• rumbu (12/12) Mar 21 2018 I tried to define a template:
• H. S. Teoh (21/37) Mar 21 2018 [...]

rumbu <rumbu rumbu.ro> writes:

I tried to define a template:

enum isFoo(alias T) =
     T.stringof.length >= 3 && T.stringof[0..3] == "abc";

int i;
pragma(msg, isFoo!i);

Error: string slice [0 .. 3] is out of bounds
Error: template object.__equals cannot deduce function from 
argument types !()(string, string), candidates are:
[...]

Is it normal that the condition is not short circuited at first 
test? Of course, I can throw there a bunch of static ifs in an 
eponymous template, but I don't see the point of this verbosity.

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

On Wed, Mar 21, 2018 at 05:42:34PM +0000, rumbu via Digitalmars-d-learn wrote:
 I tried to define a template:
 
 enum isFoo(alias T) =
     T.stringof.length >= 3 && T.stringof[0..3] == "abc";
 
 int i;
 pragma(msg, isFoo!i);
 
 Error: string slice [0 .. 3] is out of bounds
 Error: template object.__equals cannot deduce function from argument types
 !()(string, string), candidates are:
 [...]
 
 Is it normal that the condition is not short circuited at first test?
 Of course, I can throw there a bunch of static ifs in an eponymous
 template, but I don't see the point of this verbosity.

[...]

The reason it's not short-circuited is because T.stringof[0..3] is
processed *before* the && is evaluated.  Basically, this:

	enum isFoo(alias T) = T.stringof.length >= 3 && T.stringof[0..3] == "abc";

is a shorthand for this:

	template isFoo(alias T)
	{
		enum isFoo = T.stringof.length >= 3 && T.stringof[0..3] == "abc";
	}

T.stringof[0..3] is actually evaluated during AST expansion time,
because it's slicing a template argument list, but && isn't evaluated
until CTFE-time.

See: https://wiki.dlang.org/User:Quickfur/Compile-time_vs._compile-time


Now, arguably, we *could* make it so that && is also shortcircuited at
AST expansion time, i.e., basically lower && in this context into a
series of nested static if's instead of a CTFE expression.  But that
would probably involve some extensive changes in the compiler.


T

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