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
