Trying to reproduce Node.js async waterfall pattern.. (Meta-programming)

[Christian Beaumont via Digitalmars-d-learn]

> Just to be sure: whether or not an error is passed to a 
> callback, the
> final callback is always called?
> I mean, the last callback could also be called *only when 
> something
> fails*, a bit like a default case in a switch, or an 
> error-handling
> routine.

Yes, the final callback is always called, but if an error is 
passed to the callback by any of the main steps in the "sequence 
ladder", it will immediately jump to the final callback and not 
execute further steps.

> What do you mean? The compiler does deduce the type of Funcs.

If you look at where I call Waterfall() in main, you'll see I had 
to manually specify (Callback cb) instead of just (cb); since it 
didn't know that the Funcs... were of type AsyncFunc

> What do you mean by 'specialize'?

That is to say, there is no way I can see, to say that the 
variadic template parameter "Funcs..." are all AsyncFunc's.  I 
think I noticed that in Swift you can say something like 
"Funcs:AsyncFunc..." to specialize the variadic.  No swift 
expert, but was just browsing around today trying to compare how 
you might do it in C++ or other languages.

> (cb) { cb(null, "one");} is possible, but that means it's a 
> function
> template, not a function.
> You can get this syntax by making the callbacks template 
> arguments,
> which means they must be known at compile-time. Is that OK with 
> you or
> do you need the possibility to define the callbacks at runtime?

The goal was to do as much as possible at compile time.  Could 
you elaborate on this a bit.  I guess the answer is, yes, it's 
okay with me.

> I don't get it: none of your callbacks have a return type per 
> se: they
> all return 'void'.
> Do you want callbacks that really return something?

Yes, the callbacks at step0 should output a type in the result 
which is specific to step0, and then that type should be fed in 
as a secondary parameter to step1.

I didn't get that far, as I was already stuck.

> You can test if func[0] takes one or two arguments:
>
> import std.traits: isCallable, ReturnType;
>
> static if (isCallable!(Func[0]) && ReturnType!(Func[0]).length 
> == 1)

Ah, yes, that sounds reasonable, I already thought something like 
that may do the trick.

thanks for your patience!