以字符串作为键和任何值的字典

Question

I am trying to define a dictionary that takes strings as keys and any values. Thus, I try to use Dict{String, <:Any} as type. However, the return value to that expression is

> Dict{String,#s27} where #s27

Moreover, if I try to define a dictionary of that type I get an error:

• For Dict{String,<:Any}() I get ERROR: MethodError: no method matching Dict{String,#s28} where #s28()
• For Dict{String,<:Any}("aa"=>42) I get ERROR: MethodError: no method matching Dict{String,#s29} where #s29(::Pair{String,Int64})

I also tried using Dict{String} (which should be equivalent), with similar results.

What am I missing about types of dictionaries here?

Answer 1

What you are looking for is a Dict{String, Any} , not Dict{String, <:Any} . The first one is a concrete type, namely a dict that takes strings as keys and anything as values. The second, Dict{String, <:Any} is not actually a concrete type, but a unionall type. That means it is an infinite set of types . And the error you are seeing is that you can't instantiate this set of types. You can only instantiate a concrete (leaf) type.

Another way of writing Dict{String, <:Any} is Dict{String, T} where T <: Any , and that makes it a little clearer what it is. It is the set of all types of Dict that has String as they key type and a type that is a subtype of Any as their value type.

So for example we can say that Dict{String, Int} is a subtype of the infinite set Dict{String, <:Any} .

Edit: One use of unionall types is to be able to restrict the kind of type you take to fine grained level. For example, a counting function may look like this:

function count_stuff(stuff, counter::Dict{T, <:Integer}) where T
     # stuff here¨
end

The second argument here is a Dict that has some subtype of Integer as a value type and any type as a key type. That's basically what you'd need to use the dict as a counter.