On Sunday, 3 April 2016 at 10:59:47 UTC, Q. Schroll wrote:
Simple as that, suppose
uint[uint] aa;
Any range supports carrying an index. Not so does the Range
returned by byKey and byValue.
foreach (i, k; aa.byKey) { }
and
foreach (i, v; aa.byValue) { }
both don't compile.
That's incorrect. Only Random Access Ranges are indexable. The
ranges returned by aa.byKey and aa.byValue are simply Input
Ranges. Moreover, ranges do not by default allow for an index
value in a foreach loop. That only works out of the box with
arrays. To get the same for a range, you can use
std.range.enumerate:
import std.range : enumerate;
foreach(i, k; aa.byKey.enumerate) {}
Reason (I found out by chance):
If the key or value type is a std.typecons.Tuple, iteration
over aa.by* decomposes the Tuple if there is the right number
of arguments. For 2-Tuples, there cannot be both possible.
alias Tup = Tuple!(int, int);
int[Tup] it;
Tup[int] ti;
foreach (x, y; it.byKey) { }
foreach (x, y; ti.byValue) { }
Why is this undocumented? http://dlang.org/spec/hash-map.html
doesn't mention Tuples at all!
D's associative arrays don't know anything about Tuples, so
there's no reason for the aa docs to talk about them. This
behavior comes from how std.typecons.Tuple is implemented.
Why is this useful? Anyone can decompose the Tuple with .expand
if they like. I would prefer allowing an index.
If you look at the source of Tuple, alias this is used on
.expand, which is likely why they are automatically decomposed in
an aa.
