Can we have `String`s/`Cmd`s as type parameter?

General Usage
1

I am working on Julia 1.3 and find we can have Numbers, Symbols and Chars, etc., as type parameters, but they cannot be Strings, Cmds, etc.

julia> struct A{T}
       end

julia> A{333}()
A{333}()

julia> A{"fd"}()
ERROR: TypeError: in Type, in parameter, expected Type, got String
Stacktrace:
 [1] top-level scope at REPL[5]:1

julia> A{:fd}()
A{:fd}()

julia> A{1.0}()
A{1.0}()

julia> x = complex(1,2)
1 + 2im

julia> A{x}()
A{1 + 2im}()

julia> A{`run`}()
ERROR: TypeError: in Type, in parameter, expected Type, got Cmd
Stacktrace:
 [1] top-level scope at REPL[16]:1

What is the reason behind that? What other type of values can we use for type parameters?

2

I found in this post and the docs of isbitstype and Val that only values that are isbits can be used, plus Symbols. See the first post for more details.

3

Duplicate of Arguments for Value types