On 9/28/2016 6:58 AM, Timon Gehr wrote:
An excellent example of that is the std.regex package.
It's actually a bad example, because it is irrelevant: it is obviously a bad
idea to implement regex using operator overloading, because the regex operators
have no D equivalent.
Yet this "obviously bad" idea regularly resurfaces as a cool use of expression
templates and respected people in the industry like it and advocate it.
Assume I have two symbolic expressions a and b:
Expression a=variable("a"), b=variable("b");
Why should I be allowed to do
auto c = a + b; // symbolic value a + b
but not
auto d = a <= b; // symbolic value a <= b
Because there is no way to stop the former but still have operator overloading.
The string parsing approach is not useful here: how do I use existing
expressions that are accessible in the scope to build new expressions?
You seem to be asking for expression templates. They do actually work in D, but
I recommend against them.
To reiterate, operator overloading exists in D to support the inclusion of
arithmetic library types. Any other purpose is discouraged, and that includes
expression templates and things like << for iostreams.
AST macros are an alternative to expression templates. Having AST macros has
been proposed before multiple times, and has been debated at length. I haven't
seen any new information that would justify reopening the issue.
