• Elvis Maehren (20/20) Dec 21 2011 This works fine at runtime, but explodes in CTFE:
• Vladimir Panteleev (4/5) Dec 21 2011 I believe that's what __traits(compiles, ...) is for:
• Elvis Maehren (3/4) Dec 22 2011 __traits(compiles, cast(T) o) is true even if the cast blows up in CTFE,...
• Jacob Carlborg (6/26) Dec 21 2011 I don't think you can do a test like that during compile time. In your
• Elvis Maehren (5/17) Dec 22 2011 I don't think testing for equality of O and T gets me anywhere. T is usu...
• Timon Gehr (6/27) Dec 22 2011 This will be fixed in the next release:
• Elvis Maehren (8/20) Dec 22 2011 ok, did that: http://d.puremagic.com/issues/show_bug.cgi?id=7154
• Trass3r (1/1) Dec 22 2011 I'd really like to have 'if (instance is ClassType)' syntax in D.
• Andrew Wiley (3/4) Dec 22 2011 It couldn't use that syntax because 'is' means direct bitwise equality

Elvis Maehren <maehlvis googlemail.com> writes:

This works fine at runtime, but explodes in CTFE:
---
bool isInstanceOf(T, O)(O o) {
	return (cast(T) o) !is null;
}
---

CTFE doesn't like "!is null" ("cannot compare [...] at compile time"). 

Moreover, in CTFE a failed cast terminates the interpretation ("cannot 
reinterpret class from [...] to [...] at compile time"). Is this somehow 
catchable? If so, I could do something along these lines:
---
if(__ctfe) {
	try {
		auto t = cast(T) o;
		return true;
	} catch {
		return false;
	}
}
---

"Vladimir Panteleev" <vladimir thecybershadow.net> writes:

On Wednesday, 21 December 2011 at 20:15:36 UTC, Elvis Maehren 
wrote:
 Is this somehow catchable?

I believe that's what __traits(compiles, ...) is for:

http://dlang.org/traits.html#compiles

Elvis Maehren <maehlvis googlemail.com> writes:

Vladimir Panteleev wrote:
 I believe that's what __traits(compiles, ...) is for:

__traits(compiles, cast(T) o) is true even if the cast blows up in CTFE, so 
that doesn't work.

Jacob Carlborg <doob me.com> writes:

On 2011-12-21 21:15, Elvis Maehren wrote:
 This works fine at runtime, but explodes in CTFE:
 ---
 bool isInstanceOf(T, O)(O o) {
 	return (cast(T) o) !is null;
 }
 ---

 CTFE doesn't like "!is null" ("cannot compare [...] at compile time").

 Moreover, in CTFE a failed cast terminates the interpretation ("cannot
 reinterpret class from [...] to [...] at compile time"). Is this somehow
 catchable? If so, I could do something along these lines:
 ---
 if(__ctfe) {
 	try {
 		auto t = cast(T) o;
 		return true;
 	} catch {
 		return false;
 	}
 }
 ---

I don't think you can do a test like that during compile time. In your 
above function, isInstanceOf, "o" will always be of the type "O". You 
can always do a comparisons to see if O is the same type as T.

-- 
/Jacob Carlborg

Elvis Maehren <maehlvis googlemail.com> writes:

Jacob Carlborg wrote:
 On 2011-12-21 21:15, Elvis Maehren wrote:
 This works fine at runtime, but explodes in CTFE:
 ---
 bool isInstanceOf(T, O)(O o) {
 return (cast(T) o) !is null;
 }
 ---


[...]
 I don't think you can do a test like that during compile time. In your
 above function, isInstanceOf, "o" will always be of the type "O". You
 can always do a comparisons to see if O is the same type as T.
 

I don't think testing for equality of O and T gets me anywhere. T is usually 
derived from O. That makes it a different type, but still o may be castable 
to T.

Timon Gehr <timon.gehr gmx.ch> writes:

On 12/21/2011 09:15 PM, Elvis Maehren wrote:
 This works fine at runtime, but explodes in CTFE:
 ---
 bool isInstanceOf(T, O)(O o) {
 	return (cast(T) o) !is null;
 }
 ---

 CTFE doesn't like "!is null" ("cannot compare [...] at compile time").

This will be fixed in the next release:

http://d.puremagic.com/issues/show_bug.cgi?id=7143

 Moreover, in CTFE a failed cast terminates the interpretation ("cannot
 reinterpret class from [...] to [...] at compile time").

I don't think this is filed already. Maybe you want to file a bug report?

 Is this somehow
 catchable? If so, I could do something along these lines:
 ---
 if(__ctfe) {
 	try {
 		auto t = cast(T) o;
 		return true;
 	} catch {
 		return false;
 	}
 }
 ---

use traits(__compiles, {...}), where '...' is the exact code that fails. 
But that is a mere workaround. What you are doing should/will work.

Elvis Maehren <maehlvis googlemail.com> writes:

Timon Gehr wrote:
 On 12/21/2011 09:15 PM, Elvis Maehren wrote:

[...]
 Moreover, in CTFE a failed cast terminates the interpretation ("cannot
 reinterpret class from [...] to [...] at compile time").

 
 I don't think this is filed already. Maybe you want to file a bug report?

ok, did that: http://d.puremagic.com/issues/show_bug.cgi?id=7154

[...]
 use traits(__compiles, {...}), where '...' is the exact code that fails.
 But that is a mere workaround. What you are doing should/will work.

Vladimir Panteleev pointed me to __traits(compiles, ...), too. I think that 
doesn't work out:

Elvis Maehren wrote:
 Vladimir Panteleev wrote:
 I believe that's what __traits(compiles, ...) is for:

 
 __traits(compiles, cast(T) o) is true even if the cast blows up in CTFE,
 so that doesn't work.

Or am I missing something?

Trass3r <un known.com> writes:

I'd really like to have 'if (instance is ClassType)' syntax in D.

Andrew Wiley <wiley.andrew.j gmail.com> writes:

On Thu, Dec 22, 2011 at 8:35 AM, Trass3r <un known.com> wrote:
 I'd really like to have 'if (instance is ClassType)' syntax in D.

It couldn't use that syntax because 'is' means direct bitwise equality
everywhere else. Changing it in this one situation would be awkward.