Template constraints
Xinok via Digitalmars-d
digitalmars-d at puremagic.com
Sat Feb 14 11:18:12 PST 2015
On Saturday, 14 February 2015 at 17:00:33 UTC, Andrei
Alexandrescu wrote:
> There's been recurring discussion about failing constraints not
> generating nice error messages.
>
> void fun(T)(T x) if (complicated_condition) { ... }
> struct Type(T)(T x) if (complicated_condition) { ... }
>
> If complicated_condition is not met, the symbol simply
> disappears and the compiler error message just lists is as a
> possible, but not viable, candidate.
>
> I think one simple step toward improving things is pushing the
> condition in a static_assert inside type definitions:
>
> void fun(T)(T x) if (complicated_condition) { ... } // no change
> struct Type(T)(T x)
> {
> static assert(complicated_condition, "Informative message.");
> ...
> }
>
> This should improve error messages for types (only). The
> rationale is that it's okay for types to refuse compilation
> because types, unlike functions, don't overload. The major
> reason for template constraints in functions is allowing for
> good overloading.
>
>
> Andrei
I've done this myself for the same reason. I may split up the
condition into multiple 'static assert' statements which gives a
more specific error message. However, I wonder if there's a
better solution that we can incorporate in to D. The trouble with
template constraints is that, if you have complex conditions,
there's no easy way to fall back to a default state. You would
have to duplicate all the conditions and write "not this and not
this and not this and ...".
Template specializations can fall back to a default state, but
not template constraints. So what if we were to add a feature
that would allow us to do just that?
void test(Range)(Range r) if(isRandomAccessRange!Range)
{
...
}
void test(Range)(Range r) default
{
static assert(false, "descriptive error message");
}
I'm not claiming this is a better solution; I'm simply putting
the idea out there.
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