Adding Type to (working) Union{...} bricks it

skleinbo · June 2, 2022, 6:25am

Julia treats the possible absence of a value (nothing) as a special case. Calling convert on the abstract type Union{Nothing, T} basically strips the Nothing. From the source:

github.com

JuliaLang/julia/blob/5ffdc5c52bc4eb5dfba83e5f389f14902db824d0/base/some.jl#L36


      
              return Union{R, Nothing}
          end
          
          function nonnothingtype_checked(T::Type)
              R = nonnothingtype(T)
              R >: T && error("could not compute non-nothing type")
              return R
          end
          
          convert(::Type{T}, x::T) where {T>:Nothing} = x
          convert(::Type{T}, x) where {T>:Nothing} = convert(nonnothingtype_checked(T), x)
          convert(::Type{Nothing}, x) = throw(MethodError(convert, (Nothing, x)))
          convert(::Type{Nothing}, ::Nothing) = nothing
          convert(::Type{Some{T}}, x::Some{T}) where {T} = x
          convert(::Type{Some{T}}, x::Some) where {T} = Some{T}(convert(T, x.value))
          
          function show(io::IO, x::Some)
              if get(io, :typeinfo, Any) == typeof(x)
                  show(io, x.value)
              else
                  print(io, "Some(")

For MyStruct1 we have T = Dict{Union{UInt8, String}, Number} for which a conversion is possible. In the case of MyStruct2, T = Union{Dict{Union{UInt8, String}, Number}, String} for which no conversion exists. In particular because it is an abstract type.

So in short: Union{Nothing, ConcreteType} is handled conveniently.

Topic		Replies	Views
Dict Types Behavior Internals & Design	4	535	November 2, 2018
Mixing Union Types and Dictionaries New to Julia question	1	664	April 12, 2020
Dict subtype definition General Usage type , dictionary	5	973	January 31, 2022
Construct object of type whose type is UnionAll General Usage question	5	503	April 14, 2018
Compatibility of Dict{String, Any} with Dict{String, String} General Usage dictionary	3	1683	March 20, 2018

Adding Type to (working) Union{...} bricks it

Related topics