DataType and Type{T}

New to Julia
1

I don’t understand the following:

julia> String isa Type{String}
true

julia> ("abc", String) isa Tuple{String, Type{String}}   # ?
false

julia> ("abc", String) isa Tuple{String, Type{T} where T}
true

julia> ("abc", String) isa Tuple{String, DataType}
true

The second one is the one I would like. I think 3d and 4th are equivalent. The slightly broader context is a data structure which contains maps of the type :abc => (value, typeof(value)); I can type the dictionary

julia> Dict{Symbol, Tuple{T, DataType} where T}(
           :abc => (2, Int),
           :def => ("hello", String)
       )
Dict{Symbol, Tuple{T, DataType} where T} with 2 entries:
  :abc => (2, Int64)
  :def => ("hello", String)

but not

julia> Dict{Symbol, Tuple{T, Type{T}} where T}(
           :abc => (2, Int),
           :def => ("hello", String)
       )
ERROR: TypeError: in arrayset, expected Tuple{T, Type{T}} where T, got a value of type Tuple{Int64, DataType}

I can work around this / do this differently but I’m curious as to why this doesn’t work; is it the same as DataType with type parameter and the reference to the issue redesign typeof(::Type) · Issue #29368 · JuliaLang/julia · GitHub ?

Thanks

1 Like
2

Yes.

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