Union!() seems to be type-unstable, is this by design?

New to Julia
1

I’m not new to Julia, but am new to using Sets. The following MWE puzzles me (the autosearch for related topics reference some 2017 queries which aren’t relevant, AFAICS):

julia> x = Set((1,2)); y = Set(('a', 'b')); union!(y,x)
ERROR: TypeError: 2 is not a valid key for type Char
Stacktrace:
 [1] setindex!(h::Dict{Char, Nothing}, v0::Nothing, key0::Int64)
   @ Base ./dict.jl:348
 [2] push!(s::Set{Char}, x::Int64)
   @ Base ./set.jl:137
 [3] union!(s::Set{Char}, itr::Set{Int64})
   @ Base ./abstractset.jl:106
 [4] top-level scope
   @ REPL[272]:1

julia> x = Set{Any}((1,2)); y = Set{Any}(('a', 'b')); union!(y,x)
Set{Any} with 4 elements:
  2
  'a'
  'b'
  1

Should I have expected this?

2

What in particular is confusing for you?

3

When Julia choosed type, the types are a mismatch (I didn’t realise that Set elements have all to be of the same type). But had I known, I would have expected promotion, even up to Any.

4

union!(y,x) means to update y to include the elements of x. But this only works if the set type of y can actually contain those elements (and you cannot change the type of an existing object). Maybe you wanted to use union (without a !):

julia> x = Set((1,2)); y = Set(('a', 'b')); union(y,x)
Set{Any} with 4 elements:
  2
  'a'
  'b'
  1
5

Thanks, I didn’t quite catch the type issues from reading the docs. There is an art to reading as well as writing them …