[M3devel] Resummary of tagged reference proposals

[Rodney M. Bates]

Tony Hosking wrote:
> I'd prefer something like TAGGEDINT or some such.  But I am still not 
> convinced that the minimalist proposal and this proposal differ in any 
> significant way.
The minimalist proposal forces you to give up some static safety you have
in the current language, that the safe proposal allows.   In the minimalist
proposal, you cannot both use tagged pointers and have any of the 
following:

1) An abstract data type T can (and today virtually always is) some 
proper subtype
    of REFANY.  This statically restricts values to this type.  In 
minimalist, T can
    only be declared REFANY, allowing a client to provide a value that 
was neither
    tagged nor of the type the abstraction expects.   Instead, the type 
constraint will be
    checked at runtime, on every call into the abstraction.   And this 
is a more expensive
    check than just a tag bit check.  Aside from the overhead, going 
from static to
    runtime type checking requires an extensive test suite to get 
somewhere short
    of the same level of confidence that a mere compile would 
guarantee.   In the safe
    proposal, T could be replaced by TAGGED T, and the static rules 
would guarantee
    its values are always either integer or T, comparable to the current 
language.

2) If there is more than one abstraction that uses tagged types, 
minimalist loses static
    guarantee that they are not mixed up by client code.   Since Abs1.T 
and Abs2.T must
    both be REFANY,  a client can get a value from Abs1 and pass it to 
Abs2.  Again, this
    is a demotion of static to runtime type checking.  In safe, Abs1.T 
and Abs2.T could
    be tagged types built from different opaque types, giving the same 
separation that
    we get in the current language.

3) The type declared in an abstraction can currently be an opaque 
subtype of some
    proper subtype of REFANY.  This allows the abstraction writer to 
give clients the
    ability to directly access some fields/methods, while hiding 
others.  Once again,
    the restriction that only REFANY can be tagged makes this impossible 
in minimalist.
    I suppose this could be worked around by providing accessors, 
mutators, and plain
    procedures in the abstraction, but then these would have to take 
REFANY too, just
    making for more instances of loss of static checking.
>
> On 21 Apr 2009, at 22:26, hendrik at topoi.pooq.com wrote:
>
>> On Mon, Apr 20, 2009 at 08:35:02AM -0500, Rodney M. Bates wrote:
>>>
>>> ---------------------------------------------------------------------
>>> My (Rodney's) "safe" proposal:
>>>
>>> There is a new type named TAGABLE.  It is a subrange of INTEGER, whose
>>
>> Please spell it TAGGABLE.  TAG doubles its last consonant when adding
>> suffixes:  TAGGED, TAGGING.
>>
>> -- hendrik
>
>