improving the join function

[Andrei Alexandrescu]

I'm looking at http://d.puremagic.com/issues/show_bug.cgi?id=3313 and 
that got me looking at std.string.join, which currently has the sig:

string join(in string[] words, string sep);

A narrow fix:

Char[] join(Char)(in Char[][] words, in Char[] sep)
if (isSomeChar!Char);

I think it's reasonable to assume that people would want to join things 
that aren't necessarily arrays of characters, so T could be pretty much 
any type. An obvious step towards generalization is:

T[] join(T)(in T[][] items, T[] sep);

But join doesn't really need random access for words - really, an input 
range should suffice. So a generally useful join, almost worth putting 
in std.algorithm, would be:

ElementType!R1[] join(R1, R2)(R1 items, R2 sep)
if (isInputRange!R1 && isForwardRange!R2
     && is(ElementType!R2 : ElementType!R1);

Notice how the separator must be a forward range because it gets spanned 
multiple times, whereas the items need only be an input range as they 
are spanned once. This is at the same time a very general and very 
precise interface.

One thing is still bothering me: the array output type. Why would the 
"default" output range be an array? What can be done to make join() at 
the same time a general function and also one that works for strings the 
way the old join did? For example, if I want to join things into an 
already-existing buffer, or if I want to write them straight to a file, 
there's no way to do so without having an array allocation in the loop. 
I have a couple of ideas but I wouldn't want to bias yours.

I also have a question from people who dislike Phobos. Was there a point 
in the changes of signature above where you threw your hands thinking, 
"do the darn string version already and cut all that crap!"?


Thanks,

Andrei