package Sah; # just to make PodWeaver happy

our $VERSION = '0.9.7'; # VERSION

1;
# ABSTRACT: Schema for data structures (specification)


__END__
=pod

=head1 NAME

Sah - Schema for data structures (specification)

=head1 VERSION

version 0.9.7

=head1 SPECIFICATION VERSION

0.9

=head1 STATUS

In the 0.9.0 series, there will probably still be incompatible syntax changes
between revision before the spec stabilizes into 1.0 series.

=head1 ABOUT

This document specifies Sah, a schema language for validating data structures.

Data structures are written in pseudo-JSON (JSON with comments C<// ...>,
ellipsis C<...>, or some JavaScript).

=head1 SCHEMA

Although it can contain extra stuffs, a schema is essentially B<a type
definition>, stating a set of valid values for data.

Sah schemas are regular data structures, specifically arrays:

 [TYPE_NAME, CLAUSE_SET, EXTRAS]

C<TYPE_NAME> is a string, C<CLAUSE_SET> is a hash of clauses, and C<EXTRAS> is a
hash and is optional. Some examples:

 ["int", {"min": 0, "max": 100}]

 // a definition of pos_even (positive even natural numbers). "pos" is defined
 // in the EXTRAS part.
 ["pos", {"div_by": 2}, {"def": {"pos": ["int": {"min": 0}]}}]

A shortcut string form containing only the type name is allowed when there are
no clauses. It will be normalized into the array form:

 "int"

The type name can have a C<*> suffix as a shortcut for the C<< "req": 1 >>
clause. This shortcut exists because stating something is required is very
common.

 "int*"

 // equivalent to
 ["int", {"req": 1}]

 ["int*", {"min": 0}]

 // equivalent to
 ["int", {"req": 1, "min": 0}]

A flattened array form is also supported when there are no C<EXTRAS>. It will be
normalized into the non-flattened form. This shortcut exists to save a couple of
keystrokes :-) And also reduce the number of nested structure, which can get a
bit unwieldy for complex schemas.

 ["int", "min", 1, "max", 10]

 // is equivalent to
 ["int", {"min": 1, "max": 10}]

=head1 TYPE

Type classifies data and specifies the possible values of data.

Sah defines several standard types like C<bool>, C<int>, C<float>, C<str>,
C<array>, C<hash>, and a few others. Please see L<Sah::Type> for the complete
list.

Type name must match this regular expression:

 \A[A-Za-z_][A-Za-z0-9_]+(::[A-Za-z_][A-Za-z0-9_]+)*\z

A type can have B<clauses>. Most clauses declare constraints (so, B<metadata
clauses>). B<Metadata clauses> are like functions, they accept an argument, are
evaluated against data and return a value. The returned value need not strictly
be boolean, but for the clause to succeed, the return value must evaluate to
true. The notion of true/false follows Perl's notion: undefined value, empty
string (C<"">), the string C<"0">, and number 0 are considered false. Everything
else is true.

For the schema to succeed, all constraint clauses must evaluate to true.

Aside from declaring constraints, clauses can also declare other stuffs. There
is the C<default> clause which specifies default value. There are B<metadata
clauses> which specify metadata, e.g. the C<summary>, C<description>, C<tags>
clauses.

Aside from clauses, type can also have B<type properties>. Properties are
different from clauses in the following ways: 1) they are used to find out
something about the data, not to test/validate data; 2) they are allowed to not
accept any argument. A type can have a property and a clause with the same name,
for example the C<str> type have a C<len> clause to test its length against an
integer, as well as a C<len> property which returns its length. Properties are
differentiated from clauses so that compilers to human text can generate a
description like "string where its length is at least 1".

Type properties can be validated against a schema using the C<prop> or C<if>
clause.

B<Base schema>. You can define a schema, declare it as a new type, and then
write subsequent schemas against that type, along with additional clauses. This
is very much like subtyping. See L</"BASE SCHEMA"> for more information.

