4. Checking

4.1. Introduction

This documents the design of structure checking within the MPS.

4.2. History

.hist.0: Incomplete design. Gavin Matthews, 1996-08-05.

.hist.1: Converted from MMInfo database design document. Richard Brooksby, 2002-06-07.

.hist.2: Converted to reStructuredText. Gareth Rees, 2013-03-12.

4.3. Implementation

.level: There are three levels of checking:

1. .level.sig: The lowest level checks only that the structure has a valid Signature (see design.mps.sig).
2. .level.shallow: Shallow checking checks all local fields (including signature) and also checks the signatures of any parent or child structures.
3. .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:

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