What is Expressiveness in a Computer Language

[Pascal Costanza]

Matthias Blume wrote:
> Pascal Costanza <pc at p-cos.net> writes:
> 
>> - In a dynamically typed language, you can run programs successfully
>>   that are not acceptable by static type systems.
> 
> This statement is false.

The example I have given is more important than this statement.

> For every program that can run successfully to completion there exists
> a static type system which accepts that program.  Moreover, there is
> at least one static type system that accepts all such programs.
> 
> What you mean is that for static type systems that are restrictive
> enough to be useful in practice there always exist programs which
> (after type erasure in an untyped setting, i.e., by switching to a
> different language) would run to completion, but which are rejected by
> the static type system.

No, that's not what I mean.

>> Here is an example in Common Lisp:
>>
>> ; A class "person" with no superclasses and with the only field "name":
>> (defclass person ()
>>    (name))
>>
>> ; A test program:
>> (defun test ()
>>    (let ((p (make-instance 'person)))
>>      (eval (read))
>>      (slot-value p 'address)))
>>
>> (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


Pascal

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