=head1 BASE SCHEMA

As mentioned before, you can define a schema as a type and then write other
schemas against that type. For example:

 // defined as pos_int type
 ["int", {"min": 0}]

and later:

 // a positive integer, divisible by 5
 ["pos_int", {"div_by": 5}]

During data validation, base schemas will be replaced by its original
definition, and all the clause sets will be evaluated. Illustrated by the plus
sign:

 ["int", {"min": 0} + {"div_by": 5}]

You can also declare base schemas/types locally using the C<def> key in
C<EXTRAS>, for example:

 ["throws", {},
  {
      "def": {
          "single_dice_throw":  ["int": {"in": [1, 2, 3, 4, 5, 6]}],
          "sdt":                "single_dice_throw", // short notation
          "dice_pair_throw":    ["array": {"len": 2, "elems": ["sdt", "sdt"]}],
          "dpt":                "dice_pair_throw",   // short notation
          "throw":              ["any": {"of": ["sdt", "dpt"]}],
          "throws":             ["array": {"of": "throw"}],
      }
  }
 ]

The above schema describes a list of dice throws (C<throws>). Each C<throw> can
be a single dice throw (C<sdt>) which is a number between 1 and 6, or a throw of
two dices (C<dpt>) which is a 2-element array (where each element is a number
between 1 and 6).

Examples of valid data for this schema:

 [1, [1,3], 6, 4, 2, [3,5]]

Examples of invalid data:

 1                  // not an array
 [1, [2, 3], 0]     // the third throw is invalid
 [1, [2, 0, 4], 4]  // the second throw is invalid

All the base schemas names C<throw>, C<throws>, C<sdt>, etc is only declared
locally and unknown outside the schema. You can even nest this.

=head2 Optional/conditional definition

If you put a C<?> suffix after the definition name then it means that the
definition is optional and can be skipped if the type is already defined, e.g.:

  "def": {
      "emailaddr?": ["str", {"req": 1, "match": ".+\@.+"}],
      "username":   ["str", {"req": 1, "match": "^[a-z0-9_]+$"}]
  }

In the above example, if there is already an C<emailaddr> type defined at that
time, the definition will be skipped instead of a "cannot redefine type" error
being generated.

Optional definition is useful if you want to provide some defaults (e.g. a
rudimentary validation for email address) but don't mind if the validator
already has something probably better (a stricter or more precise definition of
email address).

=head1 CLAUSE AND CLAUSE SET

A clause set is a defhash (see L<DefHash>) containing a mapping of clause name
and clause values or clause attribute names and clause attribute values. Defhash
properties map to Sah clauses, while defhash property attributes map to Sah
clause attributes.

 {
     "CLAUSENAME1": CLAUSEVALUE,
     "CLAUSENAME1.ATTRNAME1": ATTRVALUE1,
     "CLAUSENAME1.ATTRNAME2": ATTRVALUE2,
     "CLAUSENAME1.ATTRNAME1.SUBATTR1": ...,
     ...
     "_IGNORED": ...,
     "CLAUSENAME1._IGNORED": ...
 }

For convenience, there are also some shortcuts:

=over 4

=item * C<&> suffix (multiple clause values, all must succeed)

 "CLAUSENAME&": [VAL, ...]

is equivalent to:

 "CLAUSENAME": [VAL, ...],
 "CLAUSENAME.is_multi": 1,
 "CLAUSENAME.max_nok":  0

=item * C<|> suffix (multiple clause values, only one must succeed)

 "CLAUSENAME|": [VAL, ...]

is equivalent to:

 "CLAUSENAME": [VAL, ...],
 "CLAUSENAME.is_multi": 1,
 "CLAUSENAME.min_ok":   1

=item * C<!> prefix (negation)

 "!CLAUSENAME": VAL

is a shortcut for this:

 "CLAUSENAME": VAL,
 "CLAUSENAME.max_ok": 0

=item * C<=> suffix (expression)

 "CLAUSENAME=": EXPR

 "CLAUSENAME.ATTRNAME1=": EXPR

are respectively equivalent to:

 "CLAUSENAME.is_expr": 1
 "CLAUSENAME": EXPR

 "CLAUSENAME.ATTRNAME1.is_expr": 1
 "CLAUSENAME.ATTRNAME1": EXPR

=back

Every clause has a B<priority> between 0 and 100 to determine the order of
evaluation (the lower the number, the higher the priority and the earlier the
clause is evaluated). Most constraint clauses are at priority 50 (normal) so the
order does not matter, but some clauses are early (like C<default> and
C<prefilters>) and some are late (like C<postfilters>). Variables mentioned in
expression also determine ordering, for example:

 ["int", {"min=": "0.5*$clause:max", "max": 10}]

In the above example, although C<max> and C<min> are both at priority 50, C<min>
needs to be evaluated first because it refers to C<max> (XXX syntax of variable
not yet finalized).

=head2 Clause name

This specification comes from DefHash: Clause names must begin with
letter/underscore and contain letters/numbers/underscores only. All clauses
which begin with an C<_> (underscore) is ignored. You can use this to embed
extra data for other purposes.

