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Sub::Spec(3pm) User Contributed Perl Documentation Sub::Spec(3pm)


Sub::Spec - Subroutine metadata specification


version 1.0.7


 [1, 0]


NOTICE: The Sub::Spec specification is deprecated as of Jan 2012. Rinci is the new specification to replace Sub::Spec, it is about 95% compatible with Sub::Spec, but corrects a few issues and is more generic. Perinci is the Perl implementation for Rinci and many of its modules can handle existing Sub::Spec sub specs.

This document specifies Sub::Spec metadata for subroutines (and methods). Sub::Spec is a language-neutral specification to describe various aspects of a subroutine. The goal is to make subroutine, which is the unit of reuse in many programming languages, more reusable and powerful.


Sub spec

A sub spec (short for specification) is a hash containing certain keys, the keys being called clauses. There are a set of known clauses specified. A spec is valid if it contains only known clauses and each clause is valid.

Normally a spec is created for each subroutine and placed alongside the code. For example, in Perl the spec is normally put in the %SPEC package variable:

 package MyPackage;
 our %SPEC;
 $SPEC{func1} = { ... }; # specification for func1()
 sub func1 {
 $SPEC{func2} = { ... }; # specification for func2()
 sub func2 {

and so on.

What can spec do for sub?

In Perl, there is a growing list of Perl modules (in the Sub::Spec::* namespace) which can utilize the information in the spec. For example:

  • Sub::Spec::Wrapper: wraps subroutines and implement many of the clauses (e.g. validate arguments specified in "args" clause, implement "timeout" clause using alarm() and eval block, etc).
  • Sub::Spec::Exporter: can be used to replace Exporter or Sub::Exporter, automatically wrap subroutines when exporting.
  • Sub::Spec::To::* modules: can generate documentation of various formats from spec, like a text usage (which one would normally see when giving --help argument to a command-line program).
  • Sub::Spec::CmdLine: run subroutines from the command line. Supply sub arguments from command line options.
  • Sub::Spec::HTTP::Server: serves subroutine call requests over HTTP.

Hopefully there will be implementations to in other languages.


Any Perl subroutine can be given sub spec, but it needs to accept hash (named) arguments and return a sub response (explained below), unless "args_as" in spec is 'array' and "result_naked" is set to 1.

 sub foo {
     my %args = @_; # accept hash/named arguments
     return [200, "OK", $data]; # return sub response instead of just data

Sub response

A sub response is a 2+-element arrayref:


STATUS is an integer number between 200-599, analogous to HTTP response code (see "Response status" for more details on the codes). MESSAGE is a string (or object) containing error message. DATA contains the result, and is optional. METADATA is a hashref containing metadata (analogous to HTTP response headers), is optional and seldom used.

This way, a subroutine can conveniently return error/status message as well as data. Also, since the codes are the same as in HTTP, converting to HTTP messages is straightforward.

Response status

  • 1xx code

    Currently not used.

  • 2xx code

    200 should be used to mean success.

    206 can be used to signal partial content, for example: a read_file() sub 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 clients will simply check for this exact code (instead of checking for range 200-299).

  • 3xx code

    301 (moved) can be used to redirect clients to alternate location, though I haven't used it yet.

    304 (not modified, nothing done). Used for example by setup subroutines to indicate that nothing is being modified (see Setup::* modules in CPAN).

  • 4xx code

    400 (bad request, bad arguments) should be returned when the sub encountered invalid input (arguments). The wrapper code will return this code when arguments fail schema validation.

    403 (forbidden, access denied).

    404 not found. Can be used for example: a get_user() sub which accepts username/userid argument and when the user is not found.

    Note: In general, a delete_object() sub should return 200 (or perhaps 304, but 200 is preferred) instead of 404 when the object specified by the user is not found, since the goal of the delete function is reached anyway.

    408 (request timeout)

    409 (conflict) can be used, for example: a create_user() function when receiving an already existing username.

    412 (precondition failed).

