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.. highlight:: none


.. index::
   pair: checking; design

.. _design-check:


Checking
========

.. mps:prefix:: design.mps.check


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

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

:mps:tag:`readership` MPS developers.


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

:mps:tag:`level` There are three levels of checking:

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

   .. _design.mps.sig: sig.html

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

#. :mps:tag:`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.

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

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

:mps:tag:`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 :c:type:`Bool`. It should check all fields in
order, using one of the macros in :mps:ref:`.macro`, or document why not.

:mps:tag:`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).

:mps:tag:`fun.return` Although the function returns a :c:type:`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
:c:macro:`TRUE`.

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

.. c:macro:: CHECKS(type, val)

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

.. c:macro:: CHECKL(cond)

:mps:tag:`macro.local` ``CHECKL(cond)`` checks a local field (depending on
level; see :mps:ref:`.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.

.. c:macro:: CHECKU(type, val)

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

.. c:macro:: CHECKD(type, val)

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

:mps:tag:`full-type` Use :c:func:`CHECKS()`, :c:func:`CHECKD()` or :c:func:`CHECKU()` on all
types that satisfy these three requirements:

:mps:tag:`full-type.pointer` The type is a pointer type.

:mps:tag:`full-type.check` The type provides a function ``Bool TypeCheck(Type
type)`` where ``Type`` is substituted for the name of the type (for
example, :c:func:`PoolCheck()`).

:mps:tag:`full-type.sig` The expression ``obj->sig`` is a valid value of
type :c:type:`Sig` whenever ``obj`` is a valid value of type ``Type``.

:mps:tag:`partial-type` Where the type satisfies :mps:ref:`.full-type.pointer` and
:mps:ref:`.full-type.check` but not :mps:ref:`.full-type.sig` because the type lacks a
signature in order to save space (this applies to small structures
that are embedded many times in other structures, for example
:c:type:`Ring`), use :c:func:`CHECKD_NOSIG()`.

:mps:tag:`hidden-type` Where the type satisfies :mps:ref:`.full-type.pointer` and
:mps:ref:`.full-type.check` but not :mps:ref:`.full-type.sig` because the structure
has a signature but the structure definition is not visible at point
of checking (for example :c:type:`Root`), use :c:func:`CHECKD_NOSIG()` and
reference this tag. The structure could be considered for addition to
``mpmst.h``.


Common assertions
-----------------

:mps:tag:`common` Some assertions are commonly triggered by mistakes in the
client program. These are listed in the section "Common assertions and
their causes" in the MPS Reference, together with an explanation of
their likely cause, and advice for fixing the problem. To assist with
keeping the MPS Reference up to date, these assertions are marked with
a cross-reference to this tag. When you update the assertion, you must
also update the MPS Reference.