Make durations interact with difftime and numeric about lubridate HOT 7 CLOSED

I got it to work for both by setting things up as
"+.duration" <- "+" <- function(e1, e2){
if(!is.POSIXt(e1) && is.duration(e2)) e1 <- as.duration(e1)
if(!is.POSIXt(e2) && is.duration(e1)) e2 <- as.duration(e2)
if (is.duration(e1) && is.duration(e2))
add_duration_to_duration(e1, e2)
else if (is.duration(e1) && is.POSIXt(e2))
add_duration_to_date(e2, e1)
else if (is.duration(e2) && is.POSIXt(e1))
add_duration_to_date(e1, e2)
else {
base::'+'(e1, e2)
}
}

just want to verify with you that this isn't a bad idea before I close this issue. (If a user then loads a package that uses a similar approach, '+' will be rewritten and '+.duration' and '+' will give incompatible methods. But on the otherhand, '+.duration' is compatible with everything until this happens.

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Try +.duration <- +.POSIXt <- .difftime and then let me know what doesn't work.

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1. if we leave the code as it is but replace +.duration <- + <- with +.duration <- +.POSIXt <- .difftime, adding two difftimes yields:

Error: evaluation nested too deeply: infinite recursion / options(expressions=)?

The same is true for adding two POSIXt dates (this should produce an error, but a different one)

1. If we expand the code to:

   "+.duration" <- "+.POSIXt" <- "+.difftime" <- function(e1, e2){
      ...
      else if (is.difftime(e1) && is.difftime(e2))
         base::Ops.difftime(e1, e2)
      else {
         base::'+'(e1, e2)
      }
   }  
   

   adding two difftimes yields

Error in switch(.Generic, < = , > = , == = , != = , <= = , >= = TRUE, :
object '.Generic' not found

from lubridate.

The following code solves all these issues except for the minor issue noted below:

"+.duration" <- "+.POSIXt" <- "+.difftime" <- function(e1, e2){
if(!is.POSIXt(e1) && is.duration(e2)) e1 <- as.duration(e1)
if(!is.POSIXt(e2) && is.duration(e1)) e2 <- as.duration(e2)
if (is.duration(e1) && is.duration(e2))
add_duration_to_duration(e1, e2)
else if (is.duration(e1) && is.POSIXt(e2))
add_duration_to_date(e2, e1)
else if (is.duration(e2) && is.POSIXt(e1))
add_duration_to_date(e1, e2)
else if (is.POSIXt(e1) && is.POSIXt(e2))
stop("binary '+' is not defined for "POSIXt" objects")
else if (is.POSIXt(e1) || is.POSIXt(e2))
base::'+.POSIXt'(e1,e2)
else if (is.difftime(e1) && is.difftime(e2))
make_difftime(as.duration(e1) + as.duration(e2))
else if (is.difftime(e1)){
e2 <- structure(e2, units = units(e1), class = "difftime")
make_difftime(as.duration(e1) + e2)
}
else if (is.difftime(e2)){
e1 <- structure(e1, units = units(e2), class = "difftime")
make_difftime(as.duration(e2) + e1)
}
else
base::'+'(e1, e2)

}

make_difftime <- function (dur) {
if (dur$months != 0)
stop("difftime does not support non-uniform durations (months)")
if (dur$seconds < 60)
units <- "secs"
else if (dur$seconds < 3600)
units <- "mins"
else if (dur$seconds < 86400)
units <- "hours"
else units <- "days"

switch(units, secs = structure(dur$seconds, units = "secs", class = "difftime"), 
    mins = structure(dur$seconds/60, units = "mins", class = "difftime"), 
    hours = structure(dur$seconds/3600, units = "hours", class = "difftime"), 
    days = structure(dur$seconds/86400, units = "days", class = "difftime"), 
    weeks = structure(dur$seconds/(7 * 86400), units = "weeks", class = "difftime"))

}

Minor issue: I don't know why, but R seems to disobey the order rule when operating with difftimes, soif k is a difftime object,

k + k + 1 would be read as k + (k + 1)

The code above would read it as (k + k) + 1

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To self: tomorrow update the subtraction code in a similar way

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It's a little weird that the precedence is different, but I don't think there's anything we can do about it.

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I've rewrote the subtraction code as well. The same precedence difference exists, but our addition and subtraction is consistent (and traditional) in this matter.

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