Parser options

[lukescott]

Parse options https://github.com/webconnex/cron/tree/parser-options
This adds Parser and NewParser with the ability to:

• Select which fields are included. So you can exclude seconds, or any other field you don't want to use.

• Set other options that might be useful, like making certain assumptions about the schedule.

  The Parse function now looks like this:

  var defaultParser = NewParser(
      Second | Minute | Hour | Dom | Month | DowOptinal | Descriptor,
  )
  
  // Parse returns a new crontab schedule representing the given spec.
  // It returns a descriptive error if the spec is not valid.
  //
  // It accepts
  //   - Full crontab specs, e.g. "* * * * * ?"
  //   - Descriptors, e.g. "@midnight", "@every 1h30m"
  func Parse(spec string) (_ Schedule, err error) {
      return defaultParser.Parse(spec)
  }

  This could be a resolution for Cron Format not the same as Cron Spec #58. All you would need to do is remove Second and it would be spec compliant. But for those who need it, they can create a new parser with the option.

  I'm not sure about Dow being optional and being "spec complaint" - but there are two options for this - Dow and DowOptional which are interchangeable - one makes it required, the other allows you to omit it.

(See #69)

[robfig]

Does #73 meet your need? If not, can you describe the use case for creating a non-standard parser? It seems like having some kind of semi-standard syntax is a big benefit, and changing the set of stuff accepted would be fairly confusing if it didn't enable some significant new use case.

That being said, it does look cool to configure the parser like that!

[lukescott]

We use it to omit the time options since we don't use them. We store a schedule associated with a subscription and scheduled the next date. We also added some non-standard options (like week of year) that was needed to accomplish a bi-weekly schedule.

You could keep the function in #73 by passing a different set of options, much like how I presented Parse above.

There are also a couple parser options that make some fields optional or required (like day of week).

I have some other features built on the parser options branch described in #69.

[lukescott]

@robfig I've brought this up to date with master. ParseStandard looks like this now:

var standardParser = NewParser(
    Minute | Hour | Dom | Month | Dow | Descriptor,
)

// ..
func ParseStandard(standardSpec string) (Schedule, error) {
    return standardParser.Parse(standardSpec)
}

Which looks almost exactly like Parse, but lacks Second and replaces DowOptional with Dow.

[robfig]

Looks good, just the two exported types should have godoc

[robfig]

godoc please

[robfig]

godoc please

[lukescott]

godocs added. Also fixed golint warning complaining about an else { return ... }.

[robfig]

LGTM thank you!!