Checking if a string is null

[Manfred Nowak]

*** superseeding previous post ***

Bruno Medeiros wrote

> But first, explain me this, what do you mean the
> "identityExpression is restricted to arrays containing at least
> one element."? That doesn't seem to make sense.

In at least most definitions for relations on arrays in D the phrase 
"all elements" is used.

Besides the fact that this phrase is informal its meaning seems to be 
different from "for all x in the set S: P(x)". This latter phrase 
clearly binds the predicate P to elements in existence.

Such existence quantor seems not to be included in the phrase "all 
the elements are equal". This impressions is supported by this code 
fragment:
   writefln( "" is ""); // prints "false"

The definition for the relationalExpression supports the restriction 
I phrased most prominently:

| For static and dynamic arrays, the result of the relational op is
| the result of the operator applied to the first non-equal element
| of the array.

I.e.: if there is no element, then there is no first element, then 
the result of the relationalExpression is not defined. According to 
the definition it is a bug that this code fragment is compilable:
  writefln( "" < ""); // prints "false"
 

Note: maybe definition _and_ implementation are both buggy :-(


> That property "if `a is b' then `a == b'" holds for arrays with
> zero elements as well.

Maybe that the implementation is holding this property. But the 
specifications does not mention it. Therefore this property might not 
be intended and the implementation is waiting for a case where this 
property does not hold.

-manfred