.. _library_types:

``types``
=========

This library implements predicates over standard Prolog term types and
also terms representing common data structures such as lists and pairs.

It also includes a user-extensible ``type`` object defining type
checking predicates over common Logtalk and Prolog term types. The types
define a hierarchy with the Prolog type ``term`` at the root (i.e.,
type-checking a predicate argument of type ``term`` trivially succeeds).
Some types are only meaningful for backend Prolog systems supporting
non-universal features (e.g., ``cyclic`` or ``char(CharSet)`` with a
Unicode character set). See the API documentation for a full list of the
types defined by default.

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

Open the
`../../docs/library_index.html#types <../../docs/library_index.html#types>`__
link in a web browser.

Loading
-------

To load all entities in this library, load the ``loader.lgt`` file:

::

   | ?- logtalk_load(types(loader)).

If your code only requires the most basic types, you can load in
alternative the file:

::

   | ?- logtalk_load(basic_types(loader)).

See the notes on the ``basic_types`` virtual library for details.

Testing
-------

To test this library predicates, load the ``tester.lgt`` file:

::

   | ?- logtalk_load(types(tester)).

Type-checking
-------------

This library ``type`` object can be used to type-check common Logtalk
and Prolog term types (see the object documentation for a listing of all
the pre-defined types). The ``valid/2`` predicate succeeds or fails if a
term is of a given type. For example:

::

   | ?- type::valid(positive_integer, 42).
   yes

   | ?- type::valid(positive_integer, -13).
   no

The ``check/2`` and ``check/3`` predicates throw an exception if a term
is not of a given type. For example:

::

   | ?- catch(type::check(integer, abc), Error, true).
   Error = type_error(integer, abc)
   yes

If we require a standard ``error/2`` exception term, the ``check/3``
predicate takes a *context* argument. For example:

::

   | ?- catch(type::check(integer, abc, foo/3), Error, true).
   Error = error(type_error(integer, abc), foo/3)
   yes

Typically, the context is provided by calling the built-in ``context/1``
method.

Defining new types
------------------

To define a custom type, define clauses for the multifile predicates
``type::type/1`` (to declare the type) and ``type::check/2`` (to
type-check values). For example:

::

   :- multifile(type::type/1).
   type::type(age).

   :- multifile(type::check/2).
   type::check(age, Term) :-
       type::check(between(non_negative_integer, 0, 150), Term).

Be careful to ensure that the new type definitions don't introduce
spurious choice-points for these predicates. The unit tests of the
``types`` library perform this check for pre-defined and loaded
user-defined ground types.

When defining a meta-type (i.e., a type with arguments that are also
types), add also a clause for the ``type::meta_type/3`` multifile
predicate. For example:

::

   :- multifile(type::meta_type/3).
   type::meta_type(tuple(Type1, Type2, Type3), [Type1, Type2, Type3], []).

This predicate is called when checking if a type is a defined type. For
meta-types, that check must extend to the sub-types.

Examples
--------

See e.g. the ``os`` library implementation of custom types for files and
directories. Or the ``expecteds`` and ``optionals`` libraries custom
types. See also the ``my_types`` programming example.
