Mergewith(dicts...)

General Usage
1

Why does this discard the first dict instead of merging? This signature is undocumented.

julia> mergewith(Dict(1=>10), Dict(2=>20))
Dict{Int64, Int64} with 1 entry:
  2 => 20
2

If the signature is undocumented this is undefined behaviour and you should not rely on it.

3

eh, you just want


julia> merge(Dict(1=>10), Dict(2=>20))
Dict{Int64, Int64} with 2 entries:
  2 => 20
  1 => 10

merge with takes an “combine” function

1 Like
4

It should give an error in this case, no?

5

maybe, but we probably have 1000:
“this works because our functions are general enough but users probably don’t want this behavior”

this particular case is just because

function mergewith!(combine, d::AbstractDict, others::AbstractDict...)
    foldl(mergewith!(combine), others; init = d)
end

we just returned init=d I guess

and the fact that this is un-typed

mergewith!(combine) = (args...) -> mergewith!(combine, args...)

maybe type this with:

julia> Base.Callable
Union{Function, Type}

?

6

Yes, that seems like it should be typed with a Callable check, to avoid this type of issue. This “working” without error is an unfortunate consequence of the fact that the combine “operator” Dict(1 => 10) is never actually used, since according to mergewith! (and its called functions) there’s only an initial value here, Dict(2=>20), so nothing to reduce using the operator. So passing anything at all as the first argument will work, as long as there’s only one other argument to the function.

julia> mergewith(Main, Dict(2=>20))
Dict{Int64, Int64} with 1 entry:
  2 => 20

julia> mergewith(Ď€, Dict(2=>20))
Dict{Int64, Int64} with 1 entry:
  2 => 20

julia> mergewith("this is free real estate", Dict(2=>20))
Dict{Int64, Int64} with 1 entry:
  2 => 20

I’m not sure if the non-specialization penalty of Function applies to Callable too. If it does, an explicit check of combine isa Callable seems worth doing to avoid this quite unintuitive behaviour.

7

Jeff says

Callable should not be used anymore.

8

exactly, so nothing can be done here, even if you error on combine::AbstractDict, there are so many other types that is “probably wrong”