pengines_io.pl -- Provide Prolog I/O for HTML clients

This module redefines some of the standard Prolog I/O predicates to behave transparently for HTML clients. It provides two ways to redefine the standard predicates: using goal_expansion/2 and by redefining the system predicates using redefine_system_predicate/1. The latter is the preferred route because it gives a more predictable trace to the user and works regardless of the use of other expansion and meta-calling.

Redefining works by redefining the system predicates in the context of the pengine's module. This is configured using the following code snippet.

:- pengine_application(myapp).
:- use_module(myapp:library(pengines_io)).
pengines:prepare_module(Module, myapp, _Options) :-
      pengines_io:pengine_bind_io_to_html(Module).

Using goal_expansion/2 works by rewriting the corresponding goals using goal_expansion/2 and use the new definition to re-route I/O via pengine_input/2 and pengine_output/1. A pengine application is prepared for using this module with the following code:

:- pengine_application(myapp).
:- use_module(myapp:library(pengines_io)).
myapp:goal_expansion(In,Out) :-
      pengine_io_goal_expansion(In, Out).

pengine_writeln(+Term)

Emit Term as Term .

pengine_nl

Emit a to the pengine.

pengine_flush_output

No-op. Pengines do not use output buffering (maybe they should though).

pengine_write_term(+Term, +Options)

Writes term as Term. In addition to the options of write_term/2, these options are processed:

class(+Class): Specifies the class of the element. Default is write.

pengine_write(+Term) is det

pengine_writeq(+Term) is det

pengine_display(+Term) is det

pengine_print(+Term) is det

pengine_write_canonical(+Term) is det

Redirect the corresponding Prolog output predicates.

pengine_format(+Format) is det

pengine_format(+Format, +Args) is det

As format/1,2. Emits a series of strings with for each newline encountered in the string.

To be done: - : handle ~w, ~q, etc using term//2. How can we do that??

pengine_listing is det

pengine_listing(+Spec) is det

List the content of the current pengine or a specified predicate in the pengine.

user:message_hook(+Term, +Kind, +Lines) is semidet[multifile]

Send output from print_message/2 to the pengine. Messages are embedded in a <pre class=msg-Kind></pre> environment.

send_html(+HTML) is det[private]

Convert html//1 term into a string and send it to the client using pengine_output/1.

pengine_module(-Module) is det[private]

Module (used for resolving operators).

pengines:event_to_json(+Event, -JSON, +Format, +VarNames) is semidet[private]

Provide additional translations for Prolog terms to output. Defines formats are:

'json-s'

Simple or string format: Prolog terms are sent using quoted write.

'json-html'

Serialize responses as HTML string. This is intended for applications that emulate the Prolog toplevel. This format carries the following data:

data

List if answers, where each answer is an object with

variables

Array of objects, each describing a variable. These objects contain these fields:

variables: Array of strings holding variable names
value: HTML-ified value of the variables
substitutions: Array of objects for substitutions that break cycles holding:
- var: Name of the inserted variable
- value: HTML-ified value

residuals

Array of strings representing HTML-ified residual goals.

pengines:event_to_json(+PrologEvent, -JSONEvent, +Format, +VarNames)[private]

If Format equals 'json-s' or 'json-html', emit a simplified JSON representation of the data, suitable for notably SWISH. This deals with Prolog answers and output messages. If a message originates from print_message/3, it gets several additional properties:

message:Kind: Indicate the kind of the message (error, warning, etc.)
location:_60632{ch:CharPos, file:File, line:Line}: If the message is related to a source location, indicate the file and line and, if available, the character location.

answer_to_json_strings(+Pengine, +AnswerDictIn, -AnswerDict)[private]

Translate answer dict with Prolog term values into answer dict with string values.

pengines:event_to_json(+Event, -JSON, +Format, +VarNames)[private]

Implement translation of a Pengine event to json-html format. This format represents the answer as JSON, but the variable bindings are (structured) HTML strings rather than JSON objects.

CHR residual goals are not bound to the projection variables. We hacked a bypass to fetch these by returning them in a variable named Residuals, which must be bound to a term '$residuals'(List). Such a variable is removed from the projection and added to residual goals.

binding_to_html(+Pengine, +Binding, -Dict) is det[private]

Convert a variable binding into a JSON Dict. Note that this code assumes that the module associated with Pengine has the same name as the Pengine. The module is needed to

Arguments:

Binding - is a term binding(Vars,Term,Substitutions)

term_html_string(+Term, +VarNames, +Module, -HTMLString, +Options) is det[private]

Translate Term into an HTML string using the operator declarations from Module. VarNames is a list of variable names that have this value.

binding_term(+Term, +Vars, +WriteOptions)// is semidet[multifile]

Hook to render a Prolog result term as HTML. This hook is called for each non-variable binding, passing the binding value as Term, the names of the variables as Vars and a list of options for write_term/3. If the hook fails, term//2 is called.

Arguments:

Vars - is a list of variable names or [] if Term is a residual goal.

subst_to_html(+Module, +Binding, -JSON) is det[private]

Render a variable substitution resulting from term factorization, in this case breaking a cycle.

map_output(+ID, +Term, -JSON) is det[private]

Map an output term. This is the same for json-s and json-html.

pengine_io_predicate(?Head)

True when Head describes the head of a (system) IO predicate that is redefined by the HTML binding.

pengine_bind_user_streams[private]

Bind the pengine user I/O streams to a Prolog stream that redirects the input and output to pengine_input/2 and pengine_output/1. This results in less pretty behaviour then redefining the I/O predicates to produce nice HTML, but does provide functioning I/O from included libraries.

pengine_output is semidet[private]

pengine_input is semidet[private]