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Package "dict_schema"

Title:Dict converter and validator
Rating:Not rated. Create the first rating!
Latest version:0.0.2
SHA1 sum:be9cca6cdc75876e2b2e25734e4ff7020e0adfba
Author:Raivo Laanemets http://rlaanemets.com/
Home page:https://github.com/rla/dict-schema


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Dict validation/conversion for Swi-Prolog. The library started as a predicate to convert certain dict (from HTTP JSON requests) entries into suitable forms (especially the string/atom conversion). A large part of this library was inspired by JSON-Schema.


Check vehicle against a schema:

?- Schema = _{
    type: dict,
    keys: _{
        year: _{ type: integer, min: 1672 },
        make: _{ type: atom, min_length: 1 },
        model: _{ type: atom, min_length: 1 }
Vehicle = vehicle{
    year: 1953,
    make: chevrolet,
    model: corvette
convert(Vehicle, Schema, Out, Errors),
Out = vehicle{
    make: chevrolet,
    model: corvette,
    year: 1953
Errors = [].

You can name the schema by registering it:

?- register_schema(vehicle, _{
    type: dict,
    keys: _{
        year: _{ type: integer, min: 1672 },
        make: _{ type: atom, min_length: 1 },
        model: _{ type: atom, min_length: 1 }

And then use it by name:

Vehicle = vehicle{
    year: 1200,
    make: chevrolet,
    model: corvette
convert(Vehicle, vehicle, Out, Errors).
Out = vehicle{
    make: chevrolet,
    model: corvette,
    year: 1200
Errors = [min(#/year, 1200, 1672)].

The last example also shows validation error for the year key. Another feature is automatic conversion from strings to atoms when the atom type is requested:

?- convert("abc", atom, Out, Errors), atom(Out).

Path indicators

Path indicators are used for locating errors in terms. They have the following meaning:

  • # - the term root (or root term).
  • key - (an atom), key of dict.
  • name(N) - N-th argument of compound with the name name.
  • [N] - N-th element in the list.
  • / - path separators.


?- Schema = _{
    type: dict,
    keys: _{
        a: _{
            type: list,
            items: _{
                type: compound,
                name: b,
                arguments: [ number ]
In = d{ a: [ b(2), b(a), b(4) ] },
convert(In, Schema, Out, Errors).
Out = d{a:[b(2), b(a), b(4)]},
Errors = [not_number(#/a/[1]/b(0), a)].

The error path `#/a/[1]/b(0)` refers here to the key a in the root dict, the 1-st item of the list (starts from 0) and the 0-th argument of the term.

Available types

All types below assume that input is either a ground or a dict with ground values. Exceptions are any and var. A dict with an unbound tag is allowed depending on its type's tag attribute.


String type has the following optional attributes:

  • min_length - specifies the minimum length of the string.
  • max_length - specifies the maximum length of the string.


  • When the input value is an atom, it is converted into a string. All other input values other than strings will produce an error not_string(Path, Value).
  • The min_length property is violated: min_length(Path, Value, MinLength).
  • The max_length property is violated: max_length(Path, Value, MaxLength).


Works similar to the string type. When the input is a string, it is converted into an atom. Has same optional attributes. When input is not a string or atom, an error term not_atom(Path, Value) is produced.


Number type has the following optional attributes:

  • min - specifies the minimum value of the number.
  • max - specifies the maximum value of the number.


  • Value is not a number: not_number(Path, Value).
  • The min property is violated: min(Path, Value, Min).
  • The max property is violated: max(Path, Value, Max).


Same as the type number but allows integers only.


The bool type only allows atoms true and false. Produces error not_bool(Path, Value) when the input is not one of those.


The enum type has attribute values that contains a list of allowed values. The list must contain atoms. If the checked value is not in the list then an error is produced. If the input value is a string then it is converted into an atom first. All other values produce an error not_enum(Path, Value).


The dict type has the following attributes:

  • tag - specifies the tag of the dict. When the input has unbound tag it will be unified with the specified tag. If the tag attribute is missing then no tag checking is performed.
  • keys - specifies dict keys and schemas for values.
  • optional - list of keys that are optional.
  • additional - specifies whether extra keys are allowed or not. Default value is false.