=head2 Clause attribute

This specification comes from DefHash: Attribute name must also only contain
letters/numbers/underscores, but it can be a dotted-separated series of parts,
e.g. C<alt.lang.id_ID>. As with clauses, clause attributes which begin with C<_>
(underscore) is ignored. You can use this to embed extra data.

Currently known general attributes:

=over 4

=item * prio : INT

Change the clause's priority for this clause set. Note that this only works for
clauses which have equal priorities. Otherwise, priority value from clause
definition takes precedence.

Example:

 // both "min" and "max" clauses have priority of 50, but we want to make sure
 // that "min" is evaluated first
 ["int*", {"min=": "some expr", "min.prio": 1, "max": 10}]

=item * is_multi : BOOL (default: 0)

This attribute can be used to signify that clause contain multiple (an array of)
values instead of a single value. Each value will be tested against the clause.
Example:

 ["int*", {"div_by.is_multi": 1, "div_by": [2, 3, 5]}]

The above schema requires an integer which is divisible by 2, 3, I<and> 5.

=item * min_ok, max_ok, min_nok, max_nok : INT

In a clause which uses multiple values (see previous C<is_multi> attribute),
these attributes regulate the {minimum, maximum} number of values that must
{pass, fail} the check for the whole clause to pass.

Analogously, in a clause set, these attributes regulate the {minimum, maximum}
number of clauses that must {pass, fail} the check for the whole clause set to
pass.

C<min_ok>'s default is undef. You can use this attribute to only require certain
number of (instead of all) passing checks.

Example:

 ["str", {"cset": {"min_len": 8, "match": "\\W"}, "cset.min_ok": 1}]

The above schema requires a string to be at least 8 characters long, I<or>
contains a non-word character. Strings that would validate include: C<abcdefgh>
or C<$> or C<$abcdefg>. Strings that would not validate include: C<abcd> (fails
both C<min_len> and C<match> clauses). Without the .min_ok attribute, by default
all checks in the C<cset> clause must pass.

Another example:

 ["str", {"match.is_multi": 1, "match": [RE1, RE2, RE3], "match.min_ok": 2}]

The above schema specifies that string must match at least two of RE1/RE2/RE3.

C<max_ok>'s default is undef. You can use this attribute to require a number of
failures in the checks.

Example:

 ["str", "min_len", 8, "match", "\\W", ".min_ok", 1, ".max_ok": 1]

The above schema states that string must either be longer than 8 characters or
contains a non-word character, I<but not both>. Strings that would validate
include: C<abcdefgh> or C<$>. Strings that would not validate include:
C<$abcdefg> (match both clauses, so max_ok is not satisfied).

Another example:

 ["str", {"match.is_multi": 1, "match": [RE1, RE2, RE3], "match.max_ok": 1}]

The above schema specifies that string must not match more than one of
RE1/RE2/RE3.

C<min_nok>'s default is undef. You can use this attribute to require a certain
number of failures.

Example:

 ["str", {"cset": {"min_len": 8, "match": "\\W"}, "cset.min_nok": 1}]

The above schema requires a string to be shorter than 8 characters or devoid of
non-word characters. Strings that would validate include: C<abcdefghi> (fails
the C<match> clause), C<$abcd> (fails C<min_len> clause), or C<a> (fails both
clauses). Strings that would not validate include: C<$abcdefg>.

Another example:

 ["str", {"match.is_multi": 1, "match": [RE1, RE2, RE3], "match.min_nok": 1}]

The above schema specifies that string must fail at least one regex match.

C<max_nok>'s default is undef, but when none of the {min,max}_{ok,nok} is
defined, the default behavior is to require all clauses to succeed, in other
words, as if C<max_nok> were 0. You can use this clause to tolerate a certain
number of failures in the checks.

Example:

 ["str", {"cset": {"min_len": 8, "match": "\\W"}, "cset.max_nok": 1}]

The above schema states that string must either be longer than 8 characters or
contains two non-word characters, I<or both>. Strings that would validate
include: C<abcdefgh>, C<$$>, C<$abcdefgh>. Strings that would not validate
include: C<abcd> (fails both C<min_len> and C<match> clauses).

Another example:

 ["str", {"match.is_multi": 1, "match": [RE1, RE2, RE3], "match.max_nok": 1}]

The above schema specifies that string can fail at most one regex match.

