element_completion => CODE

           A code to supply argument element value completion. Only applicable if argument type
           is "array". Will be explained in the examples.

       •   cmdline_src => STR

           Specify how to get the value for this argument, when function is run as a command-line
           program. Valid values include: "file" (command-line argument value will be treated as
           filename and function argument will be set to content of the file), "stdin" (means
           that program should get function argument from standard input), or "stdin_or_files"
           (means that program should get value from content of files, or if none is specified,
           from standard input). Other sources might be defined in the future.

           If function argument's type is "str" or "array", the whole standard input and files
           will be slurped into memory. If function argument's type is "stream" or "filehandle",
           program should provide standard input and files as a filehandle (like the diamond
           operator in Perl) so function can read input one record at a time. Record is line, but
           specifying the record separator should perhaps be possible in the future.

           There should only be one argument with "src" set to "stdin" or "stdin_or_files".

           TODO: Define "web_src" property and source for streaming web application.

           TODO: A way to define record separator.

       Example function metadata and its implementation in Perl:

        $SPEC{multiply2} = {
            v => 1.1,
            summary => 'Multiple two numbers',
            args => {
                a => {
                    summary => 'The first operand',
                    description => '... a longer description ...',
                    schema=>'float*',
                    pos => 0,
                    tags => ['category:operand'],
                },
                b => {
                    summary => 'The second operand',
                    description => '... a longer description ...',
                    schema => 'float*',
                    pos => 1,
                    tags => ['category:operand'],
                },
                round => {
                    summary => 'Whether to round result',
                    description => '... a longer description ...',
                    schema => [bool => {default=>0}],
                    pos => 2,
                    tags => ['category:options'],
                    cmdline_aliases => {
                        r=>{},
                        R=>{summary=>'Equivalent to --round=0',
                            code=>sub {$_[0]{round}=0}},
                    },
                },
            }
        };
        sub multiply2 {
            my %args = @_;
            my $res = $args{a} * $args{b};
            $res = int($res) if $round;
            [200, "OK", $res];
        }

       By default, without any wrapper, the function is called with a named hash style:

        multiply2(a=>4, b=>3);  # 12

       But with the information from the metadata, a wrapper tool like Perinci::Sub::Wrapper is
       able to change the calling style to positional:

        multiply2(4, 3.1, 1);  # 12

       A command-line tool will also enable the function to be called named options as well as
       positional arguments:

        % multiply2 --a 2 --b 3
        % multiply2 2 --b 3
        % multiply2 2 3

       As mentioned earlier, "cmdline_alises" is parsed by command-line option parser:

        % multiply2 2 3.5 -r ; # equivalent to multiply2 2 3 --round
        % multiply2 2 3.5 -R ; # equivalent to multiply2 2 3 --noround (--round=0)

       Aliases in "cmdline_aliases" are not recognized as real arguments:

        multiply2(a=>4, b=>3, r=>0);  # unknown argument r

       Another example (demonstrates "cmdline_aliases"):

        $SPEC{smtpd} = {
            v => 1.1,
            summary => 'Control SMTP daemon',
            args    => {
                action => {
                    schema => ['str*' => {in=>[qw/status start stop restart/]}],
                    pos    => 0,
                    req    => 1,
                    cmdline_aliases => {
                        status => {
                            schema    => [bool=>{is=>1}],
                            summary   => 'Alias for setting action=status',
                            code      => sub { $_[0]{action} = 'status' },
                        },
                        start => {
                            schema    => [bool=>{is=>1}],
                            summary   => 'Alias for setting action=start',
                            code      => sub { $_[0]{action} = 'start' },
                        },
                        stop => {
                            schema    => [bool=>{is=>1}],
                            summary   => 'Alias for setting action=stop',
                            code      => sub { $_[0]{action} = 'stop' },
                        },
                        restart => {
                            schema    => [bool=>{is=>1}],
                            summary   => 'Alias for setting action=restart',
                            code      => sub { $_[0]{action} = 'restart' },
                        },
                    },
                },
                force => {
                    schema => 'bool',
                },
            },
        };

