Defining Julia types using macros - macros

Question

Let's say I want to define a series of structs that will be used as parametric types for some other struct later on. For instance, I would like to have something like
abstract type Letter end
struct A <: Letter end
struct B <: Letter end
...etc...
The idea I've had is to define a macro which takes a string of the name of the struct I want to create and defines it as well as some basic methods. I would then execute the macro in a loop for all names and get all my structs defined at compile time. Something like this:
const LETTERS = ["A","B","C","D"]
abstract type Letter end
macro _define_type(ex)
lines = Vector{Expr}()
push!(lines, Meta.parse("export $ex"))
push!(lines, Meta.parse("struct $ex <: Letter end"))
push!(lines, Meta.parse("string(::Type{$ex}) = \"$ex\""))
return Expr(:block,lines...)
end
#_define_type "A"
for ex in LETTERS
#_define_type ex
end
The first way of executing the macro (with a single string) works and does what I want. However, when I execute the macro in a loop it does not. It tells me some variables are declared both as local and global variables.
Can someone explain what is happening? I believe it may be solved by a proper use of esc, but I can't figure out how.
Thanks!
Edit: Thank you all for the amazing responses! Got my code running!

Answer 1

I think this is what you are trying to do:
module MyModule
abstract type Letter end
const LETTERS = ["A", "B", "C", "D"]
for letter in LETTERS
sym = Symbol(letter)
#eval begin
export $sym
struct $sym <: Letter end
Base.string(::Type{$sym}) = $letter
end
end
end
using .MyModule
string(A) # "A"
See the Code Generation section for more details:
https://docs.julialang.org/en/v1/manual/metaprogramming/#Code-Generation

Answer 2

Okay, the problem here is that in Julia for loops introduce a separate, local scope, while struct definitions need to be in the global scope. So your macro fails because it creates struct definitions in the local scope of the for loop.
A way to get around this is to use #eval, to ensure your struct definitions are evaluated in the global scope. In that case, you don't need to create a macro for that, just have a simple loop like this:
abstract type Letter end
const LETTERS = [:A, :B, :C, :D, :E]
for ex in LETTERS
#eval struct $ex <: Letters end
end
You can even put that loop in a function and it will still work. The defined structs can have fields, as #eval covers the entire code block that follows it.
Note that LETTERS must contain symbols rather than strings for this to work correctly. It's easy enough to convert a vector of strings into a vector of symbols using Symbol.(vector_of_strings).

Answer 3

While there are other ways of achieving what you want to do I believe the core issue is understanding the nature of macros. From the docs (emphasis mine):
Macros are necessary because they execute when code is parsed, therefore, macros allow the programmer to generate and include fragments of customized code before the full program is run.
So the macro in your loop does not "see" the values "A", "B", "C" and "D", it sees the expression: ex. To demonstrate this try using #macroexpand:
julia> #macroexpand #_define_type ex
quote
export ex
struct ex <: Main.Letter
#= none:1 =#
end
var"#11#string"(::Main.Type{Main.ex}) = begin
#= none:1 =#
"ex"
end
end
As you can see the actual value of the variable ex does not matter. With this in mind let's look at the actual error you get. You can reproduce it like this:
julia> for ex in ["A"]
struct ex <: Letter
end
end
ERROR: syntax: variable "ex" declared both local and global
Stacktrace:
[1] top-level scope
# REPL[52]:1
You can probably see that this is not what you want, but why this specific error? The reason is that structs are implicitly global while the loop variable is local.
Here is a possible solution that uses a macro that takes a variable number of arguments instead (I also switched to providing the expression directly instead of as a string):
abstract type Letter end
macro _define_types(exprs...)
blocks = map(exprs) do ex
name = string(ex)
quote
export $ex
struct $ex <: Letter end
Base.string(::Type{$ex}) = $name
end
end
Expr(:block, blocks...)
end
#_define_types A
#_define_types B C D