TDPL: Overloading template functions

[Andrej Mitrovic]

On Sun, Aug 1, 2010 at 11:15 PM, Philippe Sigaud
<philippe.sigaud at gmail.com>wrote:

>
>
> On Sat, Jul 31, 2010 at 17:24, Andrej Mitrovic <andrej.mitrovich at gmail.com
> > wrote:
>
>> I think there really is some bug here. I tried to use your template helper
>> function, and it seems the order in which I put two constraints in a
>> template signature changes the way dmd compiles the code. Consider this:
>>
>
>
>> T[] find(T, E)(T[] haystack, E needle)
>>     if (is(typeof(haystack[0] != needle) == bool) &&
>>     areComparable!(T, E))
>>
>> ...
>
>
>> T1[] find(T1, T2)(T1[] longer, T2[] shorter)
>>     if (is(typeof(longer[0 .. 1] == shorter) : bool) &&
>> (areComparable!(T1, T2)))
>>
>
>
>
>> This will give out the error "incompatible types for comparing of string
>> and immutable(char)".
>>
>
> That's because the first part of the constraint is tested, to allow for
> short-circuiting the second part. So the bad array comparison triggers the
> problem in object._EqArray.
>
> You should always test for the most general constraint first, I guess. In
> this particular case, use areComparable!(A,B) as the first argument to && in
> both templates.
>
>
Let me see if I'm getting this right.

false && true will short circuit to false (this is normal).

true && false will short circuit to true? How can that be? It's not a
logical OR, its an AND, and as far as I know both expressions need to be
True to evaluate to True. Or is it different for template constraints?

Thanks again for the std.math.approxEqual and binaryFun tips.
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