What is Expressiveness in a Computer Language

[David Hopwood]

Pascal Costanza wrote:
> Rob Thorpe wrote:
>> Pascal Costanza wrote:
>>> Matthias Blume wrote:
>>>> Pascal Costanza <pc at p-cos.net> writes:
>>>>
>>>>> (slot-value p 'address) is an attempt to access the field 'address in
>>>>> the object p. In many languages, the notation for this is p.address.
>>>>>
>>>>> Although the class definition for person doesn't mention the field
>>>>> address, the call to (eval (read)) allows the user to change the
>>>>> definition of the class person and update its existing
>>>>> instances. Therefore at runtime, the call to (slot-value p 'adress)
>>>>> has a chance to succeed.
>>>>
>>>> I am quite comfortable with the thought that this sort of evil would
>>>> get rejected by a statically typed language. :-)
>>>
>>> This sort of feature is clearly not meant for you. ;-P
>>
>> To be fair though that kind of thing would only really be used while
>> debugging a program.
>> Its no different than adding a new member to a class while in the
>> debugger.
>>
>> There are other places where you might add a slot to an object at
>> runtime, but they would be done in tidier ways.
> 
> Yes, but the question remains how a static type system can deal with
> this kind of updates.

It's not difficult in principle:

 - for each class [*], define a function which converts an 'old' value of
   that class to a 'new' value (the ability to do this is necessary anyway
   to support some kinds of upgrade). A default conversion function may be
   autogenerated if the class definition has changed only in minor ways.

 - typecheck the new program and the conversion functions, using the old
   type definitions for the argument of each conversion function, and the
   new type definitions for its result.

 - have the debugger apply the conversions to all values, and then resume
   the program.


[*] or nearest equivalent in a non-OO language.

-- 
David Hopwood <david.nospam.hopwood at blueyonder.co.uk>