Re: Discriminated records are not the most efficient, but ...

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Mon, 23 Aug 2010 03:52:14 +0200, Yannick Duchêne (Hibou57)  
<yannick_duchene@yahoo.fr> a écrit:
> Now I've just seen about Structures and Functors (you seems to know ML,  
> do you ?), and we could say “Ada's functors” can only get functors as  
> parameter, not structures.
I need to correct something or else what I wrote will be confusing and  
incomplete.

In Ada : formal parameters may refer to a generic package. However,  
instantiation of such a package will requires a package instance in place  
of that formal parameter.

In ML : a functor has the following signature: “structure -> structure”.  
So at first sight, this may seems ML functors cannot model Ada packages.  
But as a functor takes a structure to return a structure, this is exactly  
the same as with an Ada generic instantiation. As in ML a functor cannot  
create a structure signature and can only create a structure matching a  
structure signature, finally, application of an ML functor exactly match  
the process of instantiating an Ada generic.

Conclusion: the Ada package system can be properly modeled with ML.

Note: in ML, the parameters appears only at the functor definition, not in  
the signature, thus, the signature match Ada's package specification,  
*excluding the formal part* ; thus is correspond to what is between the  
“package Name” and “end Name;”. [*]

    Application of an ML functor = Ada generic instantiation
    Definition of an ML functor = Ada generic body
    Definition of an ML signature = Ada specification of a package or  
generic package (formal part excluded)

And a one way equivalence

    Ada generic specification is the same as ML signature + ML functor's  
signature.

That is fun, we may be able to model the Ada language :) This is a new  
view on Ada, I like that. There was first the LRM, later there was Ada  
Semantic Interface Specification (ASIS), now I get the beginning of an Ada  
model in a formal language.

[*] With that in mind, the choice made for Ada to have the formal part  
preceding the package name, seems less strange! For the requirement of  
static checking on public interface, Ada simply required to have both  
signature (structure and functor) at one place and the choice to have the  
formal part preceding the generic package name, simply make these still  
clearly separated, while both at the same place. Clever syntactic choice  
they made!

CheckMe: is there is a way to have a functor signature in ML ?

-- 
* 3 lines of concise statements is readable, 10 pages of concise  
statements is unreadable ;
* 3 lines of verbose statements may looks unuseful, 10 pages of verbose  
statements will never looks too much pedantic