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Sah::FAQ(3) User Contributed Perl Documentation Sah::FAQ(3)

Sah::FAQ - Frequently asked questions

This document describes version 0.9.45 of Sah::FAQ (from Perl distribution Sah), released on 2017-03-09.

Why use a schema (a.k.a "Turing tarpit")? Why not use pure Perl?
Schema language is a specialized language (DSL) that should be more concise to write than equivalent Perl code for common validation tasks. Its goal is never to be as powerful as Perl.
90% of the time, my schemas are some variations of the simple cases like:
 ["str":   {"len_between": [1, 10], "match": "some regex"}]
 ["str":   {"in": ["a", "b", "c", ...]}]
 ["array": {"of": "some_other_type"}]
 ["hash":  {"keys": {"key1": "some schema", ...}, "req_keys": [...], ...}]
and writing schemas is faster and less tedious/error-prone than writing equivalent Perl code, plus Data::Sah can generate JavaScript code and human description text for me. For more complex validation I stay with Sah until it starts to get unwieldy. It usually can go pretty far since I can add functions and custom clauses to its types; it's for the very complex and dynamic validation needs that I go pure Perl. Your mileage may vary.
What does "Sah" mean?
Sah is an Indonesian word, meaning "valid" or "legal". It's picked because it's short.
The previous incarnation of this module uses the namespace Data::Schema, started in 2009 and deprecated in 2011 in favor of "Sah".

Comparison to JSON schema?
JSON schema limits its type system to that supported by JSON/JavaScript.
JSON schema's syntax is simpler.
Its metaschema (schema for the schema) is only about 130 lines. There are no shortcut forms.
JSON schema's features are more limited.
No expression, no function.
Comparison to Data::Rx?
Comparison to Data::FormValidator (DFV)?
Comparison to Moose types?

Why is "req" not enabled the default?
I am following SQL's behavior. A type declaration like:
in SQL means "NULL" is allowed, while:
means "NULL" is not allowed. The above is equivalent to specifying this in Sah:
One could argue that setting "req" to 1 by default is safer/more convenient to her/whatever, and "int" should mean "["int", "req", 1]" while something like perhaps "int?" means "["int", "req", 0]". But this is simply a design choice and each has its pros/cons. Nullable by default can also be convenient in some cases, like when specifying program options where most of the options are optional.
How about adding a "default_req" configuration in "Data::Sah" then?
In general I am against compiler configuration which changes language behavior (think PHP's "register_globals" or <magic_quotes_*> settings). In this case, it makes a simple schema like "int" to have ambiguous meaning (is undefined value allowed? Or not allowed? It depends on compiler configuration).
Why "int" instead of "integer"? Why "req" instead of "required"? "str" instead of "string"? Etc.
This is also a design choice. To be consistent, either we abbreviate or we don't. Although there is very little reason to abbreviate when it comes to disk/memory size (compared to the eras of early Unix or C language), there are other limited resources to consider: source code column width (usually still around 80 characters in many best practices) and developer's time/energy (typing more takes more time and effort).
I want to make it possible for short schemas to be specified on a single line. For example compare:
 [integer => {required => 1, minimum => 0, maximum => 100, divisible_by => 2}]
 [int => {req=>1, min=>0, max=>100, div_by=>2}]
The latter is not that much less readable than the first, but is less tedious to type, especially if you write lots of schemas.
Therefore, the decision is to use commonly used (and unambiguous) abbreviations for type and clause names.
How to express "not-something"? Why isn't there a "not" or "not_in" clause?
There are generally no "not_CLAUSE" clauses. Instead, a generic "!CLAUSE" syntax is provided. Examples:
 // an integer that is not 0
 ["int", {"!is": 0}]
 // a username that is not one of the forbidden/reserved ones
 ["str", {"!in": ["root", "admin", "superuser"]}]
How to state "in" as well as "!in" in the same clause set?
You can't do this since it will cause a conflict:
 ["str ", {"in": ["a","b","c"], "!in": ["x","y","z"]}]
However, you can do this:
 ["str ", {"clset&": [{"in": ["a","b","c"]}, {"!in": ["x","y","z"]}]}]
How to express mutual failure ("if A fails, B must also fail")?
You can use "if" clause and negate the clauses. For example:
 "if": [{"!div_by": 2}, {"!div_by": 5}]
How about "len_in" clause for str? Or "values_uniq" for hash? Or perhaps "len_div_by"? Or some other clauses that test a property/transform of a value?
Except for some commonly used cases like "len_between", "min_len", "max_len", "allowed_keys", "forbidden_keys", to validate a certain property of the value (instead of the raw value itself), you can use the generic "prop" clause:
 // check hash values are unique
 ["hash", {"prop": ["values", ["array", {"uniq":1}]]}]
Or, for more general use, you can apply a temporary prefilter first to the data:
 // check hash values are unique
 ["array", {"prefilters.temp":1, "prefilters":["values($_)"], "uniq":1}]
General advice when writing schemas?
Avoid "any" or "all" if you know that data is of a certain type
For performance and ease of reflection, it is better to create a custom clause than using the "any" type, especially with long list of alternatives. An example:
 // dns_record is either a_record, mx_record, ns_record, cname_record, ...
 ["any", "of", [
 // base_record
 ["hash", "keys", {
     "owner": "str*",
     "ttl": "int",
 // a_record
 ["base_record", "merge.normal.keys", {
     "type": ["str*", "is", "A"],
     "address": "str*"
 // mx_record
 ["base_record", "merge.normal.keys", {
     "type": ["str*", "is", "MX"],
     "host": "str*",
     "prio": "int"
If you see the declaration above, every record is a hash. So it is better to declare "dns_record" as a "hash" instead of an "any". But we need to select a different schema based on the "type" key. We can develop a custom clause like this:
 ["hash", "select_schema_on_key", ["type", {
     "A": "a_record",
     "MX": "mx_record",
     "NS": "ns_record",
     "CNAME": "cname_record",
This will be faster.