       Another example (demonstrates greedy):

        $SPEC{multiply_many} = {
            v => 1.1,
            summary => 'Multiple numbers',
            args    => {
                nums   => {
                    schema => ['array*' => {of=>'num*', min_len=>1}],
                    pos    => 0,
                    greedy => 1
                },
            },
        };
        sub multiply_many {
            my %args = @_;
            my $nums = $args{nums};

            my $ans = 1;
            $ans *= $_ for @$nums;
            [200, "OK", $ans];
        }

       After wrapping, in positional mode it can then be called:

        multiply_many(2, 3, 4);  # 24

       which is the same as (in normal named-argument style):

        multiply_many(nums => [2, 3, 4]);  # 24

       In command-line:

        % multiply-many 2 3 4

       in addition to the normal:

        % multiply-many --nums '[2, 3, 4]'

       completion. This argument specification key specifies how to complete argument value (e.g.
       in shell or Riap::HTTP) and is supplied an anonymous function as value. The function will
       be called with arguments: word=>... (which is the formed word so far, ci=>0|1 (whether
       completion should be done case-insensitively). The function should return an array
       containing a list of possible candidates. For an example of implementation for this, see
       Perinci::Sub::Complete in Perl which provides tab completion for argument values. Example:

        $SPEC{delete_user} = {
            v => 1.1,
            args => {
                username => {
                    schema     => 'str*',
                    pos        => 0,
                    completion => sub {
                        my %args = @_;
                        my $word = $args{word} // "";

                        # find users beginning with $word
                        local $CWD = "/home";
                        return [grep {-d && $_ ~~ /^\Q$word/} <*>];
                    },
                },
                force => {schema=>[bool => {default=>0}]},
            },
        };

       When "delete_user" is executed over the command line and the Tab key is pressed:

        $ delete-user --force --username fo<tab>
        $ delete-user fo<tab>

       then bash will try to complete with usernames starting with "fo".

       element_completion. This is like completion, but for array elements.  Argument type must
       be "array". Example:

        $SPEC{delete_users} = {
            v => 1.1,
            args => {
                usernames => {
                    schema     => ['array*' => of => 'str*'],
                    req        => 1,
                    pos        => 0,
                    greedy     => 1,
                    element_completion => sub {
                        my %args = @_;
                        my $word = $args{word} // "";

                        # find users beginning with $word
                        local $CWD = "/home";
                        my $res = [grep {-d && $_ ~~ /^\Q$word/} <*>];

                        # exclude users already mentioned by user
                        my $ary = $args{args}{usernames};
                        $res = [grep {!($_ ~~ @$ary)}] @$res;

                        return $res;
                    },
                },
            },
        };

       When "delete_users" is executed over the command line:

        $ delete-users c<tab> ; # will complete with all users beginning with c
        $ delete-users charlie c<tab> ; # will complete with users but exclude charlie
        $ delete-users charlie chucky <tab> ; # and so on

   Property: args_as => STR
       Specify in what form the function expects the arguments. The value is actually
       implementation-specific since it describes the function implementation. For example in
       Perinci for Perl, these values are recognized: "array", "hash", "arrayref", "hashref".
       This property is useful for wrapper to be able to convert one form to another.

       The default value is also left to the implementation.

       For interimplementation communication (e.g. via Riap::HTTP or Riap::TCP), named arguments
       are always used so this property is irrelevant.

   Property: result => HASH
       Specify function return value. It is a defhash containing keys:

       •   summary

           From DefHash. Like the "summary" property in variable metadata.

       •   description

           From DefHash. Like the "description" property. Suggested to be formatted to 78
           columns.

