Re: Ada202X : Adding functors

["Randy Brukardt" <randy@rrsoftware.com>]

"Peter C. Chapin" <pcc482719@gmail.com> wrote in message 
news:WdGdnd-Z6-nFHzjN4p2dnAA@giganews.com...
> On 11/15/2012 12:04 PM, Dmitry A. Kazakov wrote:
>
>>> Could you elaborate on what you mean here? There are statically typed
>>> functional languages where functions are first class and that also have
>>> a reasonable notion of type safety.
>>
>> I think they are actually weakly typed.
>
> In the functional language that I use the most (Scala) lambda terms are 
> statically typed and fully checked at compile time, just like any other 
> expression.

There's more to a function contract than just the type-and-result-profile! 
To truly statically check such things, you also have to match the 
precondition, the postcondition, the exception contract, the global in/out 
usage, and so on. (Matching doesn't necessarily mean exact equivalence, of 
course, which makes it even harder.)

For Ada 2012, we simply punted on these issues. But that makes 
access-to-subprogram (or any sort of lambda, which would be semantically 
similar to access-to-subprogram) fragile: you can get Assertion_Error from 
any access-to-subprogram value, and there isn't much you can do about it.

One hopes that Ada 2020 will do better in this area, but I'm dubious that it 
can be done well for access-to-subprogram (and lambdas, and formal 
subprograms, all semantically similar). Complete reliance on dynamic 
contracts can be useful, but it certainly isn't my idea of strong types 
(that is, enforced at compile-time).

                                              Randy.