  • 5xx code

    500 is the general code to use when a failure occurs during the execution of sub. For example, when a delete_file() sub fails to delete specified file (though it can return 403 which is more specific).

    501 (not implemented)

    503 (service unavailable)

    507 (insufficient storage)

    53x (bad spec) is used when there is something wrong with the spec.


A clause must be of lowercase letters, numbers, or underscores. The next sections describe known clauses. A specific application/project can add to this list, but in general it is advisable to stick with the standard.

Clause: type => STR (default 'sub')

Type of subroutine. Either 'sub' for normal subroutine, 'method' for object method (class instance method), or 'class_method' for class method.

Clause: scope => STR (default 'public')

Subroutine scope. Either 'public' or 'private'.

Clause: name => STR

The name of the subroutine. Useful for generating help/usage information, or when aliasing subroutines (and reusing the spec) and finding out the canonical/original name of the subroutine.

Clause: summary => STR

A one-line summary. It should be plain text without any markup. It is like the "summary" Data::Sah clause.

Clause: description => STR

A longer description. It should be text in Org format. See for details on the Org format. It is like the "description" Data::Sah clause.

Clause: timeout => NUM

A number (in seconds), specifying execution time limit.

Clause: args => HASHREF

Args is a hash of argument names and schemas. Argument must only contain letters, numbers, and underscores. Schemas are Data::Sah schema. Example:

 args => {
     a => 'str*',
     b => ['int*' => {
         summary => 'The second argument',
         description => '... a longer description ...',
         min => 0,
         max => 100,
         ... # other schema clauses for 'int'

Sub::Spec adds a few schema clauses:

  • arg_pos => INT, 0+

    Specify the order of argument when specified in a positional order. Example:

     $SPEC{multiply2} = {
         summary => 'Multiple two numbers (a & b)',
         args    => {
             a      => ['num*' => {arg_pos=>0}],
             b      => ['num*' => {arg_pos=>1}],
             digits => 'int',

    This allows calling using named arguments as well as positional:

     multiply2(a=>4, b=>3);
     multiply2(4, 3); # after being wrapped
     # In command line:
     % cmd --a 2 --b 3
     % cmd 2 --b 3
     % cmd 2 3
  • arg_greedy => BOOL

    Specify whether, in positional arguments, this argument should gobble up the rest of the arguments into array.


     $SPEC{multiply} = {
         summary => 'Multiple numbers',
         args    => {
             nums   => ['num*[]*' => {arg_pos=>0, arg_greedy=>1, min_len=>1}],
     sub multiply {
         my %args = @_;
         my $nums = $args{nums};
         my $ans = 1;
         $ans *= $_ for @$nums;
         [200, "OK", $ans];

    In positional mode it can then be called:

     multiply(2, 3, 4);

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

     multiply(nums => [2, 3, 4]);

    In command-line:

     % cmd 2 3 4

    instead of:

     % cmd --nums '[2, 3 ,4]'
  • arg_complete => CODEREF

    Specifies how to complete argument value. CODEREF will be given arguments: word=>..., args=>.... and should return an arrayref containing a list of possible candidates. For an example of implementation for this, see Sub::Spec::BashComplete to provide tab completion for argument value. Example:

     $SPEC{delete_user} = {
        args => {
            username => ['str*' => {
                arg_complete => sub {
                    my %args = @_;
                    my $word = $args{word} // "";
                    # find users beginning with $word
                    local $CWD = "/home";
                    return [grep {-d && $_ ~~ /^\Q$word/} <*>];

Clause: required_args => LIST

List of argument names that are required.

Clause: args_as => STR (default 'hash')

Specify what kind of arguments sub accepts. The default is 'hash', which means arguments will be passed as %args into Perl subroutines. The other options are 'hashref', 'array', 'arrayref', 'object'.

Setting args_as to 'array' is useful if you have a "legacy" sub which accepts arguments directly from @_, like most normal Perl subroutines.