       •   schema => SCHEMA

           A Sah schema to validate the result (the third element in the envelope result).  This
           schema should only be tested if status is 200. See also: "statuses".

       •   statuses => HASH

           Can be used to specify different result schema for different statuses. For example:

            statuses => {
                206 => {
                    schema => 'str*',
                },
            }

       Note that since functions normally return enveloped result, instead of returning:

        RESULT

       your functions normally have to return an enveloped result:

        [STATUS, MESSAGE, RESULT, METADATA]

       Examples:

        # result is an integer
        result => {schema => 'int*'}

        # result is an integer starting from zero
        result => {schema => ['int*' => {ge=>0}]}

        # result is an array of records
        result => {
            summary => 'Matching addressbook entries',
            schema => ['array*' => {
                summary => 'blah blah blah ...',
                of      => ['hash*' => {allowed_keys=>[qw/name age address/]} ]
            }]
        }

   Property: result_naked => BOOL
       If set to true, specify that function does not envelope its results. The default is false,
       to encourage functions to create envelopes. However, wrapper should be able to create or
       strip envelope if needed. For example, if you have "traditional" functions which does not
       do envelopes, you can set this property to true, and the wrapper can generate the envelope
       for the functions.

   Property: examples => ARRAY
       This property allows you to put examples in a detailed and structured way, as an
       alternative to putting everything in "description".

       Each example is a defhash, it specifies what arguments are used, what the results are, and
       some description. It can be used when generating API/usage documentation, as well as for
       testing data. It can also be used for testing (function will be run with specified
       arguments and the result will be matched against expected result). Known properties:

       •   args => HASH

           Arguments used to produce result. Can be converted to "argv" by tool, e.g. when
           displaying command-line eamples

       •   argv => ARRAY

           An alternative to "args", for example when function is run from the command-line. Can
           be converted to "args" most of the time when wanting to display examples in Perl
           instead of command-line.

       •   src => STR

           An alternative to "args" or "argv", to provide raw source code. See also: "src_plang".
           This can be used to show more general examples. For example, you can show how a
           function is used in an expression or code block, or how a command-line program is used
           in a shell script.

           Exactly one of "args", "argv", or "src" must be specified.

       •   src_plang => STR

           The programming language the examples source code "src" is written in. Valid values
           include: "perl", "bash".

           Command-line interface tools will typically only show examples written in "bash" or
           other shells, while Perl module tools will typically only show "perl" examples.

           Required if "src" is specified.

       •   status => INT (default: 200)

           Status from envelope. If unspecified, assumed to be 200.

       •   result => DATA

           Expected result.

       •   summary => STR

           From DefHash. A one-line summary of the example You should describe, in one phrase or
           sentence, what the example tries to demonstrate. You can skip the summary if the
           example is pretty basic or things are already clear from the "args" alone.

       •   description => STR

           From DefHash. Longer marked up text about the example (e.g. discussion or things to
           note), suggested to be formatted to 72 columns.

       •   tags => ARRAY

           From DefHash.

       •   test => BOOL (default: 1)

           Whether to actually test example or not. Examples are by default run as tests by a
           test module (e.g. Perl module Test::Rinci. Setting this to 0 disables this example
           from being included in a test.

           TODO: more detailed testing instruction (e.g. only test in release candidate, or under
           certain environment flag, etc).

       Example:

        # part of metadata for Math::is_prime function
        examples => [
            {
                args => {num=>10},
                result => 0,
                # summary no needed here, already clear.
            },
            {
                args => {},
                result => 400,
                summary => 'Num argument is required',
            },

            {
                argv => [-5],
                result => 1,
                summary => 'Also works for negative integers',
            },
        ],

       Another example demonstrating "src" for a function called "list_countries":

        examples => [
            {
                src => 'for c in `list-countries`; do wget http://flags.org/country/$c; done',
                src_plang => 'bash',
            },
            {
                src => <<'EOT',
        my $res = list_countries(detail => 1, sort=>['-popsize']);
        die "Can't list countries: $res->[0] - $res->[1]" unless $res->[0] == 200;
        my $i = 0;
        for my $c (@{ $res->[2] }) { $i++; say "$i. $_->{name}'s population: $_->{popsize}";
        EOT
                src_plang => 'perl',
            },
        ],

   Property: features => HASH
       The "features" property is a deffhash. It allows functions to express their features. Each
       hash key contains feature name, which must only contain letters/numbers/underscores.

