Allow date format yyyy-mm-dd

[wucas]

This is related to gap-system/gap#3355 in gap. If there are more places to change I am happy to change them as well. It seems there was something wrong in line 348, I added some brackets to correct the if statement.

[wucas]

I have not tested my changes though because I don't know how to make packages and I don't have one I could tests it with. The code that checks whether a date is valid I pushed to gap as well and there I tested it.

[wucas]

This is also related to Issue #20

[olexandr-konovalov]

Thank you! I will deal with this after gap-system/gap#3355 will stabilise.

[wucas]

I added the check for dates in the mm/dd/yyyy format. Different to the checks in ValidatePackageInfo it will print the "invalid date" error additionally. I don't see a nice way to avoid this but I also think it's not too bad to have both errors printed since it is still clear what the problem ist.

[fingolfin]

@wucas to make a package, you can use https://github.com/gap-packages/PackageManager :-)

[olexandr-konovalov]

Thanks @wucas - is this now ready to be reviewed in conjunction with gap-system/gap#3355 ?

[wucas]

I use if IsBound( infon.Date ) now but as mentioned above I'am not entirely sure that this is how I should do it.

[wucas]

I've made a few more changes based on @fingolfin suggestions. The idea is the following:
If no date is bound an according message is printed. Otherwise I check whether it is a string of the correct length, containing digits and -- or // at the correct positions. If this is not the case a message is printed telling the user that Date should be a string of the format dd/mm/yyyy or yyyy-mm-dd representing a date since 1999.
If this was the case then I split the string at // or -- accordingly to get a list with which I can easily check the validity of the date.
In the case of // I check whether it is plausible from the date that the format is mm/dd/yyyy and if so print a message telling the user specifically that they might switched month and day and that the format should be dd/mm/yyyy or yyyy-mm-dd.
If either of this messages is printed no other message should be printed thus I set has_error := true and do Unbind( date ).
If everything went well so far I check if the date is valid with a helper function for better readability. I the date is invalid another different message is printed.

tldr; My goal was to give the user an as specific as possible hint to what they might have done wrong. Thus four times Print now all with different messages.

[fingolfin]

I am completely happy with this now, thank you @wucas . So now it's up to @alex-konovalov to decide whether and when to merge this :-)

[olexandr-konovalov]

For the update, gap-system/gap#3355 is now merged.