• Borden Rhodes <jrvp bordenrhodes.com> Sep 14 2010
• bearophile <bearophileHUGS lycos.com> Sep 15 2010
• bearophile <bearophileHUGS lycos.com> Sep 15 2010

Borden Rhodes <jrvp bordenrhodes.com> writes:

Good morning, list,

I know that D has support for ranges in for-each statements and in array 
bounds checking, but I'm curious if it also has a facility for compile-time 
range checking or assertions on individual variables.

For example, using the Java Tutorial's venerable Bicycle class, say I 
had a property <code>int gear;</code> which indicated the bicycle's current 
gear.  Let's say all bicycles of this class have 6 gears.  Typically, I would 
write a changeGear method as:

<code>
void changeGear(int newGear) {
        if ( 1 <= newGear && newGear <= 6)
                gear = newGear;
        else
                throw SomeKindOfException();
}
</code>

which would include the if statement to make sure that newGear would be passed 
a number in the correct range and a throw statement to deal with cheaters.

Does D allow - or would there be logic in creating - an ability to define the 
acceptable ranges when the variable is initialised?  So, say I wanted gear 
only to accept values between 1 and 6 inclusive.  Could I declare something to 
the effect of:

<code>int gear {1...6} = 1;</code>

Which could read, "create a new int, called gear, which only accepts values 
between 1 and 6, inclusive, and initialise it to 1."  Then, if I, or some 
coder who forgot how many gears my Bicycle has, tried to call:

</code>bicycle.changeGear( 12 );</code>

the compiler would catch that <code>gear = newGear;</code> is assigning a 
value out of range and throw a compile-time error.  Can D do this?  Should it 
do this?

With thanks,

Borden

bearophile <bearophileHUGS lycos.com> writes:

Borden Rhodes:
 <code>int gear {1...6} = 1;</code>


I think Walter said that Ranged Types are a "failed feature" of Pascal/Ada
lineage, but I don't agree and I think they are useful to increase code safety,
a possible syntax from yours:

typedef int { 1 ... 6 } Gear;

Using a struct with "alias this" and a struct invariant that enforces the
range, plus properties, is possible, but such struct:
- Doesn't act transparently as a int subclass;
- Syntax-wise it's longer to define;
- The invariant is not enforced in all situations.

struct Gear {
    int g_ = 1;
    this(int g) { this.g_ = g; }
    invariant() { assert(this.g_ >= 1 && this.g_ <= 6); }
     property void g(int x) { this.g_ = x; }
     property int g() { return this.g_; }
    alias g this;
}

Bye,
bearophile

bearophile <bearophileHUGS lycos.com> writes:

 - Doesn't act transparently as a int subclass;
 - Syntax-wise it's longer to define;
 - The invariant is not enforced in all situations.


And the compiler in most situations doesn't "run" the invariant at compile time
(unless you define an enum).

Well, a test shows that it doesn't work (dmd 2.048) even if you use enum, I
don't know why:

struct Foo {
    int x;
    this(int y) { this.x = y; }
}
void main() {
    enum Foo f = Foo(1);
}

Bye,
bearophile