Fix a single weekday grammar callback

[oxidase]

Issue

The PR adds a constructor callback for a single weekday condition like Su 00:00-24:00

Tasklist

• add regression / cucumber cases (see docs/testing.md)
• review
• adjust for comments

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?

[MoKob]

The part around the single central line of code looks solid and makes me believe that the fix does what it is supposed to.

However, I feel that we could do a better job of documenting what spirit syntax does. I can't even begin to tell what the function does and why these two braces change it from false to correct.

[MoKob]

@oxidase I have a very hard time understanding this magic syntax. Given that these changes are not that obvious, do you think that we could add some commentary to make it easier to follow what is supposed to happen here?

[oxidase]

@MoKob i can explain what happened in this case: without parenthesis the semantic action [_val = ph::construct<OpeningHours::WeekdayRange>(_a, _b)] that constructs a WeekdayRange was invoked only in a case if both the first and the second part of the rule succeed.

In case Mo-Fr local rule variable _a will be Mo and _b will be Fr. So the actions will be called as ph::construct<OpeningHours::WeekdayRange>(1, 5).

If parsing of the second rule failed but the first part succeeded then both _a and _b will be initialized to let say Sa, but construction of a ph::construct<OpeningHours::WeekdayRange>(6, 6) will not be called. So WeekdayRange will be a default initialized object with an empty range.

Adding parenthesis around both rules makes invocation of ph::construct<OpeningHours::WeekdayRange>(_a, _b) in both cases:

• if the first part succeeds, like 'Mo' -> ph::construct<OpeningHours::WeekdayRange>(1,1)
• if the first and the second optional part succeeds, like 'Tu-Fr' -> ph::construct<OpeningHours::WeekdayRange>(2,5)

Idk if such lengthy comments would make sense, but I can make a step-by-step intro how to add semantic to unimplemented parts like nth_entry or day_offset, For example, with nth_entry it is possible to define Oct Su[1] (the first Sunday in October) like in the Oktoberfest example.

[MoKob]

@oxidase thank you for this explanation. This makes a lot more sense to me now.
I believe simply having a bit more context on what kind of format the grammar parses might already help here (Parsing Mo-Fr into ph::construct<>(1,5) or Sa into ph:construct<6,6>). I agree that the longer explanation here might be a bit much, but it is already accessible now via git-blame, so I guess we are golden here.