Consider the following:

>> library(Rmpfr)

>> seq(mpfr(1,10),mpfr(5,10),by=1)
5 'mpfr' numbers of precision  10   bits 
[1] 1 2 3 4 5
>> seq(1,mpfr(5,10),by=1)
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

[[4]]
[1] 4

[[5]]
[1] 5

>> seq(mpfr(1,10),5,by=1)
Error in max(abs(to), abs(from)) : invalid 'type' (list) of argument
>> methods(seq)
[1] seq.Date    seq.default seq.POSIXt  seq.units* 
see '?methods' for accessing help and source code

We see there is no method for mpfr objects, yet with a little coaxing, seq() will provide the correct result. But the class of the result depends on just what gets coerced when.

Anyone know what’s going on inside seq ? Am I just getting “lucky” that this works at all