start missing for Nothing

[Nosferican]

julia> setdiff!([1, 2, nothing], nothing)
ERROR: MethodError: no method matching start(::Nothing)
Closest candidates are:
  start(::Core.SimpleVector) at essentials.jl:533
  start(::Base.MethodList) at reflection.jl:684
  start(::ExponentialBackOff) at error.jl:170
  ...
Stacktrace:
 [1] mapfoldl(::Function, ::Function, ::Set{Union{Nothing, Int64}}, ::Nothing) at ./reduce.jl:59
 [2] foldl at ./reduce.jl:90 [inlined]
 [3] setdiff! at ./abstractset.jl:149 [inlined]
 [4] _shrink! at ./array.jl:2338 [inlined]
 [5] setdiff!(::Array{Union{Nothing, Int64},1}, ::Nothing) at ./array.jl:2343
 [6] top-level scope

Needs an implementation of:

start(::Nothing)

and a test.

[KristofferC]

help?> setdiff
search: setdiff setdiff! selectdim setrounding searchsortedfirst

  setdiff(s, itrs...)

  Construct the set of elements in s but not in any of the iterables in itrs.

nothing is not an iterable so why do you expect this to work?

[StefanKarpinski]

Probably because numbers are iterable so it works if you replace the second nothing with 2.

[JeffBezanson]

nothing should not be iterable.

[Nosferican]

It seems to be a bit arbitrary,

setdiff([1,2,3], 1)
setdiff(['a','b','c'], 'c')

work, but

setdiff(["a","b","c"], "b")
setdiff(["a","b",missing], missing)

doesn't. I thought the String example would work after the efforts to make "a" == "a" return true (and it works when enclosed in a Vector). What is the rule for making Number, Char, and String (even if it doesn't work as intended) iterables, but Nothing and Missing not? I thought these where valid values to have in collections to represent values (at least missing). If missing and nothing are treated differently for some issues I guess people can fall back to,

setdiff(["a","b",missing], [missing])
setdiff(["a","b",nothing], [nothing])
setdiff(["a","b","c"], ["c"])

which actually work are intended.

[StefanKarpinski]

#7903; long story short, don't do setdiff([1,2,3], 1) either—it happens to work because of the iterability of numbers but you really shouldn't rely on that if you can avoid it.