On Friday, 27 July 2012 at 18:47:34 UTC, Stuart wrote:
[snip]
Ah, but that depends upon the pre-existence of the dirEntries()
function. I think perhaps you're missing the point - which is
that "Yield" allows you to WRITE a function synchronously which
will then be executed lazily, as an iterator. What you have
demonstrated there is USING a lazy function. How would I write,
in D, a function that would lazily assemble some data and
return it as a lazy collection? I mean, without calling
existing lazy functions.
Short answer: you'd implement an InputRange, just as dirEntries is
implemented.
When I started learning D, the lack of first-class coroutines
("things
that yield") struck me as a real drawback. I had grown very used
to
them in Python and in Scheme.
So I learned how to write ranges. I did not totally fall in love
with
them. I agree with you that there is a conceptual elegance in
writing
a coroutine as if it were a regular function, just one that
happens to
include a "yield" keyword that seems to "remember its place"
between
calls. I'd like to see language-level support for them in D some
day.
Having said that, I'm satisfied with ranges. I've seen the fiber
and
opApply ways of implementing yield, and they are okay. But the
fact
is, it's usually not that hard to write an input range. Just make
a
struct that implements these operations:
http://dlang.org/phobos/std_range.html#isInputRange
Decide what state you need to maintain between calls; determine
meaningful definitions for empty, front, and popFront that read
and
manipulate that state.
Write a few of those, and the idiom will become natural to you.
It will feel inside-out with respect to C# enumerators. But it's
not
inherently bad -- certainly not bad enough to dismiss without
deeper
study, and certainly not bad enough to immediately jump to fibers
or
an opApply-with-mixin trick before you've at least attempted a
range-based solution.
There is a load of reusable library code in D that can be brought
to
bear on ranges. It's well worth a bit of mental rewiring to
benefit
from that.
Best,
Graham
