IsExpression

[Kirk McDonald]

Luís Marques wrote:
> Hello,
> 
> The documentation of IsExpression lists the following valid forms:
> 
> is ( Type )
> is ( Type : TypeSpecialization )
> is ( Type == TypeSpecialization )
> is ( Type Identifier )
> is ( Type Identifier : TypeSpecialization )
> is ( Type Identifier == TypeSpecialization )
> 
> What about the form "expression is expression"? I cannot find 
> documentation for that form. What are its semantics? "exp is null" is an 
> obvious one. But what about other forms?
> 
> - for classes it seems that "obj1 is obj2" is equivalent to "&obj1 == 
> &obj2". Can I rely on that? I have a singleton called "nil", so it's 
> great being able to say "obj is nil".
> 
> - for structs "structvar1 is structvar2" is always true.
> 
> - for PODs it seems that values must be semantically equivalent. For 
> instance, "1L is 1.0+0i" is true.
> 
> Could this be documented?
> 

The 'is' keyword is used in two completely separate contexts: The first 
is the IsExpression, which has the grammar you outlined above. This is 
used for compile-time type checking.

The other use of 'is' is in the IdentityExpression.
http://www.digitalmars.com/d/expression.html#IdentityExpression

"For operand types other than class objects, static or dynamic arrays, 
identity is defined as being the same as equality."

Thus for structs I would expect it to perform a bitwise comparison of 
the two instances if the struct doesn't overload opEquals.

> Also, the IsExpression docs refer to a "Type", which can be an 
> identifier (e.g. "int x; writefln(is(x));" is valid, even though the 
> form in question is "is(Type)"). Perhaps that could be made more clear?
> 

It is indeed valid. As you point out, a "Type" can be an identifier, and 
it is a property of the IsExpression that it returns false if the Type 
provided is not a semantically valid type. Therefore, you must be able 
to provide it semantically /invalid/ types. (Which must still be 
/syntactically/ valid.)

-- 
Kirk McDonald
http://kirkmcdonald.blogspot.com
Pyd: Connecting D and Python
http://pyd.dsource.org