       Below is the list of defined features. New feature names may be defined by extension.

       •   feature: reverse => BOOL (default: 0)

           If set to true, specifies that function supports reverse operation. To reverse, caller
           can add special argument "-reverse". For example:

            $SPEC{triple} = {
                v => 1.1,
                args     => {num=>{schema=>'num*'}},
                features => {reverse=>1}
            };
            sub triple {
                my %args = @_;
                my $num  = $args{num};
                [200, "OK", $args{-reverse} ? $num/3 : $num*3];
            }

            triple(num=>12);              # => 36
            triple(num=>12, -reverse=>1); # =>  4

       •   feature: tx => HASH

           Default is none. Specify transactional support, as specified in Rinci::Transaction.
           Value is a hash containing these keys: "v" (int, protocol version, default if not
           specified is 1).

           Please see Rinci::Transaction for more details on transaction.

       •   feature: dry_run => BOOL (default: 0)

           Default is false. If set to true, specifies that function supports dry-run
           (simulation) mode. Example:

            use Log::Any '$log';

            $SPEC{rmre} = {
                summary  => 'Delete files in curdir matching a regex',
                args     => {re=>{schema=>'str*'}},
                features => {dry_run=>1}
            };
            sub rmre {
                my %args    = @_;
                my $re      = qr/$args{re}/;
                my $dry_run = $args{-dry_run};

                opendir my($dir), ".";
                while (my $f = readdir($dir)) {
                    next unless $f =~ $re;
                    $log->info("Deleting $f ...");
                    next if $dry_run;
                    unlink $f;
                }
                [200, "OK"];
            }

           The above Perl function delete files, but if passed argument "-dry_run" => 1
           (simulation mode), will not actually delete files, only display what files match the
           criteria and would have be deleted.

           Specifying a function as supporting dry_run means, among others:

           •   If dry_run is requested, function will have no side effects

               It will behave like a pure function, and thus have the properties of a pure
               function.

       •   feature: pure => BOOL (default: 0)

           If set to true, specifies that function is "pure" and has no "side effects" (these are
           terms from functional programming / computer science). Having a side effect means
           changing something, somewhere (e.g. setting the value of a global variable, modifies
           its arguments, writing some data to disk, changing system date/time, etc.) Specifying
           a function as pure means, among others:

           •   it can safely be inculded in transaction without recording in journal;

           •   it can safely be included during dry run;

       •   feature: immutable => BOOL

           Default is false. If set to true, specifies that function always returns the same
           result when given the same argument values. This enables optimization like
           memoization. An example of an immutable function is "sub { $_[0]+$_[1] }" where its
           results only depend on the arguments. Example of a mutable function would be "rand()"
           or "read()" that reads contents from a file.

       •   feature: idempotent => BOOL

           Default is false. If set to true, specifies that function is idempotent.  Idempotency
           means that repeated invocation of a function (each with the same arguments) will have
           the same effect as a single invocation. In other words, extra invocation will not have
           any effect.

           Some operations, like reading a database row or a file's content, is inherently
           idempotent (or to be exact nullipotent). Another example is setting or updating an
           entity to some specific value, or deleting some entity. Repeated invocation of the
           operation will still sets the entity to the same value, or still deletes the entity.

           Some other operations are inherently non-idempotent, for example sending an email.
           Repeated invocation will cause multiple emails to be sent.

