[M3devel] range analysis?

[Rodney M. Bates]

On 05/23/2011 09:31 AM, Tony Hosking wrote:
> An empty subrange gives a warning in the compiler.

By my experiments, empty subrange, empty enumeration, and arrays
thereof don't warn.

> But, the semantics is that it is empty, and has no values of the type.
> You cannot allocate (NEW) an empty type.
> You cannot declare a field of empty type.

It seems odd that we have this rule for fields, but no corresponding
prohibition against arrays with empty element types.  By experiments,
cm3 allows this, for both fixed and open arrays, but treats such array
types as empty types, something the language also does not say.

At the least, the language and the compiler should agree with each other.
Maybe records/objects and arrays should be treated consistently here?


> Nor can you declare a variable of empty type.

Again, oddly, there is no such prohibition in the language against
declaring a formal parameter of empty type.  Cm3 prohibits it,
and calls it a 'variable' in the error message.

Again, language and compiler should agree.

The language also prohibits applying FIRST and LAST to the empty
enumeration, but they are OK applied to an empty subrange.
This makes sense, because they return values of the base type
of the argument type, and such values exist for an empty subrange
but not the empty enumeration, whose base type is only itself.

>
> On May 23, 2011, at 8:41 AM, Hendrik Boom wrote:
>
>> On Sun, May 22, 2011 at 11:18:02PM +0000, Jay K wrote:
>>>
>>> Well, I can build the whole tree and see how many times it helps.
>>> I think this is a pretty standard optimization technique.
>>> Though it'd work better with compiler-derived additional information.
>>>
>>> I initially happened upon this idea developing test cases.
>>>
>>>> The code assumes the minimum of a type is less than or equal to its maximum.
>>>
>>> I'll change it not have that problem -- to just fall back to usual pessimistic code for such types, I guess.
>>
>> What *is* the semantics of a range whose minimum is greater than its
>> maximum?  There plainly can't be any values in this range.  How is a
>> variable of this tyoe initialized?  Not to some arbitrary value of the
>> type, because there aren't any.  I can see this type being useful to
>> admit convenient generalizations -- for example, an array with n
>> elements can still exist if n happens to be zero, but it seems to me
>> that any code involving a value of the range for subscripts for this
>> array must be simple unexecutable.
>>
>> Or is there some latitude available in the principle that a value of a
>> variable must always be of the correct type?
>>
>> -- hendrik
>
>