Re: Ada 2012: In-out parameters for functions

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

Le Wed, 01 May 2013 18:28:07 +0200, <dptrash@arcor.de> a écrit:

> Why does Ada 2012 have in-out parameters for functions?
>
> Quote from the book 'Ada 2005 Rationale' (page 4): "[..] Indeed many  
> other changes were rejected as really unnecessary. These include old  
> chestnuts such as in out and out parameters for functions (ugh), [..]".
>
> So, when do I use functions or procedures. Functions have return values.  
> Any other differences?
>
> - Dennis

This is just personal opinion, I don't speak for anyone else:

I did not enjoy neither the out mode parameters for functions in Ada 2012,  
but after some though, had the conviction it's not that much bad.

On one hand, you were already able to have functions with side effects  
prior to Ada 2012, which is even worse than out parameters. If a function  
modifies an outer level variable which is visible from its body, and that  
variable may be interpreted later from outside of the body of this  
function, that's less clear than using an out parameter.

Then, there is what a function do and how it is implemented. Out mode  
parameters is just implementation after‑all, and sometime, you may really  
need out parameters to complete the purpose of a rally pure function: Ada  
function does not returns tuples like SML's functions may do, and to  
achieve this in Ada, you need out parameters, or else you may have to  
define multiple types everywhere (if all elements of the tuples have  
different types, you do not really need to define a record type for that,  
as there is no way to take one element for another).

Semantic is one thing, implementation is another. And if you want  
functional programming in Ada, you need out parameters.

In/out (both way) parameters may be another story, and may more suggest  
this is a procedure and not a function, but still the same: semantic is  
one thing, implementation is another, and there is not always an exact  
match of one on the other.


-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University