Document Qo::PatternMatchers::PatternMatch#default as private API

[bolshakov]

I want my Option matcher raise error if nothing matcher instead of returning nil (because the whole point of having Option is to avoid dealing with nils.

I managed to achieve this by redefining @default ivar but not really happy with using undocumented internals:

  module ExhaustivePatternMatch
    def initialize(**)
      super
      @default ||= self.else { fail MatchError }
    end
  end

So, I documented PatternMatch#default accessor as private API to make it valid extension point.

[baweaver]

I'd had an idea earlier to mark branches as required. Perhaps I'll look into implementing that back in, as I feel that'd be a more explicit way to do it.

[bolshakov]

Required branch is not the same. For example this matcher doesn't require else branch

option.match do |m|
  m.some { |x| x * 2 }
  m.none { 'noop' }
end

I want to customise behaviour if nothing matched

[baweaver]

#26 addresses some of this.