           Yet some other operations are non-idempotent, but can be made idempotent simply by
           checking whether the target object(s) has (have) reached the final desired state,
           (optionally additionally also checking whether they are in the correct original state
           to begin with). For example, a function that renames a file can record the original
           file that was renamed (its MD5 checksum, size, or what not) or perhaps record the
           action in a history database or flag file, and refuse to rename again if the file to
           be renamed is not the original file.

   Property: deps => HASH
       This property specifies function's dependencies to various things. It is a hash of dep
       types and values. Some dep types are special: "all", "any", and "none".

        deps => {
            DEPTYPE => DEPVALUE,
            ...,
            all => [
                {DEPTYPE=>DEPVALUE, ...},
                ...,
            },
            any => [
                {DEPTYPE => DEPVALUE, ...},
                ...,
            ],
            none => [
                {DEPTYPE => DEPVALUE, ...},
                ....,
            ],
        }

       A dependency can be of any type: another function, environment variables, programs, OS
       software packages, etc. It is up to the dependency checker library to make use of this
       information.

       For the dependencies to be declared as satisfied, all of the clauses must be satisfied.

       Below is the list of defined dependency types. New dependency type may be defined by an
       extension.

       •   dep: env => STR

           Require that an environment variable exists and is true, where true is in the Perl
           sense (not an empty string or "0"; " " and "0.0" are both true). Example:

            env => 'HTTPS'

       •   dep: prog => STR

           Require that a program exists. If STR doesn't contain path separator character '/' it
           will be searched in PATH. Windows filesystem should also use Unix-style path, e.g.
           "C:/Program Files/Foo/Bar.exe".

            prog => 'rsync'   # any rsync found on PATH
            prog => '/bin/su' # won't accept any other su

       •   dep: code => CODE

           Require that anonymous function returns a true value after called, where the notion of
           true depends on the host language. Example in Perl:

            code => sub {$>}  # i am not being run as root

           Example in Ruby:

            "code" => Proc.new { Process.euid > 0 }  # i am not being run as root

       •   dep: tmp_dir => BOOL

           If set to 1, specify that function requires temporary directory. Caller should provide
           path to this using special argument "-tmp_dir".

       •   dep: trash_dir => BOOL

           If set to 1, specify that function requires trash directory. Trash is not unlike a
           temporary directory. Caller should provide path to trash directory using special
           argument "-trash_dir".

           Trash directory can be provided, e.g. by transaction manager (see Rinci::Transaction).

       •   dep: all => [DEPHASH, ...]

           A "meta" type that allows several dependencies to be joined together in a logical-AND
           fashion. All dependency hashes must be satisfied. For example, to declare a dependency
           to several programs and an environment variable:

            all => [
                {prog => 'rsync'},
                {prog => 'tar'},
                {env  => 'FORCE'},
            ],

       •   dep: any => [DEPHASH, ...]

           Like "all", but specify a logical-OR relationship. Any one of the dependencies will
           suffice. For example, to specify requirement to alternative Perl modules:

            or => [
                {perl_module => 'HTTP::Daemon'},
                {perl_module => 'HTTP::Daemon::SSL'},
            ],

       •   dep: none => [DEPHASH, ...]

           Specify that none of the dependencies must be satisfied for this type to be satisfied.
           Example, to specify that the function not run under SUDO or by root:

            none => [
                {env  => 'SUDO_USER'   },
                {code => sub {$> != 0} },
            ],

           Note that the above is not equivalent to below:

            none => [
                {env => 'SUDO_USER', code => sub {$> != 0} },
            ],

           which means that if none or only one of 'env'/'code' is satisfied, the whole
           dependency becomes a success (since it is negated by 'none'). Probably not what you
           want.

