what's the best (and DRY) way to achieve:

```
static if(__traits(compiles, expr))
  fun(expr);
```

ie, without repeating the expression inside expr?

eg:

```
static if(__traits(compiles, foo.bar[2])){
  counter++;
  writeln(" expr = ", foo.bar[2]);
}

```

On 14.12.2016 00:00, Timothee Cour via Digitalmars-d-learn wrote:
> what's the best (and DRY) way to achieve:
>
> ```
> static if(__traits(compiles, expr))
>   fun(expr);
> ```
>
> ie, without repeating the expression inside expr?
>
> eg:
>
> ```
> static if(__traits(compiles, foo.bar[2])){
>   counter++;
>   writeln(" expr = ", foo.bar[2]);
> }
>
> ```
>

I usually do

enum code = q{expr};
static if(__traits(compiles,mixin(code)))
    fun(mixin(code));

On Tuesday, 13 December 2016 at 23:37:59 UTC, Timon Gehr wrote:
> On 14.12.2016 00:00, Timothee Cour via Digitalmars-d-learn wrote:
>> what's the best (and DRY) way to achieve:
>>
>> ```
>> static if(__traits(compiles, expr))
>>   fun(expr);
>> ```
>>
>> ie, without repeating the expression inside expr?
>>
>> eg:
>>
>> ```
>> static if(__traits(compiles, foo.bar[2])){
>>   counter++;
>>   writeln(" expr = ", foo.bar[2]);
>> }
>>
>> ```
>>
>
> I usually do
>
> enum code = q{expr};
> static if(__traits(compiles,mixin(code)))
>     fun(mixin(code));

Strangely if i put this in a templated enum that doesn't work.
If instead i use a delegate literal it works.

enum Compiles(string code) = is(typeof((){mixin(code);}));

enum Compiles2(string code) = __traits(compiles, mixin(code));

unittest
{
    static assert(Compiles!q{auto a = 1+1;});
    static assert(!Compiles!q{1;});
    static assert(!Compiles!q{a = 1;});

    static assert(Compiles2!q{auto a = 1+1;});
    static assert(!Compiles2!q{1;});
    static assert(!Compiles2!q{a = 1;});
}

wtf ?

On 12/14/2016 09:25 AM, Basile B. wrote:
> On Tuesday, 13 December 2016 at 23:37:59 UTC, Timon Gehr wrote:

>> I usually do
>>
>> enum code = q{expr};
>> static if(__traits(compiles,mixin(code)))
>>     fun(mixin(code));
>
> Strangely if i put this in a templated enum that doesn't work.
> If instead i use a delegate literal it works.
>
> enum Compiles(string code) = is(typeof((){mixin(code);}));
>
> enum Compiles2(string code) = __traits(compiles, mixin(code));

When you do that, the code does not match the syntax of __traits(compiles). Putting the code inside a scope works at least in this case:

enum Compiles2(string code) = __traits(compiles, mixin('{' ~ code ~ '}'));

Ali

On Wednesday, 14 December 2016 at 22:06:35 UTC, Ali Çehreli wrote:
> On 12/14/2016 09:25 AM, Basile B. wrote:
> > On Tuesday, 13 December 2016 at 23:37:59 UTC, Timon Gehr
> wrote:
>
> >> I usually do
> >>
> >> enum code = q{expr};
> >> static if(__traits(compiles,mixin(code)))
> >>     fun(mixin(code));
> >
> > Strangely if i put this in a templated enum that doesn't work.
> > If instead i use a delegate literal it works.
> >
> > enum Compiles(string code) = is(typeof((){mixin(code);}));
> >
> > enum Compiles2(string code) = __traits(compiles, mixin(code));
>
> When you do that, the code does not match the syntax of __traits(compiles). Putting the code inside a scope works at least in this case:
>
> enum Compiles2(string code) = __traits(compiles, mixin('{' ~ code ~ '}'));
>
> Ali

I see, it makes sense. Anyway the two templates have a common problem (protection attributes: data used in the code must be visible to the outside), so using them as a shortcut is a false good idea.

On 15.12.2016 01:38, Basile B. wrote:
> On Wednesday, 14 December 2016 at 22:06:35 UTC, Ali Çehreli wrote:
>> On 12/14/2016 09:25 AM, Basile B. wrote:
>> > On Tuesday, 13 December 2016 at 23:37:59 UTC, Timon Gehr
>> wrote:
>>
>> >> I usually do
>> >>
>> >> enum code = q{expr};
>> >> static if(__traits(compiles,mixin(code)))
>> >>     fun(mixin(code));
>> >
>> > Strangely if i put this in a templated enum that doesn't work.
>> > If instead i use a delegate literal it works.
>> >
>> > enum Compiles(string code) = is(typeof((){mixin(code);}));
>> >
>> > enum Compiles2(string code) = __traits(compiles, mixin(code));
>>
>> When you do that, the code does not match the syntax of
>> __traits(compiles). Putting the code inside a scope works at least in
>> this case:
>>
>> enum Compiles2(string code) = __traits(compiles, mixin('{' ~ code ~
>> '}'));
>>
>> Ali
>
> I see, it makes sense. Anyway the two templates have a common problem
> (protection attributes: data used in the code must be visible to the
> outside), so using them as a shortcut is a false good idea.

Shortcut that works:

enum ifCompiles(string code)=`static if(__traits(compiles,{ `~code~` })){ `~code~` }`;

mixin(ifCompiles!q{ ... });