Bug 4729 – std.algorithm: strange iota behaviour

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 }

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))); }

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

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.

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?

(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

The problem here is that no start value is given.

(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

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.

Looks good: exception thrown if no seed, seed returned if it exists.