http://d.puremagic.com/issues/show_bug.cgi?id=4729

           Summary: std.algorithm: atrange iota behaviour
           Product: D
           Version: D2
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Optlink
        AssignedTo: nobody@puremagic.com
        ReportedBy: necroment@gmail.com


--- Comment #0 from Max Klyga <necroment@gmail.com> 2010-08-26 05:16:40 PDT ---
import std.stdio, std.algorithm, std.range;

void main() {
    filter!((int n) { write(n, ' '); return 0; })(iota(1, 10));
    //writes: 1 2 3 4 5 6 7 8 9

    writeln();

    reduce!"0"(
        filter!((int n) { write(n, ' '); return 1; })(iota(1, 10))
    ); //writes: 1 2 3 4 5 6 7 8 9

    writeln();

    reduce!"0"(
        filter!((int n) { write(n, ' '); return 0; })(iota(1, 10))
    ); //Never stops writing numbers
}

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc
            Summary|std.algorithm: atrange iota |std.algorithm: strange iota
                   |behaviour                   |behaviour


--- Comment #1 from bearophile_hugs@eml.cc 2010-08-26 06:04:08 PDT ---
It seems a iota() problem.


import std.algorithm: reduce, filter;
import std.range: iota;
void main() {
    reduce!"0"(filter!((int a){ return false; })(iota(1)));
}

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--- Comment #2 from Max Klyga <necroment@gmail.com> 2010-08-26 07:50:27 PDT ---
This bug was introduced in 2.048, as is 2.047 iota stops but fails shortly
after:
src/phobos/std/algorithm.d(279): Enforcement failed

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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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Brad Anderson <eco@gnuk.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eco@gnuk.net


--- Comment #3 from Brad Anderson <eco@gnuk.net> 2012-02-01 13:21:38 PST ---
2.057 seems to handle this situation (trying to reduce an empty range but not offering a seed) by throwing an appropriate exception:

object.Exception@C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(662): Cannot reduce an empty range w/o an explicit seed value.

I believe this should be closed. Also, not sure why this is set as an optlink issue.

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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #4 from hsteoh@quickfur.ath.cx 2013-01-03 20:24:38 PST ---
In latest git head, throws an exception. Is this acceptable (should the bug be
closed)?

Or is it better to have reduce return (ElementType!R).init?

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--- Comment #5 from bearophile_hugs@eml.cc 2013-01-04 17:53:04 PST ---
(In reply to comment #4)
> In latest git head, throws an exception. Is this acceptable (should the bug be
> closed)?
> 
> Or is it better to have reduce return (ElementType!R).init?

The reduce of Python returns the start value:

>>> reduce(lambda a,b: a * b, [], 0)
0

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--- Comment #6 from hsteoh@quickfur.ath.cx 2013-01-04 18:17:16 PST ---
The problem here is that no start value is given.

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--- Comment #7 from bearophile_hugs@eml.cc 2013-01-04 18:36:53 PST ---
(In reply to comment #6)
> The problem here is that no start value is given.

In this case Python raises an error. I think this is acceptable:

>>> reduce(lambda a,b: a * b, [])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: reduce() of empty sequence with no initial value

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--- Comment #8 from hsteoh@quickfur.ath.cx 2013-01-04 22:11:08 PST ---
In that case, we should probably close the bug, since the latest Phobos throws an exception upon calling reduce() with an empty range and no start value.

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