.. _library_wrapper:

``wrapper``
===========

This is a prototype tool to help port a plain Prolog application to
Logtalk. It can also be used to enable applying other Logtalk developer
tools, such as the documenting and diagramming tools, to plain Prolog
code.

The tool takes a directory of Prolog files or a list of Prolog files,
loads and wraps the code in each file using an object wrapper, and
advises on missing directives to be added to those objects by using the
compiler lint checker and the reflection API. The user can then either
save the generated wrapper objects or copy and paste the printed advice
into the Prolog files (updating them to Logtalk files by adding the
object opening and closing directives to the Prolog files). The wrapper
objects use ``include/1`` directives to include the Prolog files and can
be loaded for testing and for use with other tools. The wrapped Prolog
files are not modified and thus require only read permission.

API documentation
-----------------

This tool API documentation is available at:

`../../apis/library_index.html#wrapper <../../apis/library_index.html#wrapper>`__

Loading
-------

This tool can be loaded using the query:

::

   | ?- logtalk_load(wrapper(loader)).

Workflows
---------

The typical porting workflow is:

::

   | ?- wrapper::rdirectory(root_directory_of_prolog_code).
   ...
   | ?- wrapper::save.
   ...

See the next section on how to customize the API calls for more flexible
processing.

Customization
-------------

The tool can be customized by extending the ``wrapper`` object. A common
scenario is when wrapping plain Prolog code just to take advantage, for
example, of the documenting tool or for generating cross-referencing
diagrams. In this case, we can workaround any compiler errors by
specializing the inherited definitions for the ``term_expansion/2`` and
``goal_expansion/2`` predicates and then load the wrapper objects for
further processing by using the ``include_wrapped_files(false)`` option
described below.

The API predicates also accept a set of options for customization:

- | ``prolog_extensions(Extensions)``
  | list of file name extensions used to recognize Prolog source files
    (default is ``['.pl', '.pro', '.prolog']``)

- | ``logtalk_extension(Extension)``
  | Logtalk file name extension to be used for the generated wrapper
    files (default is ``'.lgt'``)

- | ``exclude_files(Files)``
  | list of Prolog source files to exclude (default is ``[]``)

- | ``exclude_directories(Files)``
  | list of sub-directories to exclude (default is ``[]``)

- | ``include_wrapped_files(Boolean)``
  | generate ``include/1`` directives for the wrapped Prolog source
    files (default is ``true``)

The use, by default, of ``include/1`` directives to wrap the code in
Prolog source files facilitates running in parallel the Logtalk port and
the original Prolog code. This is specially useful when the Prolog code
being ported lacks a comprehensive set of tests that could be adapted to
verify the Logtalk port. The generated Logtalk files can also take
advantage of ``uses/2`` directives, including predicate aliases and
predicate shorthands to minimize the changes to the original Prolog code
and help verify if replacement calls to Logtalk library predicates
provide the same semantics as the original calls.

Current limitations
-------------------

- The tool cannot deal with syntax errors in the Prolog files. These
  errors usually occur when using a backend Prolog system different from
  the one used to compile the original plain Prolog code. A common cause
  of syntax errors is operator definitions. These can often be solved by
  defining those operators for the Prolog backend used to run Logtalk
  and this tool. An alternative is to preload the Prolog files where
  those operators are declared. Preloading the plain Prolog application
  can also help in wrapping it by ensuring that its dependencies are
  also loaded.

- The tool assumes that all files to be wrapped have different names
  (even if found in different directories). If that is not the case, the
  name conflicts must be manually solved before using the tool.

- There's only preliminary support for dealing with meta-predicates and
  advise on missing meta-predicate directives.
