Using macros to generate NamedTuples

General Usage
,
1

I am a bit confused by using macros to generate expressions. I would like to have a macro to make NamedTuples as follows:

a = 1
b = 2
@make_nt a b

which should return an expression that evaluates to NamedTuple{(:a, :b)}((a, b)) or (a=1, b=2)

however

macro make_nt_try(args...)
    return :(NamedTuple{$args}($args))
end

returns (a = :a, b = :b)

and

macro make_nt_expr(args...)
    return Expr(:(NamedTuple{$args}), args)
end

raises TypeError: in Expr, expected Symbol, got Expr

and

macro make_nt(args...)
    return :(NamedTuple{$args}(tuple($(args...))))
end

works outside of functions, but not for variables in the local scope of functions.
I have read the metaprogramming section of the documentation, but did not understand when exactly evaluation happens and why it does not happen within the local scope.

@macroexpand @make_nt(a, b)

always gives

:(Main.NamedTuple{(:a, :b)}(Main.tuple(Main.a, Main.b)))

even when it is called inside a function.

Is it possible to make a macro that works properly inside function, or is my mental model for how macros should be used wrong in some way?

2

Looks like like you’re missing an esc. See the macro hygiene section of the documentation.

Does this do what you want?

macro make_nt(args...)
    return :(NamedTuple{$args}(tuple($(esc.(args)...))))
end
2 Likes
3

It does, thank you for the quick reply! I will look deeper in the escaping and macro hygiene parts to understand it.