.. mode: -*- rst -*-

Checking
========

:Tag: design.mps.check
:Author: Gavin Matthews
:Date: 1996-08-05
:Status: incomplete design
:Revision: $Id: //info.ravenbrook.com/project/mps/version/1.113/design/check.txt#1 $
:Copyright: See section `Copyright and License`_.
:Index terms: pair: checking; design


Introduction
------------

_`.intro`: This documents the design of structure checking within the
MPS.

_`.readership`: MPS developers.


Implementation
--------------

_`.level`: There are three levels of checking:

#. _`.level.sig`: The lowest level checks only that the
   structure has a valid ``Signature`` (see
   design.mps.sig).

#. _`.level.shallow`: Shallow checking checks all local fields
   (including signature) and also checks the signatures of any parent
   or child structures.

#. _`.level.deep`: Deep checking checks all local fields
   (including signatures), the signatures of any parent structures,
   and does full recursive checking on any child structures.

_`.level.control`: Control over the levels of checking is via the
definition of at most one of the macros ``TARGET_CHECK_SHALLOW``
(which if defined gives `.level.shallow`_), ``TARGET_CHECK_DEEP``
(which if defined gives `.level.deep`_). If neither macro is defined
then `.level.sig`_ is used. These macros are not intended to be
manipulated directly by developers, they should use the interface in
impl.h.target.

_`.order`: Because deep checking (`.level.deep`_) uses unchecked
recursion, it is important that child relationships are acyclic
(`.macro.down`_).

_`.fun`: Every abstract data type which is a structure pointer should
have a function ``<type>Check`` which takes a pointer of type
``<type>`` and returns a ``Bool``. It should check all fields in
order, using one of the macros in `.macro`_, or document why not.

_`.fun.omit`: The only fields which should be omitted from a check
function are those for which there is no meaningful check (for
example, an unlimited unsigned integer with no relation to other
fields).

_`.fun.return`: Although the function returns a ``Bool``, if the assert
handler returns (or there is no assert handler), then this is taken to
mean "ignore and continue", and the check function hence returns
``TRUE``.

_`.macro`: Checking is implemented by invoking four macros in
impl.h.assert:

``CHECKS(type, val)``

_`.macro.sig`: ``CHECKS(type, val)`` checks the signature only, and
should be called precisely on ``type`` and the received object
pointer.

``CHECKL(cond)``

_`.macro.local`: ``CHECKL(cond)`` checks a local field (depending on
level; see `.level`_), and should be called on each local field that
is not an abstract data type structure pointer itself (apart from
the signature), with an appropriate normally-true test condition.

``CHECKU(type, val)``

_`.macro.up`: ``CHECKU(type, val)`` checks a parent abstract data
type structure pointer, performing at most signature checks
(depending on level; see `.level`_). It should be called with the
parent type and pointer.

``CHECKD(type, val)``

_`.macro.down`: ``CHECKD(type, val)`` checks a child abstract data
type structure pointer, possibly invoking ``<type>Check`` (depending
on level; see `.level`_). It should be called with the child type
and pointer.

_`.full-type`: ``CHECKS()``, ``CHECKD()``, ``CHECKU()``, all operate
only on fully fledged types. This means the type has to provide a
function ``Bool TypeCheck(Type type)`` where ``Type`` is substituted
for the name of the type (for example, ``PoolCheck()``), and the
expression ``obj->sig`` must be a valid value of type ``Sig`` whenever
``obj`` is a valid value of type ``Type``.

_`.type.no-sig`: This tag is to be referenced in implementations
whenever the form ``CHECKL(ThingCheck(thing))`` is used instead of
``CHECK{U,D}(Thing, thing)`` because ``Thing`` is not a fully fledged
type (`.full-type`_).


Document History
----------------

- 1996-08-05 Gavin Matthews Incomplete design.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-03-12 GDR_ Converted to reStructuredText.

.. _RB: http://www.ravenbrook.com/consultants/rb/
.. _GDR: http://www.ravenbrook.com/consultants/gdr/


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---------------------

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