What is Expressiveness in a Computer Language

[David Hopwood]

Pascal Bourguignon wrote:
> Pascal Costanza <pc at p-cos.net> writes:
>>Andreas Rossberg wrote:
>>>Pascal Costanza wrote:
>>>
>>>>Consider a simple expression like 'a + b': In a dynamically typed
>>>>language, all I need to have in mind is that the program will
>>>>attempt to add two numbers. In a statically typed language, I
>>>>additionally need to know that there must a guarantee that a and b
>>>>will always hold numbers.
>>>
>>>I'm confused. Are you telling that you just write a+b in your
>>>programs without trying to ensure that a and b are in fact numbers??
>>
>>Basically, yes.
>>
>>Note that this is a simplistic example. Consider, instead, sending a
>>message to an object, or calling a generic function, without ensuring
>>that there will be applicable methods for all possible cases. When I
>>get a "message not understood" exception, I can then decide whether
>>that kind of object shouldn't be a receiver in the first place, or
>>else whether I should define an appropriate method. I don't want to be
>>forced to decide this upfront, because either I don't want to be
>>bothered, or maybe I simply can't because I don't understand the
>>domain well enough yet, or maybe I want to keep a hook to be able to
>>update the program appropriately while it is running.
> 
> Moreover, a good proportion of the program and a good number of
> algorithms don't even need to know the type of the objects they
> manipulate.
> 
> For example, sort doesn't need to know what type the objects it sorts
> are.  It only needs to be given a function that is able to compare the
> objects.

But this is true also in a statically typed language with parametric
polymorphism.

[...]
> Why should adding a few functions or methods, and providing input
> values of a new type be rejected from a statically checked  point of
> view by a compiled program that would be mostly bit-for-bit the same
> with or without this new type?

It usually wouldn't be -- adding methods in a typical statically typed
OO language is unlikely to cause type errors (unless there is a naming
conflict, in some cases). Nor would adding new types or new functions.

(*Using* new methods without declaring them would cause an error, yes.)

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