Bug in ListX with empty arguments

[laurentbartholdi]

The test on line 1846 of colls.gi seems to harsh:

Observed behaviour

gap> ListX([],function(args) return args; end);
Error, gens[1] must be a collection, a boolean, or a function called from
ListXHelp0( result, arg, 0 ); called from

Expected behaviour

gap> ListX([],function(args) return args; end);
[ ]

Copy and paste GAP banner (to tell us about your setup)

GAP, Version 4.7.6 of 15-Nov-2014 (free software, GPL)
(but also the most recent github pull)

[ChrisJefferson]

I agree. Just for completeness, the same bug occurs in SetX, ProductX and SumX.

The problem is that the X family of functions demand a collection, so this also fails:

ListX([2,"abc"], x -> x), which I see no reason to fail either.

I think the fix is just to turn IsCollection into "can be iterated over" (because that's what we actually care about, and do). What I'm not sure about is if we have a function to do that. We could (at the risk of losing the error message), just try iterating, and then the error would move to no 1st choice method found for Iterator.

[ChrisJefferson]

Should probably just be IsListOrCollection, forgot about that function.

[ChrisJefferson]

Now fixed in #903 (along with general other improvements).

[olexandr-konovalov]

@laurentbartholdi any reasons to use GAP 4.7.6 of 15-Nov-2014 btw?

[laurentbartholdi]

Yes, it's the gap on my office computer - no space for a private install.

On Tue, Sep 27, 2016, 21:52 Alexander Konovalov notifications@github.com
wrote:

@laurentbartholdi https://github.com/laurentbartholdi any reasons to
use GAP 4.7.6 of 15-Nov-2014 btw?

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Laurent Bartholdi
DMA, École Normale Supérieure, 45 rue d'Ulm, 75005 Paris. +33 14432 2060
Mathematisches Institut, Universität Göttingen, Bunsenstrasse 3-5, D-37073
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