Move away from using span value as a type parameter

[omus]

Moving away from using the span value as a type parameter as this does not allow us to check properties of the type like "ending" (span is < 0) or "beginning" (span is > 0).

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #12   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines         177    188   +11     
=====================================
+ Hits          177    188   +11

Impacted Files	Coverage Δ
src/endpoint.jl	100% <ø> (ø)	⬆️
src/description.jl	100% <100%> (ø)	⬆️
src/anchoredinterval.jl	100% <100%> (ø)	⬆️

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[omus]

The current commit switches AnchorInterval{P, T} to AnchorInterval{T, P} in preparation for additional commits which end up changing AnchorInterval to its final form of AnchorInterval{T, S, E} where S is the type of the span and E is an instance of Direction (either Ending or Beginning) which indicates the direction of the endpoint.

[omus]

Currently the span can field can be positive/negative which is strange as the span accessor only returns a positive value. I believe the code can be simplified if we restrict the span parameter passed into AnchoredInterval to be always positive.

[omus]

Note that with these changes since the value of the span is no longer part of the type things like HourEnding have spans of Hour but not necessarily one hour:

julia> HourEnding(now(), Hour(3))
HourEnding{DateTime}(2018-03-08T14:44:00.085, 3 hours, Inclusivity(false, true))

This does have benefits as it makes our intersect function always return the same type:

julia> dt = now()
2018-03-08T14:44:46.966

julia> intersect(IntervalEnding(dt, Hour(3)), HourEnding(dt - Hour(1), Hour(3)))
HourEnding{DateTime}(2018-03-08T13:44:46.966, 2 hours, Inclusivity(false, true))

julia> ans isa IntervalEnding{DateTime, Hour}
true

[spurll]

I am eying this PR with a mix of excitement and trepidation. I look forward to reviewing this at a later date.

[omus]

Fair enough. I would like to get this merge next week sometime so let me know if you want to go through it together.

[spurll]

That would be good. I'm not sure I'll have time next week, but I'll let you know.