Escaping expression in macro-generated-macro in Julia

user_407879 0
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I want to write a macro in Julia that generates a number of macros

@register_attribute foo

should generate the macros

@set_foo
@get_foo

which in turn should be defined as 

@set_foo x 5 == set_attr!(x, :foo, 5)

I've managed to get the first part of this down fine, however @macroexpand shows that the inner variable x is not escaped properly, resulting in UndefVarErrors if I call the method with a local variable.

@macroexpand @set_foo x 5 =
    :(Main.set_attr!(Main.x, (Symbol)("foo"), 5))

whereas what I want is 

@macroexpand @set_foo x 5 =
    :(set_attr!(x, (Symbol)("foo"), 5))

My code is

macro register_attribute(name)
    setn = Symbol("set_", string(name))
    getn = Symbol("get_", string(name))
    arg = gensym()
    arg2 = gensym()
    nsym = string(name)
    return Expr(:block, 
    esc(:(
        macro $(setn)($arg, $arg2)
            :(set_attr!($$arg, $$Symbol($$nsym), $$arg2)) 
        end)
        ),
    esc(:(
        macro $(getn)($arg, $arg2)
            :(get_attr!($$arg, $$Symbol($$nsym), $$arg2)) 
        end)
        ))
    e2
end
julia macros metaprogramming
1 Answers

Here is how I would approach this whole problem:

# for purely generative tasks like this, where the generated code is
# (presumably?) destined to be evalled at top level, I prefer using @eval
for attr in [:foo, :bar]
    get_attr_name = Symbol("get_", attr)
    set_attr_name = Symbol("set_", attr)
    quoted_attr   = QuoteNode(attr)

    @eval begin
        $get_attr_name(x)      = getproperty( x, $quoted_attr)
        $set_attr_name(x, val) = setproperty!(x, $quoted_attr, val)
    end
end

# should generate code like:
#   get_foo(x) = getproperty(x, :foo)

julia> nt = (foo=1, bar=2);

julia> get_foo(nt)
1

julia> get_bar(nt)
2

# Let's test in local scope too
julia> let nt2 = (foo=42,)
           get_foo(nt2)
       end
42
user_12256070
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