  • Value not a dict: not_dict(Path, Value).
  • When the additional property is missing or its value is false and every key is not listed in keys: additional_key(Path, Key).
  • When a key in the input is missing: no_key(Path, Key).
  • When the tag property is specified and the input's tag does not match it: invalid_tag(Path, Tag, RequiredTag).

When the tag property is specified and the input has no tag then input's tag is unified with the tag property value.


The list type has the following attributes:

  • items - specifies the type of the list items.
  • min_length - specifies the minimum number of items (optional).
  • max_length - specifies the maximum number of items (optional).


  • When the input is not a list: not_list(Path, Value).
  • When the min_length property is violated: min_length(Path, Value, MinLength).
  • When the max_length property is violated: max_length(Path, Value, MaxLength).


The compound type has the following attributes:

  • name - the compound name.
  • arguments - the compound arguments.


  • When the input is not a compound: invalid_compound(Path, In).
  • When number of arguments does not match: compound_args_length(Path, ActualLen, RequiredLen).
  • When the name does not match: compound_name(Path, ActualName, Name).


Union of types can be expressed with using a list. The first schema and the conversion result that matches is used. When no schema matches then an error union_mismatch(Path, Reasons) is produced.


?- convert(123, [ number, atom ], Out, Errors).
Out = 123,
Errors = [].

?- convert(a(1), [ number, atom ], Out, Errors).
Out = a(1),
Errors = [union_mismatch(#, [
    [not_atom(#, a(1))],
    [not_number(#, a(1))]


Type any marks the value non-checked and non-converted.


Type var is for variables in the input. When the input is not a variable then an error term not_variable(Path, Value) is produced.

Named schemas

Named schema can be added with register_schema(Name, Schema) and removed with unregister_schema(Name). Schema names must be atoms.

Validating trees

The following example validates binary tree that has integers in leafs. The top-level schema is an union of branch and leaf schemas.

?- register_schema(tree, [
        type: compound,
        name: branch,
        arguments: [ tree, tree ]
        type: integer,
        min: 0

?- convert(branch(32, branch(13, 56)), tree, Out, Errors).
Out = branch(32, branch(13, 56)),
Errors = [].

?- convert(branch(32, a), tree, Out, Errors).
Out = branch(32, a),
Errors = [union_mismatch(#,[
    [union_mismatch(# / branch(1),[
        [not_integer(# / branch(1),a)],
        [invalid_compound(# / branch(1),a)]

Using with HTTP

This was the main motivation and use case for this library. The following example registers a schema for JSON input documents that must contain a property from. The value of the property must be an atom with length minimally 3.

:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_json)).
:- use_module(library(dict_schema)).

% Schema for hello input.

:- register_schema(hello, _{
    type: dict,
    keys: _{ from: _{ type: atom, min_length: 3 } }

:- http_handler('/hello', receive_data, []).

    http_read_json_dict(Request, Dict),
    % Convert/validate input:
    convert(Dict, hello, Hello, Errors),
    format('Content-type: text/plain; charset=UTF-8~n~n'),
    (   Errors = []
    ->  get_dict(from, Hello, From),
        format('~w said hello~n', [From])
    ;   format('Input errors: ~w~n', [Errors])).

:- http_server(http_dispatch, [port(8080)]).

Example calls using curl:

curl -X POST -H 'Content-Type: application/json' \
-d '{"from": "RLa"}' http://localhost:8080/hello

RLa said hello

curl -X POST -H 'Content-Type: application/json' \
-d '{"to": "RLa"}' http://localhost:8080/hello

Input errors: [additional_key(#,to),no_key(#,from)]


The schema is validated on-the-fly during the validation of the input term. It is checked for valid attributes. Schema errors are thrown as exceptions and are not placed into the Errors output list.


  • 2014-04-22 - replace get_dict_ex/3 with dot operator.

API documentation

Generated API documentation can be found from here: http://packs.rlaanemets.com/dict-schema/doc.


This package requires Swi-Prolog 7.x.


Running tests

In the package root, insert into swipl:


Or if you cloned the repo:

make test

Bug reports/feature requests

Please send bug reports/feature request through the GitHub project page.


The MIT License. See the LICENSE file.

Contents of pack "dict_schema"

Pack contains 5 files holding a total of 34.6K bytes.