[fpc-pascal] Parse strings like "day-1" to convert in a time stamp

[Marcos Douglas B. Santos]

On Mon, Oct 15, 2018 at 9:33 AM Michael Van Canneyt
<michael at freepascal.org> wrote:
>
> > On Sun, Oct 14, 2018 at 8:19 PM Michael Van Canneyt
> > <michael at freepascal.org> wrote:
> >>
> >>
> >>
> >> On Mon, 15 Oct 2018, Sven Barth via fpc-pascal wrote:
> >>
> >> > Marcos Douglas B. Santos <md at delfire.net> schrieb am So., 14. Okt.
> >> 2018,
> >> > 16:15:
> >> >
> >> >> Do you know any Pascal lib to parse strings to convert in date/time
> >> values?
> >> >> The user is supposed to type human-friendly values like:
> >> >> - "day"    => trunc(now)
> >> >> - "day-2" => trunc(now-2)
> >> >> - "week" => trunc(now-7)
> >> >> - "yesterday" => trunc(now-1)
> >> >> - etc...
> >> >> But they could type even their computer date format or ISO as well.
> >> >>
> >> >> A lib in C, but very complex:
> >> >> - https://github.com/gagern/gnulib/blob/master/lib/parse-datetime.y
> >> >>
> >> >> I would like to start with something simpler, if possible. Thank you.
> >> >>
> >> >
> >> > I don't know a specific library that fulfills your needs, but maybe
> >> you can
> >> > build something atop of FpExprPars:
> >> > http://wiki.lazarus.freepascal.org/How_To_Use_TFPExpressionParser
> >> > At least according to the source it also supports datetime handling.
> >
> > Thanks, Sven.
> >
> >>
> >> It does, it's used (amongst other things) in the reporting engine;
> >
> > Michael,
> > But does it work using just date values - accordingly with date format
> > in OS - or also with "day", "tomorrow", etc?
> > If not, can I extend those classes without change the original ones?
>
> All depends on the exact format you want to use.
> day -1
> can be easily implemented by registering a function day, which returns teh
> same Date()

So I can register those identifiers as functions, cool!

> double quotes are supported as delimiters for identifiers, so "Day" should
> work, but a date literal is going to be tricky.

Maybe I can test the value first if it is a valid date format, before
try to use FpExprPars on it...
It looks like a good start, though. Thanks.

regards,
Marcos Douglas