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yap-6.3/packages/http/json_convert.pl

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2010-06-23 11:52:34 +01:00
/* $Id$
Part of SWI-Prolog
Author: Jan Wielemaker
E-mail: wielemak@science.uva.nl
WWW: http://www.swi-prolog.org
Copyright (C): 1985-2007, University of Amsterdam
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
As a special exception, if you link this library with other files,
compiled with a Free Software compiler, to produce an executable, this
library does not by itself cause the resulting executable to be covered
by the GNU General Public License. This exception does not however
invalidate any other reasons why the executable file might be covered by
the GNU General Public License.
*/
:- module(json_convert,
[ prolog_to_json/2, % :Term, -JSON object
json_to_prolog/2, % +JSON, :Term
(json_object)/1, % +Definition
op(1150, fx, (json_object))
]).
:- use_module(library(error)).
:- use_module(json).
:- meta_predicate
prolog_to_json(:, -),
json_to_prolog(+, :).
/** <module> Convert between JSON terms and Prolog application terms
The idea behind this module is to provide a flexible high-level mapping
between Prolog terms as you would like to see them in your application
and the standard representation of a JSON object as a Prolog term. For
example, an X-Y point may be represented in JSON as =|{"x":25,
"y":50}|=. Represented in Prolog this becomes json([x=25,y=50]), but
this is a pretty non-natural representation from the Prolog point of
view.
This module allows for defining records (just like library(record)) that
provide transparent two-way transformation between the two
representations.
==
:- json_object
point(x:integer, y:integer).
==
This declaration causes prolog_to_json/2 to translate the native Prolog
representation into a JSON Term:
==
?- prolog_to_json(point(25,50), X).
X = json([x=25, y=50])
==
A json_object/1 declaration can define multiple objects separated by a
comma (,), similar to the dynamic/1 directive. Optionally, a declaration
can be qualified using a module. The converstion predicates
prolog_to_json/2 and json_to_prolog/2 first try a conversion associated
with the calling module. If not successful, they try conversions
associated with the module =user=.
JSON objects have no _type_. This can be solved by adding an extra field
to the JSON object, e.g. =|{"type":"point", "x":25, "y":50}|=. As Prolog
records are typed by their functor we need some notation to handle this
gracefully. This is achieved by adding +Fields to the declaration. I.e.
==
:- json_object
point(x:integer, y:integer) + [type=point].
==
Using this declaration, the conversion becomes:
==
?- prolog_to_json(point(25,50), X).
X = json([x=25, y=50, type=point])
==
The predicate json_to_prolog/2 is often used after http_read_json/2 and
prolog_to_json/2 before reply_json/1. For now we consider them seperate
predicates because the transformation may be too general, too slow or
not needed for dedicated applications. Using a seperate step also
simplifies debugging this rather complicated process.
@tbd Ignore extra fields. Using a partial list of _extra_?
@tbd Consider a sensible default for handling JSON =null=. Conversion
to Prolog could translate @null into a variable if the desired type
is not =any=. Conversion to JSON could map variables to =null=,
though this may be unsafe. If the Prolog term is known to be
non-ground and JSON @null is a sensible mapping, we can also use
this simple snipit to deal with that fact.
==
term_variables(Term, Vars),
maplist(=(@null), Vars).
==
*/
%% current_json_object(Term, Module, Fields)
%
% Multifile predicate computed from the json_object/1
% declarations. Term is the most general Prolog term representing
% the object. Module is the module in which the object is defined
% and Fields is a list of f(Name, Type, Var), sorted by Name. Var
% is the corresponding variable in Term.
:- multifile
json_object_to_pairs/3, % Term, Module, Pairs
current_json_object/3. % Term, Module, Fields
%% json_object(+Declaration)
%
% Declare a JSON object. The declaration takes the same format as
% using in record/1 from library(record). E.g.
%
% ==
% ?- json_object
% point(x:int, y:int, z:int=0).
% ==
json_object(Declaration) :-
throw(error(context_error(nodirective, json_object(Declaration)), _)).
%% compile_json_objects(+Spec, -Clauses) is det.
%
% Compiles a :- json_object directive into Clauses. Clauses are of
% the form:
%
% ==
% json_object_to_pairs(Term, Module, Pairs) :-
% <type-checks on Term>,
% <make Pairs from Term>.
