• FeepingCreature (5/5) Oct 17 2022 Trolling a bit: Should this invariant always hold?
• ag0aep6g (3/5) Oct 17 2022 No, it shouldn't. The first expression might consume `range`. So
• FeepingCreature (3/10) Oct 17 2022 Ah, good idea!
• Dukc (10/12) Oct 17 2022 Posting this without having looked at other replies.
• FeepingCreature (7/12) Oct 17 2022 Solution, or rather, Phobos code that violates it (even the
• =?UTF-8?Q?Ali_=c3=87ehreli?= (4/7) Oct 17 2022 Not an entirely unique case. ;)

FeepingCreature <feepingcreature gmail.com> writes:

Trolling a bit: Should this invariant always hold?

assert(equal(range.retro.array, range.array.retro));

Can you find a definition of `range`, using only `std.algorithm` 
verbs (ie. no custom ranges), that violates it?

(And no peeking at my recently-filed bug reports!)

ag0aep6g <anonymous example.com> writes:

On Monday, 17 October 2022 at 11:02:07 UTC, FeepingCreature wrote:
 Trolling a bit: Should this invariant always hold?

 assert(equal(range.retro.array, range.array.retro));

No, it shouldn't. The first expression might consume `range`. So 
it might be empty in the second expression.

FeepingCreature <feepingcreature gmail.com> writes:

On Monday, 17 October 2022 at 11:46:35 UTC, ag0aep6g wrote:
 On Monday, 17 October 2022 at 11:02:07 UTC, FeepingCreature 
 wrote:
 Trolling a bit: Should this invariant always hold?

 assert(equal(range.retro.array, range.array.retro));

 No, it shouldn't. The first expression might consume `range`. 
 So it might be empty in the second expression.

Ah, good idea!

assert(equal(range.save.retro.array, range.save.array.retro));

Dukc <ajieskola gmail.com> writes:

On Monday, 17 October 2022 at 11:02:07 UTC, FeepingCreature wrote:
 Trolling a bit: Should this invariant always hold?

 assert(equal(range.retro.array, range.array.retro));

Posting this without having looked at other replies.

No. The range could be a reference range, in which case this does 
not hold unless the range is already empty.

The invariant would have to be 
`assert(equal(range.save.retro.array, range.save.array.retro));` 
instead.

Also this won't hold if the range contains floating point NaNs, 
or other elements that are not equal to themselves.

There may be other corner cases too I didn't think of.

FeepingCreature <feepingcreature gmail.com> writes:

On Monday, 17 October 2022 at 11:02:07 UTC, FeepingCreature wrote:
 Trolling a bit: Should this invariant always hold?

 assert(equal(range.retro.array, range.array.retro));

 Can you find a definition of `range`, using only 
 `std.algorithm` verbs (ie. no custom ranges), that violates it?

 (And no peeking at my recently-filed bug reports!)

Solution, or rather, Phobos code that violates it (even the 
strengthened `assert(equal(range.save.retro.array, 
range.save.array.retro))`):

[rot13](https://rot13.com/): `nhgb enatr = bayl(1, 
2).havd!"gehr"; // havd onpxjneq vgrengvba purpxf ryrzragf sbe 
havdhrarff va gur onpxjneq beqre bs gur haqreylvat enatr.`

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 10/17/22 23:19, FeepingCreature wrote:

 [rot13](https://rot13.com/): `nhgb enatr = bayl(1, 2).havd!"gehr"; //
 havd onpxjneq vgrengvba purpxf ryrzragf sbe havdhrarff va gur onpxjneq
 beqre bs gur haqreylvat enatr.`

Not an entirely unique case. ;)

   https://issues.dlang.org/show_bug.cgi?id=16588

Ali