unique() for Interval?

[mmuurr]

unique() doesn't appear to be defined for class Interval, yet it silently runs and produces results that (I think) don't make sense.

example:

foo <- Sys.time() + days(1:4)
ints <- rep(interval(head(foo, -1), tail(foo, -1)), 2)
## each interval is the same _duration_, and unique(ints) simply returns the unique durations (in seconds).
## would (reasonably) expect the 3 unique _intervals_, instead.
unique(ints)
## [1] 86400

[mmuurr]

quick add on why this could be useful: stratification by interval, e.g. by creating interval factors:

## continuing the example code from above...
factor(ints)
## [1] <NA> <NA> <NA> <NA> <NA> <NA>
## Levels: 86400

foo <- data.frame(x = 1:6, ints = ints)
## if dlply-ing over ints, would expect a length-3 vector of 2s... instead it's a length-1 vector of 6:
unlist(dlply(foo, .(ints), nrow))