% ==
compile_json_objects(Spec, Clauses) :-
phrase(compile_objects(Spec), Clauses).
compile_objects(A) -->
{ var(A), !,
instantiation_error(A)
}.
compile_objects((A,B)) --> !,
compile_objects(A),
compile_objects(B).
compile_objects(Term) -->
compile_object(Term).
compile_object(ObjectDef) -->
{ prolog_load_context(module, CM),
strip_module(CM:ObjectDef, M, Def),
extra_defs(Def, Term, ExtraFields),
Term =.. [Constructor|Args],
defaults(Args, _Defs, TypedArgs),
types(TypedArgs, Names, Types)
},
record_to_json_clause(Constructor, M, Types, Names, ExtraFields),
current_clause(Constructor, M, Types, Names, ExtraFields),
[ (:- json_convert:clear_cache) ].
extra_defs(Term+Extra0, Term, Extra) :- !,
must_be(list, Extra0),
maplist(canonical_pair, Extra0, Extra).
extra_defs(Term, Term, []).
canonical_pair(Var, _) :-
var(Var), !,
instantiation_error(Var).
canonical_pair(Name=Value, Name=Value) :- !,
must_be(atom, Name).
canonical_pair(Name-Value, Name=Value) :- !,
must_be(atom, Name).
canonical_pair(NameValue, Name=Value) :-
NameValue =.. [Name,Value], !.
canonical_pair(Pair, _) :-
type_error(pair, Pair).
%% record_to_json_clause(+Constructor, +Module, +Type, +Names)
%
% Create a clause translating the record definition into a pairs
% representation.
record_to_json_clause(Constructor, Module, Types, Names, Extra) -->
{ type_checks(Types, VarsHead, VarsBody, Body0, Module),
clean_body(Body0, Body),
Term =.. [Constructor|VarsHead],
make_pairs(Names, VarsBody, Pairs, Extra),
Head =.. [json_object_to_pairs,Term,Module,json(Pairs)]
},
[ (json_convert:(Head :- Body)) ].
%% type_checks(+Types, -VarsIn, -VarsOut, -Goal, +Module) is det.
%
% Goal is a body-term that validates Vars satisfy Types. In
% addition to the types accepted by must_be/2, it accepts =any=
% and Name/Arity. The latter demands a json_object term of the
% given Name and Arity.
%
% @tbd Compile list(Type) specification. Currently Type is
% handled like =any=
type_checks([], [], [], true, _).
type_checks([Type|T], [IV|IVars], [OV|OVars], (Goal, Body), M) :- !,
type_check(Type, IV, OV, M, Goal),
type_checks(T, IVars, OVars, Body, M).
type_check(any, IV, OV, M, prolog_to_json(IV, OV, M)) :- !.
type_check(Name/Arity, IV, OV, M, prolog_to_json(IV, OV, M)) :- !,
functor(IV, Name, Arity).
type_check(boolean, IV, OV, _, prolog_bool_to_json(IV, OV)) :- !.
type_check(list, IV, OV, M, prolog_list_to_json(IV, OV, M)) :- !.
type_check(list(any), IV, OV, M, prolog_list_to_json(IV, OV, M)) :- !.
type_check(list(_Type), IV, OV, M, prolog_list_to_json(IV, OV, M)) :- !.
type_check(Type, V, V, _, Goal) :-
type_goal(Type, V, Goal).
%% prolog_bool_to_json(+Prolog, -JSON) is semidet.
%
% JSON is the JSON boolean for Prolog. It is a flexible the Prolog
% notation for thruth-value, accepting one of =true=, =on= or =1=
% for @true and one of =false=, =fail=, =off= or =0= for @false.
%
% @error instantiation_error if Prolog is unbound.
prolog_bool_to_json(Var, _) :-
var(Var),
instantiation_error(Var).
prolog_bool_to_json(true, @(true)).
prolog_bool_to_json(false, @(false)).
prolog_bool_to_json(fail, @(false)).
prolog_bool_to_json(0, @(false)).
prolog_bool_to_json(on, @(true)).
prolog_bool_to_json(off, @(false)).
prolog_bool_to_json(1, @(false)).
prolog_bool_to_json(@(True), True) :-
prolog_bool_to_json(True, True).
%% type_goal(+Type, +Var, -BodyTerm) is det.
%
% Inline type checking calls.
type_goal(Type, Var, Body) :-
clause(error:has_type(Type, Var), Body),
primitive(Body), !.
type_goal(Type, Var, is_of_type(Type, Var)).
primitive((A,B)) :- !,
primitive(A),
primitive(B).
primitive((A;B)) :- !,
primitive(A),
primitive(B).
primitive((A->B)) :- !,
primitive(A),
primitive(B).
primitive(G) :-
predicate_property(system:G, built_in).
%% clean_body(+BodyIn, -BodyOut) is det.
