Use a macro inside a function's definition

New to Julia
, ,
1

Hello,

I have a function called f′ and I want to use a macro called @diff inside the function’s definition:

macro diff(expression)
    return :(2x)
end

f′(x) = @diff x^2

print(f′(10))

Yet once I run this code, this error message is shown:

ERROR: LoadError: UndefVarError: x not defined
Stacktrace:
 [1] f′(x::Int64)
   @ Main ~/Code/julia/script.jl:5
 [2] top-level scope
   @ ~/Code/julia/script.jl:7
in expression starting at /Users/jeroen/Code/julia/script.jl:7

And this is the code that the macro produces (using @macroexpand @diff x^2):

2 * Main.x

I think the problem is that Julia binds the variable x to the main scope, instead of the function scope.

How can I overcome this problem and use macros inside function definitions?

Thank you!

2

The macro definition is missing an esc call to indicate that you want to make the macro unhygienic and potentially corrupt the caller’s namespace:

macro diff(expression)
   esc(:(2x))
end

With that, the rest of your code works.

2 Likes
3

@johnmyleswhite thank you, this was stumping me.