Bugs in template constraints

[Andrej Mitrovic]

Oh and there's a shorter way to write this example, by using isInputRange
from std.range, like so:

if (isInputRange!R && is(typeof({x = fun(x, range.front);})))

This was in TDPL (except the {}'s which are missing).

On Tue, Aug 3, 2010 at 10:01 PM, Andrej Mitrovic <andrej.mitrovich at gmail.com
> wrote:

>
>
> On Tue, Aug 3, 2010 at 9:34 PM, Philippe Sigaud <philippe.sigaud at gmail.com
> > wrote:
>
>>
>>
>> On Tue, Aug 3, 2010 at 20:59, Andrej Mitrovic <andrej.mitrovich at gmail.com
>> > wrote:
>>
>>> There seem to be some bugs with template constraints. Here's a reduce
>>> example (from TDPL) which will not compile:
>>>
>>>
>>> V reduce(alias fun, V, R)(V x, R range)
>>>    if (is(typeof(x = fun(x, range.front)))
>>>        && is(typeof(range.empty) == bool)
>>>        && is(typeof(range.popFront())))
>>>
>>
>>
>>> I'm filing a bug unless something else is to blame here.
>>>
>>
>> I think that's because you cannot directly take the type of a statement.
>> The assignment in the first typeof() is to blame. To make a statement into
>> an expression, transform it into an anonymous void delegate(): put it in
>> braces (with a semicolon at the end) and call it like a function, like this:
>>
>> V reduce(alias fun, V, R)(V x, R range)
>>     if (is({ typeof(x = fun(x, range.front);}()))
>>         && is(typeof(range.empty) == bool)
>>         && is(typeof(range.popFront())))
>> {...}
>>
>>
> The { comes after "typeof(" as in your second example, and then it
> compiles.
>
>
>> Or, more readable, wrap all the code you want to test into curly brackets
>> and evaluates its global return type:
>>
>>
>> V reduce(alias fun, V, R)(V x, R range)
>>    if (is(typeof({                              // I want to be able to do
>> that with an R and a V:
>>                  x = fun(x, range.front);
>>                  if (range.empty) {};
>>                  range.popFront();
>>    }())))
>>
>> {...}
>>
>> It's a D idiom you'll see in many places in the standard library. I
>> personally find it a _bit_ heavy on parenthesis, even though I like Lisp.
>>
>>
>> Philippe
>>
>
> Yeah, those paranthesis are getting a bit scary now. :) I guess this one
> goes to the TDPL errata.
>
> Thanks for your help Philippe.
>
>
>
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