%
% Cleanup a body goal. Eliminate redundant =true= statements and
% perform partial evaluation on some commonly constructs that are
% generated from the has_type/2 clauses in library(error).
clean_body(Var, Var) :-
var(Var), !.
clean_body((A0,B0), G) :- !,
clean_body(A0, A),
clean_body(B0, B),
conj(A, B, G).
clean_body(ground(X), true) :- % Generated from checking extra fields.
ground(X), !.
clean_body(memberchk(V, Values), true) :- % generated from oneof(List)
ground(V), ground(Values),
memberchk(V, Values), !.
clean_body((integer(Low) -> If ; Then), Goal) :- % generated from between(Low,High)
number(Low), !,
( integer(Low)
-> Goal = If
; Goal = Then
).
clean_body(A, A).
conj(T, A, A) :- T == true, !.
conj(A, T, A) :- T == true, !.
conj(A, B, (A,B)).
make_pairs([], [], L, L).
make_pairs([N|TN], [V|TV], [N=V|T], Tail) :-
make_pairs(TN, TV, T, Tail).
%% current_clause(+Constructor, +Module, +Type, +Names)
%
% Create the clause current_json_object/3.
current_clause(Constructor, Module, Types, Names, Extra) -->
{ length(Types, Arity),
functor(Term, Constructor, Arity),
extra_fields(Extra, EF),
Term =.. [_|Vars],
mk_fields(Names, Types, Vars, Fields0, EF),
sort(Fields0, Fields),
Head =.. [current_json_object, Term, Module, Fields]
},
[ json_convert:Head ].
extra_fields([], []).
extra_fields([Name=Value|T0], [f(Name, oneof([Value]), Value)|T]) :-
extra_fields(T0, T).
mk_fields([], [], [], Fields, Fields).
mk_fields([Name|TN], [Type|TT], [Var|VT], [f(Name, Type, Var)|T], Tail) :-
mk_fields(TN, TT, VT, T, Tail).
/* The code below is copied from library(record) */
%% defaults(+ArgsSpecs, -Defaults, -Args)
%
% Strip the default specification from the argument specification.
defaults([], [], []).
defaults([Arg=Default|T0], [Default|TD], [Arg|TA]) :- !,
defaults(T0, TD, TA).
defaults([Arg|T0], [_|TD], [Arg|TA]) :-
defaults(T0, TD, TA).
%% types(+ArgsSpecs, -Defaults, -Args)
%
% Strip the default specification from the argument specification.
types([], [], []).
types([Name:Type|T0], [Name|TN], [Type|TT]) :- !,
must_be(atom, Name),
types(T0, TN, TT).
types([Name|T0], [Name|TN], [any|TT]) :-
must_be(atom, Name),
types(T0, TN, TT).
/*******************************
* PROLOG --> JSON *
*******************************/
%% prolog_to_json(:Term, -JSONObject) is det.
%
% Translate a Prolog application Term into a JSON object term.
% This transformation is based on :- json_object/1 declarations.
% If a json_object/1 declaration declares a field of type
% =boolean=, commonly used thruth-values in Prolog are converted
% to JSON booleans. Boolean translation accepts one of =true=,
% =on=, =1=, @true, =false=, =fail=, =off= or =0=, @false.
%
% @error type_error(json_term, X)
% @error instantiation_error
prolog_to_json(Module:Term, JSON) :-
prolog_to_json(Term, JSON, Module).
prolog_to_json(Var, _, _) :-
var(Var), !,
instantiation_error(Var).
prolog_to_json(Atomic, Atomic, _) :-
atomic(Atomic), !.
prolog_to_json(List, JSON, Module) :-
is_list(List), !,
prolog_list_to_json(List, JSON, Module).
prolog_to_json(Record, JSON, Module) :-
record_to_pairs(Record, JSON, Module), !.
prolog_to_json(Term, Term, _) :-
is_json_term(Term), !.
prolog_to_json(Term, _, _) :-
type_error(json_term, Term).
record_to_pairs(T, _, _) :-
var(T), !,
instantiation_error(T).
record_to_pairs(T, JSON, M) :-
object_module(M, Module),
json_object_to_pairs(T, Module, JSON), !.
object_module(user, user) :- !.
object_module(M, M).
object_module(_, user).
prolog_list_to_json([], [], _).
prolog_list_to_json([H0|T0], [H|T], M) :-
prolog_to_json(H0, H, M),
prolog_list_to_json(T0, T, M).
/*******************************
* JSON --> PROLOG *
*******************************/
:- dynamic
json_to_prolog_rule/3, % Module, Pairs, Term
created_rules_for_pairs/2. % Module, Pairs
clear_cache :-
retractall(json_to_prolog_rule(_,_,_)),
retractall(created_rules_for_pairs(_,_)).
