Re: Certified C compilers for safety-critical embedded systems

[Marin David Condic <nobody@noplace.com>]

What exactly would be the meaning of:

A := Func_X (A) + Func_Y (A) ;

if functions had in-out parameters and A could be modified while 
evaluating the RHS? I suppose one might be able to come up with some 
evaluation rules that at least meant determinism for the end result, but 
staring at it would not make it intuitively obvious to even the most 
casual observer. Sure, the functions *could* modify A if they had 
visibility to the right scope, but how often does someone do that in 
real-world use? The current style - while not guaranteeing a lack of 
side effects - tends to discourage it and leads to more easily 
comprehended code.

I don't understand what the headache is here anyway. If one wants to 
modify parameters, use a procedure. If one wants some kind of 
math-oriented thingie to return a result, use a function. Only 
interfacing to squirly languages like C seems to be an issue - and I 
think that ought to be a difficult and painful thing to discourage it 
from happening. :-)

MDC

Jean-Pierre Rosen wrote:
> 
> I've seen this argument quite often (especially by RBKD), but it is totally bogus.
> Nobody claimed that functions should not have side effects, but only that functions compute a result given some parameters and the
> global state of the program. It was felt that it was OK for a function to modify the global state, but modifying the parameters
> would be surprising and error-prone.
> 
> BTW, Ada79 had functions that were required to not have side effects, and value returning procedures that did not have this
> constraint. This was felt to be too extreme (or maybe difficult to enforce) and the current solution to be an acceptable trade-off.
> 


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Marin David Condic
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