On Wednesday, 25 July 2012 at 07:23:51 UTC, Jacob Carlborg wrote:
On 2012-07-25 07:30, xenon325 wrote:
For this to be useful you really need to create a template
for every condition
[snip]
With my approach the constraint will have a name which should
result in a lot better error messages.
How is it different from creating a template for every
condition ? ;)
As long as you actually _do_ that, it might not be so much
different.
But you would have to create a constraint for every condition as
well!
And if someone implements Don's idea, error messages would be as
good.
Honestly, I don't quite get what's the difference between
current
template constraints and ones you're proposing:
[snip]
on the call site it's as convenient and clear as yours.
On the definition site, yes, syntax maybe is a tad worse,
but it's not *that* bad to require introduction of a new
language construct.
Well it depends on that syntax is used. It's clearer if this
syntax would be possible:
void foo (InputRange range);
1. How would you signify `foo` is generic ?
For complier it's probably possible - by type of `InputRange`,
but that will be one more special case
What about user ?
2. How would you put 2 constraints on `range` ?
3. Also you forgot to address:
template isInputRange(R)
{
enum bool isInputRange = is(typeof(
(inout int _dummy=0)
{
R r = void; /*** how would you express this
b.t.w. ? ***/
range.d comment on this line is "can define a range object".
I think it's very little chance of a new language construct
being added. It's more like I don't necessarily think the way
the template constraints were implemented was the best way.
Sorry, but I still fail to see how you way is (significantly)
better.
Actually the only thing constraint writer needs to know is that
`is(typeof(<code>))`
means "compiles". All the rest is perfectly clear.
And why is "(inout int _dummy=0)" there? I would guess that
std.algorithm and other parts of Phobos is filled with strange
things like this.
Yeah :) My thoughts (have really little D experience):
1. why is it lambda
2. why does it have argument
3. why is arg `inout`
4. why is *dummy* arg initialized
I would think it's some kind of temporary compiler workaround.
But if it's something smart, it could definitely use
better syntax, no objections here (__traits( compiles, ..) ?).
Then the conclusion would at least be that std.algorithm could
use some enhancement how these template constraints are written.
Agree, `(inout int _dummy=0)` is confusing. There well may be
other (as confusing) examples but probably there are easy ways to
fix them.
Maybe you should create enhancement request or discuss it with
community ?
Wait! We were talking about how to make compiler error messages
better.
