• Jason House (4/49) May 18 2009 Andrei didn't voice a complaint about is expressions. I did. If you take...
• bearophile (9/10) May 18 2009 When I use my dlibs in D1 to solve a problem like that I use:
• Bill Baxter (6/8) May 18 2009 Hmm, 'is' isn't supposed to accept values is it? But it quietly

Jason House <jason.james.house gmail.com> writes:

grauzone Wrote:

 Robert Fraser wrote:
 Jason House wrote:
 Andrei Alexandrescu Wrote:

 Consider:

      foreach (x; 1.0 .. 4.1) writeln(x);
      foreach_reverse (x; 1.0 .. 4.1) writeln(x);

 This sucks. foreach with interval for floating-point types should be 
 disallowed.


 Andrei

 I agree that makes little sense.

 I'm curious, why the sudden pruning of features? Can you please 
 consider removing SFINAE? I'd also love to see is expressions go the 
 way of the dinosaur. I don't know what their replacement should look 
 like, but there _has_ to be a better and less error prone syntax!

 
 I agree about SFINAE, but what's so bad about is()? If it does get 
 removed, I hope whatever replaces it can do all the weird things is can 
 (e.x. is(typeof({}())))

 
 Isn't is() also a kind of SFINAE? SFINAE is Substitution Failure Is Not 
 An Error (or so). That means, if there's an error trying to apply  a 
 template, the error is silently ignored, and the compiler tries the next 
 template declaration. (Correct me if I'm wrong.) is() does something 
 similar: it tries to compile the expression inside is(), and if it 
 fails, the compiler silently ignores the error and returns "false" to 
 the if.
 
 The problem is: you can't really know if that what you intended to test 
 with is() was false, or if another random semantic error happened inside 
 the is() expression.
 
 Look at this for example:
  > writefln("%s", is(typeof(rtzx) == char));
 This compiles even if rtzx doesn't exist. But you probably wanted to 
 check the type of rtzx, not it if rtzx exists. If you mistyped rtzx, the 
 compiler will never tell you.
 
 Now Andrei didn't really explain his problem with is()...

Andrei didn't voice a complaint about is expressions. I did. If you take a look
at the documentation on them, you'll see 8 different flavors. Can you honestly
reproduce that list without looking it up? Here's a hint: is(typeof(...)) isn't
listed in the docs. Here's a simpler question: how do you test for an
associative array of string types indexes by a numerical type while exposing a
type alias for the index type?

Here's a recent misuse by Andrei that made its way into Phobos:
if (is(T == struct) && is(T.toString))

bearophile <bearophileHUGS lycos.com> writes:

Jason House:
how do you test for an associative array of string types indexes by a numerical
type while exposing a type alias for the index type?<

When I use my dlibs in D1 to solve a problem like that I use:

static if (IsAA!(T1) && IsNumeric!(KeyType!(T1))) {
    alias KeyType!(T1) Tkey;
    ...
}

Easy and not not much error prone. I have had no need to look into the docs.

Bye,
bearophile

Bill Baxter <wbaxter gmail.com> writes:

On Mon, May 18, 2009 at 5:46 AM, Jason House
<jason.james.house gmail.com> wrote:

 Here's a recent misuse by Andrei that made its way into Phobos:
 if (is(T == struct) && is(T.toString))

Hmm, 'is' isn't supposed to accept values is it?  But it quietly
ignores anything wrong and returns false.  Is that the misuse you're
talking about?

--bb