 $SPEC{is_palindrome} = {
     summary => 'Check whether a string is a palindrome',
     args    => {str => ['str*' => {arg_pos=>0},
                 ci  => [bool   => {arg_pos=>1, default=>0}]},
     args_as => 'array',
     result  => 'bool*',
 sub is_palindrome {
     my ($str, $ci) = @_;
     $str = lc($str) if $ci;
     [200, "OK", $str eq reverse($str) ? 1:0];

Setting args_as to 'object' is useful if you want to catch typo mistakes in your subs:

 package My::Palindrome;
 $SPEC{is_palindrome} = {
     summary => 'Check whether a string is a palindrome',
     args    => {str => 'str*', ci=>[bool=>{default=>0}]},
     args_as => 'object',
     result  => 'bool*',
 sub is_palindrome {
     my $args = shift;
     my $str  = $args->str;
     $str = lc($str) if $args->ci;
     [200, "OK", $str eq reverse($str) ? 1:0];

How this works: The wrapper (e.g. "Sub::Spec::Wrapper) will create a class for this argument, with accessors. Special arguments are accessed via _NAME (like $args-"_undo_action, $args->_reverse, etc) so make sure you do not define arguments with those names if you want to use object argument passing.

Clause: result => SCHEMA

The 'result' clause specifies sub's result value. The value of the clause is a Sah schema.

Note that since 'result_naked' by default is false, instead of just returning:


you'll normally have to return a proper response sub:



 # result is an integer
 result => 'int*'
 # result is an integer starting from zero
 result => ['int*' => {ge=>0}]
 # result is an array of records
 result => ['array*' => {
             summary => 'blah blah blah ...',
             of      => ['hash*' => {allowed_keys=>[qw/name age address/]} ]

Clause: result_naked => BOOL (default 0)

Setting 'result_naked' to 1 is useful if you have a "legacy" sub which does not return sub response. Example:

 $SPEC{is_palindrome} = {
     summary => 'Check whether a string is a palindrome',
     args    => {str => 'str*'},
     result  => 'bool*',
 sub is_palindrome {
     my %args = @_;
     my $str  = $args{str};
     [200, "OK", $str eq reverse($str) ? 1:0];


 $SPEC{is_palindrome} = {
     summary      => 'Check whether a string is a palindrome',
     args         => {str => 'str*'},
     result       => 'bool*',
     result_naked => 1,
 sub is_palindrome {
     my %args = @_;
     my $str  = $args{str};
     $str eq reverse($str);

Clause: retry => INT

Specify automatic retry upon failure. Not yet specified.

Clause: features => HASHREF

The "features" clause allows subroutines 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 clause may be defined by extension.

  • reverse => BOOL (default 0)

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

     $SPEC{triple} = {args=>{num=>'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

    PLANNED: Conditional reversibility.

  • undo => BOOL

    If set to true, specifies that subroutine supports undo operation. Undo is similar to 'reverse' but needs some state to be saved and restored for do/undo operation, while reverse can work solely from the arguments.

    Caller must provide one or more special arguments: -undo_action, -undo_hint, -undo_data when dealing with do/undo stuffs.

    To perform normal operation, caller must set -undo_action to 'do' and optionally pass -undo_hint for hints on how to save undo data. You should consult each function's documentation as undo hint depends on each function (e.g. if undo_data is to be saved on a file, -undo_hint can contain filename or base directory). Function must save undo data, perform action, and return result along with saved undo data in the response metadata (4th argument of response), example:

     return [200, "OK", $result, {undo_data=>$undo_data}];

    Undo data should contain information (or reference to information) to restore to previous state later. This information should be persistent (e.g. in a file/database) when necessary. For example, if undo data is saved in a file, undo_data can contain the filename. If undo data is saved in a memory structure, undo_data can refer to this memory structure, and so on. Undo data should be serializable. Caller should store this undo data in the undo stack (note: undo stack management is the caller's task).

    If -undo_action is false/undef, sub must assume caller want to perform action but without saving undo data.

