Re: constant string array

[Jeff Carter <jrcarter@acm.org>]

Robert Dewar wrote:
> There is a conceptual difference between using a boolean
> expression in a test, and using it as a general value (indeed
> it is this conceptual difference that leads ABC to allow
> "boolean" expressions ONLY in the context of a test.

ABC?

> I think the argument is even stronger in comparing:
> 
>    a := b > c;
> 
> with
> 
>    if b > c then
>       a := true;
>    else
>       a := false;
>    end if;
> 
> I always used to criticize students for the spelled out version
> giving the canonical advice to replace it with the use of a
> boolean expression. But in fact I think the spelled out version
> is often clearer in its intent.
> 
> You can get some feeling for this by supposing that someone
> looking at the above code decides to replace
> 
>    A : boolean;
> 
> by
> 
>    type motor_status is (on, off);
>    a : motor_status;
> 
> with a revision history saying
> 
>    use ON/OFF for motor status instead of True/False, makes
>    code clearer.

Hopefully your style checker will flag your inconsistent capitalization.

Obviously, I disagree with your premise. In all of your examples I find
the use of a Boolean expression as a value clearer than the version with
an if. If the type of a variable changes, you should expect to have to
make changes everywhere it is used. I certainly expect some of those
changes to be non-trivial.

What is important is to use a language that flags any places where the
use of the variable has been missed, rather than one that silently
compiles the meaningless code.

I have found that this kind of construct generally indicates a lack of
understanding of Booleans, frequently from people with experience with
languages that do not have a Boolean type. Such people frequently do
other irritating things, like putting unnecessary parentheses around the
condition of an if, and forgetting the "then". Learning that Boolean is
a type like any other enumeration type, and its values no different than
any other enumeration type, is often a major step in grasping the Ada
mind set.

-- 
Jeff Carter
"I blow my nose on you."
Monty Python & the Holy Grail