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

[Christian Beaumont via Digitalmars-d-learn]

Just an idea that popped into my head... Maybe I can use variant 
for the input/output types?  I haven't looked at it yet, so I'm 
not sure what it does, or the performance costs.

I realized that because the final callback always gets called, 
and the types of the intermediate steps may be different, there 
is actually no way at all to define a completely uniformly type 
safe final callback...

although the point is a little moot, because the only way you end 
up in the final callback is either by an Exception or a Result 
from the finalCallback - 1 step.

So you can actually never get both an Exception and a Result