=item * is_expr : BOOL

Signify that clause contains expression (see L</"EXPRESSION">) instead of
literal value. Example:

 // a string, minimum 4 characters
 ["str", {"min_len": 4}]

 // same thing, albeit a bit fancier
 ["str", {"min_len.is_expr": 1, "min_len": "2*2"}]

 // same thing, shortcut notation
 ["str", {"min_len=": "2*2"}]

 // for default, we pick a random number between 1 and 10
 ["int", {"default=": "int(10*rand())+1"}]

Expression is useful for more complex schema, when a clause/attribute value
needs to be calculated in terms of other values, and/or using functions.

Note that an implementation might not support expression in some clauses or
attributes, especially clauses that accept argument containing schemas as
dynamically generated schemas needs the compiler to embed an interpreter or
compiler in the generated code.

When C<is_expr> attribute is true, and C<is_multi> is also true, then the
expression is expected to return an array of values. Otherwise, the clause will
fail. Example:

 // number which must be divisible by 2, 3, 5
 ["int", {"div_by.is_expr": 1, "div_by.is_multi": 1, "div_by": "[2, 3, 5]"}]

 // string must not match any of the blacklist
 ["str", {
     "contains.is_expr": 1,
     "contains.is_multi": 1,
     "contains.max_ok": 0,
     "contains": "get_blacklist()"
 }]

=item * err_level : STR (default: error)

Valid value: C<error>, C<warn>. Normally, when clause checking fails, an error
is generated and it causes validation of the whole schema to fail. If
C<err_level> is set to C<warn>, however, this only generates a warning and does
not cause the validation to fail.

 // password
 ["str*", {"cset&": [
   {"min_len": 4},
   {"min_len": 8,
    "min_len.err_level": "warn",
    "min_len.err_msg":   "Although a password less than 8 letters are " +
                         "valid it's highly recommended that a password is " +
                         "at least 8 letters long, for security reasons"}
 ]}]

In the above example, the C<err_level> and C<err_msg> are attributes for the
C<min_len> clause. The second clause set basically adds an optional restriction
for the password: when the C<min_len> clause is not satisfied, instead of making
the data fails the validation, only a warning is issued.

=item * err_msg[.LANGCODE]

This tells the compiler that instead of the default error message from the type
handler, a custom error message is supplied. You can add translations by adding
more attributes. For example:

 ["str", {"match":                "[^A-Za-z0-9_-]",
          "match.err_msg"         "Must not contain naughty characters",
          "match.err_msg.id_ID": "Tidak boleh mengandung karakter aneh-aneh"
 }]

=item * human[.LANGCODE]

This is also ignored when validating data, but will be used by the human
compiler to supply description. You can add translations by adding more
attributes.

 ["str", {"match":             "[^A-Za-z0-9_-]",
          "match.human"        "Must not contain naughty characters",
          "match.human.id_ID": "Tidak boleh mengandung karakter aneh-aneh"
 }]

=item * alt

This comes from DefHash, mainly used to store translations for C<name>,
C<summary>, C<description>.

=item * result_var : VARNAME (EXPERIMENTAL)

Specify variable name to store results in.

Aside from pass/failure, a clause or clause set can also produce some value.
This attribute specifies where to put the results in. The value can then be
used by referring to the variable in expression. Example:

 ["any", {
     "of": [
         ["str*":    {"min_len": 1, "max_len": 10}], // 1
         ["str*":    {"min_len": 11}],               // 2
         ["array*":  {}],                            // 3
         ["hash*":   {}]                             // 4
     ],
    "of.result_var": "a"
 }]

Aside from passing/failing the validation, the C<of> clause above also produces
an index to the schema in the list which matches. So if you validate an array,
C<$a> in the schema will be set to 3. If you validate a string with length 12,
C<$a> will be set to 2. If you pass an empty string (which does not pass the
C<of> clause, C<$a> will not be set.

Refer to each clause's documentation to find out what value the clause returns.

=item * input : [KIND, ...]

Normally, a clause tests data. With this attribute, you can change the input to
something else. Possible values:

 ["prop", PROP] // test data's property instead of data itself
 ["expr", EXPR] // test arbitrary expression
 ["schema", SCHEMA] // test return value of schema
 ["clause", CLAUSE, ARG] // test return value of a clause

Examples:

 // string whose length is a prime number
 ["str", "check.input", ["prop", "len"], "check", "is_prime($_)"]

Without C<check.input> set to the C<len> property, C<$_> in the expression will
still refer to the data (the string).

 // same thing but using expr input
 ["str", "check.input", ["expr", "len($_)"], "check", "is_prime($_)"]

 // it's basically a fancy way of saying ...
 ["str", "check", "is_prime(len($_))"]

=back

Aside from the above general attributes, each clause might recognize its own
specific attributes. See documentation of respective clauses.

