What is Expressiveness in a Computer Language

[Joachim Durchholz]

Andreas Rossberg schrieb:
> Joachim Durchholz wrote:
>>>
>>>> It's worth noting, too, that (in some sense) the type of an object 
>>>> can change over time[*].
>>>
>>> No. Since a type expresses invariants, this is precisely what may 
>>> *not* happen.
>>
>> No. A type is a set of allowable values, allowable operations, and 
>> constraints on the operations (which are often called "invariants" but 
>> they are invariant only as long as the type is invariant).
> 
> The purpose of a type system is to derive properties that are known to 
> hold in advance.

That's just one of many possible purposes (a noble one, and the most 
preeminent one in FPLs I'll agree any day, but it's still *not the 
definition of a type*).

 > A type is the encoding of these properties. A type
> varying over time is an inherent contradiction (or another abuse of the 
> term "type").

No. It's just a matter of definition, essentially.
E.g. in Smalltalk and Lisp, it does make sense to talk of the "type" of 
a name or a value, even if that type may change over time.
I regard it as a highly dubious practice to have things change their 
types over their lifetime, but if there are enough other constraints, 
type constancy may indeed have to take a back seat.

Regards,
Jo