       If you add a new language-specific dependency type, please prefix it with the language
       code, e.g. "perl_module", "perl_func", "ruby_gem", "python_egg".  These dependency types
       have also been defined by some existing tools: "deb" (dependency to a Debian package),
       "rpm" (dependency to an RPM package), "js_url" (loading a remote JavaScript script URL),
       "file" (existence of a), "perl_run_func" (running a Perl subroutine and getting a
       successful enveloped result). Some of these might be declared as part of the core
       dependency types in the future.

FAQ

   What is the difference between "summary" or "description" in the Sah schema and arg
       specification?
       Example:

        {
            args => {
                src => {
                    summary => "Source path",
                    description => "...",
                    schema => ["str*", {
                        summary => "...",
                        description => "...",
                        ...
                    }],
                    ...
                },
                dest => {
                    summary => "Target path",
                    description => "...",
                    schema => ["str*", {
                        summary => "...",
                        description => "...",
                        ...
                    }],
                    ...
                },
                ...
            },
        }

       As you can see, each argument has a "summary" and "description", but the schema for each
       argument also has a "summary" and "description" schema clauses. What is the difference and
       which should be put into which?

       The argument specification's "summary" (and "description") describe the argument itself,
       in this example it says that "src" means "The source path" and "dest" means "The target
       path". The argument schema's "summary" (and "description") describe the data type and
       valid values. In this example it could say, e.g., "a Unix-path string with a maximum
       length of 255 characters".  In fact, "src" and "dest" are probably of the same type ("Unix
       path") and can share schema.

        {
            ...
            args => {
                src => {
                    ...
                    schema => "unix_path",
                },
                dest => {
                    ...
                    schema => "unix_path",
                },
                ...
            },
        }

   What is the difference between setting req=>1 in the argument specification and req=>1 in
       schema?
       Example:

        # Note: remember that in Sah, str* is equivalent to [str => {req=>1}]
        args => {
            a => {         schema=>"str"  },
            b => {         schema=>"str*" },
            c => { req=>1, schema=>"str"  },
            d => { req=>1, schema=>"str*" },
        }

       In particular look at "b" and "c". "b" is not a required argument (no req=>1 in the
       argument spec) but if it is specified, than it cannot be undef/null (since the schema says
       [str=>{req=>1}], a.k.a "str*"). On the other hand, "c" is a required argument (req=>1 in
       the argument spec) but you can specify undef/null as the value. The following are valid:

        func(c=>undef, d=>1);

       But the following are not:

        func(b=>1, d=>1);  # c is not specified
        func(b=>undef, c=>1, d=>1);  # b has undef value
        func(b=>1, c=>1, d=>undef);  # d has undef value

   Should I add a new metadata property, or add a new feature name to the "features" property, or
       add a new dependency type to the "deps" property?
       If your property describes a dependency to something, it should definitely be a new
       dependency type. If your property only describes what the function can do and does not
       include any wrapper code, then it probably goes into "features".  Otherwise, it should
       probably become a new metadata property.

       For example, if you want to declare that your function can only be run under a certain
       moon phase (e.g. full moon), it should definitely go as a new dependency type, so it
       becomes: deps => { moon_phase => 'full' }.

       Another example, "reverse" is a feature name, because it just states that if we pass
       "-reverse" => 1 special argument to a reversible function, it can do a reverse operation.
       It doesn't include any wrapper code, all functionality is realized by the function itself.
       On the other hand, "timeout" is a metadata property because it involves adding adding some
       wrapping code (a timeout mechanism, e.g. an eval() block and alarm() in Perl).

SEE ALSO

       Related specifications: Sah, HTTP/1.1 (RFC 2068)

       Rinci

HOMEPAGE

       Please visit the project's homepage at <https://metacpan.org/release/Rinci>.

SOURCE

       Source repository is at <https://github.com/sharyanto/perl-Rinci>.

BUGS

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

       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.

AUTHOR

       Steven Haryanto <stevenharyanto@gmail.com>

COPYRIGHT AND LICENSE

       This software is copyright (c) 2013 by Steven Haryanto.

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