=head2 Clause set merging

Clause set merging happens when a schema is based on another schema and the
child schema's clause set contains merge prefixes (explained later) in its keys.
For example:

 // schema1
 [TYPE1, CSET1]

 // schema2, based on schema1
 [schema1, CSET2]

 // schema3, based on schema2
 [schema2, CSET3]

When compiling/evaluating C<schema2>, Sah will check against C<TYPE1> and
C<CSET1> and then C<CSET2>. However, when C<CSET2> contains a merge prefix
(marked with an asterisk here for illustration), then Sah will check against
C<TYPE1> and C<< merge(CSET1, *CSET2) >>.

When compiling/evaluating C<schema3>, Sah will check against C<TYPE1> and
C<CSET1> and then C<CSET2> and then C<CSET3>. However, when C<CSET2> contains a
merge prefix, then Sah will check against C<TYPE1>, C<< merge(CSET1, *CSET2) >>,
and then C<CSET3>. When C<CSET2> and C<CSET3> contains merge prefixes, Sah will
check against C<TYPE1> and C<< merge(CSET1, *CSET2, *CSET3) >>. So merging will
be done from left to right.

The base schema's clause set must not contain any merge prefixes.

Merging is done using L<Data::ModeMerge>, with merge prefixes changed to
'merge.add.', 'merge.delete.' and so on. In merging, Data::ModeMerge allows keys
on the right side hash not only to replace but also add, subtract, remove keys
from the left side. This is powerful because it allows schema definition to not
only add clauses (restrict types even more), but also replace clauses (change
type restriction) as well as delete clauses (relax type restriction). For more
information, refer to the Data::ModeMerge documentation.

Illustration:

 int + {"div_by": 2} + {"div_by": 3}               // must be divisible by 2 & 3

 int + {"div_by": 2} + {"merge.normal.div_by": 3} // will be merged and become:
 int + {"div_by": 3}                              // must be divisible by 3 ONLY

 int + {"div_by": 2} + {"merge.delete.div_by": 0}  // will be merged and become:
 int + {}                                          // need not be divisible

 int + {"in": [1,2,3,4,5]} + {"in": [6]}           // impossible to satisfy

 int + {"in": [1,2,3,4,5]} + {"merge.add.in": [6]} // will be merged and become:
 int + {"in": [1,2,3,4,5,6]}

 int + {"in": [1,2,3,4,5]}, {"merge.subtract.in": [4]}  // will become:
 int + {"in": [1,2,3,  5]}

Merging is performed before schema is normalized.

Merging is not recursive.

=head1 EXPRESSION

XXX: Syntax of variables not yet finalized.

Sah supports expressions, using L<Language::Expr> minilanguage. See
L<Language::Expr::Manual::Syntax> for details on the syntax. You can specify
expression in the C<check> clause, e.g.:

 ["int", {"check": "$_ >= 4"}]

Alternatively, expression can also be specified in any clause's attribute:

 ["int", {"min=": "floor(4.9)"}]

The above three schemas are equivalent to:

 ["int", {"min", 4}]

Expression can refer to elements of data and (normalized) schema, and can call
functions, enabling more complex schema to be defined, for example:

 ["array*", {"len": 2, "elems": [
   ["str*", {"match": "^\w+$"}],
   ["str*", {"match=": "${../../0/clause_sets/0/match}",
             "min_len=": "2*length(${data:../0})"}]
 ]}]

The above schema requires data to be a two-element array containing strings,
where the length of the second string has to be at least twice the length of the
first. Both strings have to comply to the same regex, C<^\w+$> (which is
declared on the first string's clause and referred to in the second string's
clause).

=head1 FUNCTION

Functions can be used in expressions. The syntax of calling function is:

 func()
 func(ARG, ...)

Functions in Sah can sometimes accept several types of arguments, e.g.
len(ARRAY) will return the number of elements in the ARRAY, while len(STR) will
return the number of characters in the string. However, when an inappropriate
argument is given, an exception will be thrown.

=head1 HISTORY

2012-07-21 split specification to Sah

2011-11-23 Data::Sah

2009-03-30 Data::Schema (first CPAN release)

Previous incarnation as Schema-Nested (internal)

=head1 SEE ALSO

L<DefHash>

L<Sah::Type>, L<Sah::FAQ>

=head1 AUTHOR

Steven Haryanto <stevenharyanto@gmail.com>

=head1 COPYRIGHT AND LICENSE

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.

=cut

