Can't create a struct with a macro from a string of types

General Usage
1

Hello. I am trying to create a struct through a macro with types that will be know at runtime. It would work something like this: the user would pass a string “a::A b::B” and the macro would create a struct:

struct MyStruct
a::A
b::B
end

The problem is I don’t know how to break the types from the string to create the struct in the macro. Any help would be appreciated.

2

I’d suggest avoiding strings entirely. There’s no need to manually construct or parse strings for metaprogramming in Julia, since we have much better tools for working directly with expressions.

For example, you could do something like this:

julia> macro make_struct(fields...)
         quote
           struct MyStruct
             $(esc.(fields)...)
           end
         end
       end
@make_struct (macro with 2 methods)

julia> @make_struct(x::Int, y::Float64)

We can use @macroexpand to verify the macro’s behavior:

julia> @macroexpand @make_struct(x::Int, y::Float64)
quote
    #= REPL[7]:3 =#
    struct MyStruct
        #= REPL[7]:4 =#
        x::Int
        y::Float64
    end
end

The @make_struct macro takes the field definitions as expressions, so there’s no need to try to split up the string or extract the types because the Julia parser has already done that for you.

2 Likes
3

Right, but the problem is, the types will come from a TOML file, and so will live as a string inside the Julia program.

4

Ok, then I’d suggest:

  • Use Meta.parse to turn a string into an expression
  • Don’t use a macro at all–just write a function that calls eval, something like:
julia> function make_struct(fields)
           eval(quote
             struct MyStruct
               $(fields...)
             end
           end)
       end

where fields is a collection of Exprs from Meta.parse

4 Likes
5

Alright, I will try that. Thanks!