Interfaces, traits, concepts, and my idea for a DIP

[jmh530 via Digitalmars-d]

On Thursday, 30 July 2015 at 10:40:59 UTC, Atila Neves wrote:
>
> You still wouldn't get a better error message here than with:
>
> struct MyRange {
>     ...
>     static assert(isInputRange!MyRange);
> }
>
> It's less typing, but you still wouldn't know why the static 
> assertion failed. Now, if we make Adam's idiom in my 
> aforementioned pull request the go-to thing, then this would be 
> good:
>
> @satisfies!(checkInputRange, MyRange) struct MyRange { ... }
>
> There's still the problem of having two names for each 
> constraint: checkInputRange and isInputRange. But... we could 
> also establish the convention of checkXXX and use string mixins 
> to turn this:
>
> @satifies!(isInputRange, MyRange)
>
> into (which works cos I just tried it):
>
> template satisfies(alias Constraint, R) {
>     enum check = "check" ~ Constraint.stringof[2..$-3] ~ "!(R)";
>     enum assert_ = "static assert("~ check ~ ");";
>     mixin(assert_); //mixes in "static 
> assert(checkInputRange!R)"
> }
>
>
> @satisfies!(isInputRange, Zeroes)
> struct Zeroes {
>     enum empty = false;
>     void popFront() {}
>     @property int front() { return 0; }
> }
>
>
> Now, _this_ I could go for.
>
> Atila

I might be overthinking this.

You would still need to define something for checkInputRange. Are 
you thinking something simple like

template checkInputRange(R)
{
     enum bool checkInputRange = isInputRange!R;
}

In this case, the error message is still referring to 
checkInputRange instead of isInputRange. So the user would still 
need to refer to that and see what that functions conditions are. 
Alternately, you could write checkInputRange to provide the user 
information about which functions are not implemented (like no 
pop, no popFront, no empty). That seems more difficult, but would 
be convenient for the user.

The benefit of something simple like

template satisfies_alt(alias Constraint, R) {
     static assert(Constraint!R);
}

is that at least you don't have to write extra functions. You 
only need isInputRange. The downside is that the error message 
references Constraint instead of isInputRange. That's less 
intuitive.