A pass() identity range?

[bearophile]

I sometimes write code like:

auto data = File("data.txt")
             .byLine()
             .map!(r => r.strip().dup)()
             .filter!(r => !r.empty)()
             .array();


To help writing/improving/debugging such code maybe it's useful 
to create a pass() range in Phobos. It's like a "identity range" 
that just passes its inputs to its output, but also runs a given 
lambda to the items that pass through it (the lambda receives a 
const item, to avoid modifications of the data passing through 
the chain):


auto data = File("data.txt")
             .byLine()
             .pass(r => writeln("A: ", r))
             .map!(r => r.strip().dup)()
             .pass(r => writeln("B: ", r))
             .filter!(r => !r.empty)()
             .array();


This usage of pass() allows to introduce debug prints inside very 
long chains of such iterables, with no need to break them apart 
and assign the parts to temporary variables just to see what's 
going on in that chain.

But it's not hard to do it with a not pure map:

                 .map!((r){ writeln("B: ", r); return r; })()

Or even with an evil comma operator:

                 .map!(r => (writeln("B: ", r), r))()

So I don't know how much useful that pass() is.

Bye,
bearophile