DESIGN OF CHECKING IN MPS
                            design.mps.check
                           incomplete design
                           gavinm 1996-08-05


INTRODUCTION:

This documents the design of structure checking within the MPS


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 (e.g. 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 <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 (eg, 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 whenver the form 
CHECKL(ThingCheck(thing)) is used instead of CHECK{U,D}(Thing, thing) because 
Thing is not a fully fledged type (.full-type).