:- clear_cache.
%% json_to_prolog(+JSON, -Term) is det.
%
% Translate a JSON term into an application term. This
% transformation is based on :- json_object/1 declarations. An
% efficient transformation is non-trivial, but we rely on the
% assumption that, although the order of fields in JSON terms is
% irrelevant and can therefore vary a lot, practical applications
% will normally generate the JSON objects in a consistent order.
%
% If a field in a json_object is declared of type =boolean=, @true
% and @false are translated to =true= or =false=, the most
% commonly used Prolog representation for truth-values.
json_to_prolog(JSON, Module:Term) :-
json_to_prolog(JSON, Term, Module).
json_to_prolog(json(Pairs), Term, Module) :- !,
( pairs_to_term(Pairs, Term, Module)
-> true
; json_pairs_to_prolog(Pairs, Prolog, Module),
Term = json(Prolog)
).
json_to_prolog(List, Prolog, Module) :-
is_list(List), !,
json_list_to_prolog(List, Prolog, Module).
json_to_prolog(@(Special), @(Special), _).
json_to_prolog(Atomic, Atomic, _).
json_pairs_to_prolog([], [], _).
json_pairs_to_prolog([Name=JSONValue|T0], [Name=PrologValue|T], Module) :-
json_to_prolog(JSONValue, PrologValue, Module),
json_pairs_to_prolog(T0, T, Module).
json_list_to_prolog([], [], _).
json_list_to_prolog([JSONValue|T0], [PrologValue|T], Module) :-
json_to_prolog(JSONValue, PrologValue, Module),
json_list_to_prolog(T0, T, Module).
%% json_object_to_prolog(+JSONObject, ?Term, +Module) is semidet.
%
% Translate a JSON json(Pairs) term into a Prolog application term.
json_object_to_prolog(json(Pairs), Term, Module) :-
pairs_to_term(Pairs, Term, Module).
%% pairs_to_term(+Pairs, ?Term, +Module) is semidet.
%
% Convert a Name=Value set into a Prolog application term based on
% json_object/1 declarations.
%
% @tbd Ignore extra pairs if term is partially given?
pairs_to_term(Pairs, Term, Module) :-
object_module(Module, M),
( json_to_prolog_rule(M, Pairs, Term)
-> !
; created_rules_for_pairs(M, Pairs)
-> !, fail
; pairs_args(Pairs, PairArgs),
sort(PairArgs, SortedPairArgs),
forall(create_rule(SortedPairArgs, Module, M, Term0, Body),
asserta((json_to_prolog_rule(M, PairArgs, Term0) :- Body))),
asserta(created_rules_for_pairs(M, PairArgs)),
json_to_prolog_rule(M, Pairs, Term), !
).
pairs_args([], []).
pairs_args([Name=_Value|T0], [Name=_|T]) :-
pairs_args(T0, T).
%% create_rule(+PairArgs, +Vars, -Term, -Body) is det.
%
% Create a new rule for dealing with Pairs, a Name=Value list of a
% particular order. Here is an example rule:
%
% ==
% json_to_prolog_rule([x=X, y=Y], point(X,Y)) :-
% integer(X),
% integer(Y).
% ==
create_rule(PairArgs, Module, M, Term, Body) :-
current_json_object(Term, M, Fields),
match_fields(PairArgs, Fields, Body0, Module),
clean_body(Body0, Body).
match_fields([], [], true, _).
match_fields([Name=JSON|TP], [f(Name, Type, Prolog)|TF], (Goal,Body), M) :- !,
match_field(Type, JSON, Prolog, M, Goal),
match_fields(TP, TF, Body, M).
match_field(any, JSON, Prolog, M, json_to_prolog(JSON,Prolog,M)) :- !.
match_field(F/A, JSON, Prolog, M, json_to_prolog(JSON,Prolog,M)) :- !,
functor(Prolog, F, A).
match_field(boolean, JSON, Prolog, _, json_bool_to_prolog(JSON, Prolog)) :- !.
match_field(list(Type), JSON, Prolog, M, json_list_to_prolog(JSON, Prolog, M)) :-
current_json_object(Term, M, _Fields),
functor(Term, Type, _), !.
match_field(Type, Var, Var, _, Goal) :-
type_goal(Type, Var, Goal).
json_bool_to_prolog(@(True), True).
/*******************************
* EXPANSION *
*******************************/
:- multifile
system:term_expansion/2.
:- dynamic
system:term_expansion/2.
system:term_expansion((:- json_object(Spec)), Clauses) :-
compile_json_objects(Spec, Clauses).