http://d.puremagic.com/issues/show_bug.cgi?id=4468

           Summary: std.string.join() for lazy iterable of strings
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-07-15 17:24:25 PDT ---
This Python code generates a lazy sequence of the first 20 integers (starting from 0), then maps them lazily to strings, and then joins them in a single string:


from itertools import imap
r = imap(str, xrange(20))
result = "".join(r)
print result



The same code can be written in D2:

import std.stdio, std.algorithm, std.conv, std.range, std.string;
void main() {
    auto r = map!("to!string(a)")(iota(20));
    //string result = join(array(r), ""); // OK
    string result = join(r, ""); // error
    writeln(result);
}


But I'd like a std.string.join() able to accept a Range too, so I can use
join(r, "") instead of join(array(r), ""), because it's shorter, simpler,
natural enough and reduces memory used (and probably increases code performance
too).

When the total size of the resulting string is not known because the input is a
lazy sequence of strings, then probably join() has to use something like
appender() to improve its performance.

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |andrei@metalanguage.com
         AssignedTo|nobody@puremagic.com        |andrei@metalanguage.com


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--- Comment #1 from bearophile_hugs@eml.cc 2011-02-07 14:33:28 PST ---
See also bug 5542

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--- Comment #2 from bearophile_hugs@eml.cc 2011-07-30 06:54:17 PDT ---
This is a very common need. Another example:


import std.range;
void main() {
    auto aa = [1:["hello", "red"], 2:["blue", "yellow"]];
    auto r1 = join(aa.values); // OK
    auto r2 = join(aa.byValue()); // error
}

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


--- Comment #3 from bearophile_hugs@eml.cc 2011-12-21 10:33:37 PST ---
Now in DMD 2.057 this works:

import std.stdio, std.algorithm, std.conv, std.range, std.string;
void main() {
    auto r = map!("to!string(a)")(iota(20));
    //string result = join(array(r), ""); // OK
    string result = join(r, ""); // error
    writeln(result);
}


This line:
auto r = map!("to!string(a)")(iota(20));
is also better written:
auto r = map!text(iota(20));


But this doesn't work still:

import std.range;
void main() {
    auto aa = [1:["hello", "red"], 2:["blue", "yellow"]];
    auto r2 = join(aa.byValue()); // error
}


To make this work there are two solutions:
1) Change byValue() to return a Range.
2) Change join() to accept an opApply too.

The first solution allows to use byValue in many other cases, so I think join() is OK, and I close this bug report.

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