How does Sah check allowed/unallowed keys?
If "keys" clause is specified, then by default only keys defined in "keys" clause is allowed, unless the ".restrict" attribute is set to false, in which case no restriction to allowed keys is done by the clause. The same case for "re_keys".
If "allowed_keys" and/or "allowed_keys_re" clause is specified, then only keys matching those clauses are allowed. This is in addition to restriction placed by other clauses, of course.
How do I specify schemas for some keys, but still allow some other keys?
Set the ".restrict" attribute for "keys" or "re_keys" to false. Example:
 ["hash", {
     "keys": {"a": "int", "b": "int"},
     "keys.restrict": 0,
     "allowed_keys": ["a", "b", "c", "d", "e"]
The above schema allows keys "a, b, c, d, e" and specifies values for "a, b". Another example:
 ["hash", {
     "keys": {"a": "int", "b": "int"},
     "keys.restrict": 0,
     "allowed_keys_re": "^[ab_]",
The above schema specifies values for "a, b" but still allows other keys beginning with an underscore.
What is the difference between the "keys" and "req_keys" clauses?
"req_keys" require keys to exist, but their values are governed by the schemas in "keys" or "keys_re". Here are four combination possibilities, each with the schema:
To require a hash key to exist, but its value can be undef:
 ["hash", "keys", {"a": "int"}, "req_keys": ["a"]]
To allow a hash key to not exist, but when it exists it must not be undef:
 ["hash", "keys", {"a": "int*"}]
To allow a hash key to not exist, or its value to be undef when exists:
 ["hash", "keys", {"a": "int"}]
To require hash key exist and its value must not be undef:
 ["hash", "keys", {"a": "int*"}, "req_keys": ["a"]]
Merging and hash keys?
XXX (Turn off hash merging using the '' Data::ModeMerge options key.

Please visit the project's homepage at <>.

Source repository is at <>.

Please report any bugs or feature requests on the bugtracker website <>
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.

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This software is copyright (c) 2017, 2016, 2015, 2014, 2013, 2012 by
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.
2017-03-09 perl v5.28.1

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