    To perform an undo, caller must set -undo_action to 'undo' and pass back the undo data in -undo_data. Sub must restore previous state using undo data (or return 412 if undo data is invalid/unusable). After a successful undo, sub must return 200. Sub should also return undo_data, to undo the undo (effectively, redo):

     return [200, "OK", undef, {undo_data=>...}];

    Example (in this example, undo data is only stored in memory):

     use Cwd qw(abs_path);
     use File::Slurp;
     $SPEC{lc_file} = {
         summary  => 'Convert the *content* of file into all-lowercase',
         args     => {path=>'str*'},
         features => {undo=>1},
     sub lc_file {
         my %args        = @_;
         my $path        = $args{path};
         my $undo_action = $args{-undo_action} // '';
         my $undo_data   = $args{-undo_data};
         $path = abs_path($path)
             or return [500, "Can't get file absolute path"];
         if ($undo_action eq 'undo') {
             write_file $path, $undo_data->{content}; # restore original content
             utime undef, $undo_data->{mtime}, $path; # as well as original mtime
             return [200, "OK"];
         } else {
             my @st = stat($path)
                 or return [500, "Can't stat file"];
             my $content = read_file($path);
             my $undo_data = {mtime=>$st[9], content=>$content};
             write_file $path, lc($content);
             return [200, "OK", undef, {undo_data=>$undo_data}];

    To perform action, caller calls lc_file() and store the undo data:

     my $res = lc_file(path=>"/foo/bar", -undo_action=>"do");
     die "Failed: $res->[0] - $res->[1]" unless $res->[0] == 200;
     my $undo_data = $res->[3]{undo_data};

    To perform undo:

     $res = lc_file(path=>"/foo/bar", -undo_action=>"undo", -undo_data=>$undo_data);
     die "Can't undo: $res->[0] - $res->[1]" unless $res->[0] == 200;
  • dry_run => BOOL

    If set to 1, specifies that subroutine supports dry-run (simulation) mode. Example:

     use Log::Any '$log';
     $SPEC{rmre} = {
         summary  => 'Delete files in curdir matching a regex',
         args     => {re=>'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"];
  • pure => BOOL

    If set to true, specifies that subroutine 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:

  • the function needs not be involved in undo operation;
  • you can safely include it during dry run;

Clause: deps => HASHREF

This clause specifies subroutine's dependencies. It is a hashref of dep clause names and values. Some dep clauses are special: "all", "any", and "none".

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

A dependency can be anything: another subroutine, Perl version and module, environment variables, program, 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 clauses. New dependency clause may be defined by an extension.

  • sub => STR

    Require that subroutine exists. STR is the name of the subroutine and will be assumed to be in the 'main' package if unqualified.


     sub => 'foo'   # == main::foo
     sub => '::foo' # == main::foo
     sub => 'Package::foo'
  • mod => STR

    Require that module is loadable. Example:

     mod => 'Moo'
  • env => STR

    Require that an environment variable exists and has a true value. Example:

     env => 'HTTPS'
  • exec => STR

    Require that an executable exists. If STR doesn't contain path separator character '/' it will be searched in PATH.

     exec => 'rsync'   # any rsync found on PATH
     exec => '/bin/su' # won't accept any other su
  • code => CODEREF

    Require that CODEREF returns a true value after called. Example:

     code => sub {$>}  # i am not being run as root
  • all => [DEPCLAUSES, ...]

    A "meta" clause that allows several dependencies to be joined together in a logical-AND fashion. All dependencies must be satisfied. For example, to declare a dependency to several subroutines:

     all => [
         {sub => 'Package::foo1'},
         {sub => 'Package::foo2'},
         {sub => 'Another::Package::bar'},
  • any => [DEPCLAUSES, ...]

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

     or => [
         {mod => 'HTTP::Daemon'},
         {mod => 'HTTP::Daemon::SSL'},
  • none => [DEPCLAUSES, ...]

    Specify that none of the dependencies must be satisfied for this clause to be satisfied. Example, to specify that the subroutine 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.

Special arguments

These are arguments prefixed by - (dash) that have special meanings.



Python Decorators, , .


Steven Haryanto <>


This software is copyright (c) 2012 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.

2012-02-20 perl v5.40.0