More precision on missing with Dicts

[udohjeremiah]

The statement made here about the in operator with Dicts is too strong and it has edge cases where it fails:

Some collections follow a slightly different definition. For example, Sets check whether the item isequal to one of the elements. Dicts look for key=>value pairs, and the key is compared using isequal. To test for the presence of a key in a dictionary, use haskey or k in keys(dict). For these collections, the result is always a Bool and never missing.

On Sets the above is utterly true, but the statement fails when the value in key=>value in Dict is missing:

julia> X = Dict(1=>10, 2=>20, 3=>30, missing=>NaN);

julia> (missing=>10) in X
false

julia> (missing=>NaN) ∈ X
false

julia> (1=>missing) ∈ X
missing

julia> (missing=>missing) ∈ X
missing

As the above shows, for Dicts, a Bool is not always returned. So its best to state this out.

Moreover, since only the key is compared using isequal for Dicts. key should be “key (not the value)” so emphasis is seen as being placed on key only.