Fix esc bug with kernel auxiliary arguments in kernel DSL.

[femtomc]

No description provided.

[femtomc]

One other comment: postwalk was actually walking into tr ~ k(tr, args...) twice, applying Gen.reversal two times.

Would like to add tests for the DSL somehow.

[ztangent]

I think this is mostly making the right fixes, but we have to be careful about how escaping the final Expr affects name resolution for names and references we're injecting into the generated code.

For example, if you escape a block of code which contains a line like Gen.reversal(...), Gen.reversal may not always end up pointing to the right function, depending on what Gen refers to in the context where the macro is called. To avoid this, we should either avoid escaping lines like that, or if easier, replace Gen.reversal with $(GlobalRef(Gen, :reversal)).

[ztangent]

I'm confused why the escs are being removed here. I think they should stay...? What's the result of calling @macroexpand on the version without the escs?

[ztangent]

Replace Gen.check_is_kernel with GlobalRef.

[femtomc]

Also, here's a reference expansion:

quote
    function myk(tr::Trace; var"##check#282" = false, var"##observations#283" = EmptyChoiceMap())
        var"##check#282" && Gen.check_is_kernel(mh)
        tr = (mh(tr, select(:a), var"##check#282" = var"##check#282", var"##observations#283" = var"##observations#283"))[1]
        var"##check#282" && Gen.check_observations(get_choices(tr), var"##observations#283")
        metadata = nothing
        (tr, metadata)
    end
    Gen.check_is_kernel(::typeof(myk)) = true
    function var"##reverse_kernel#284"(tr::Trace; var"##check#282" = false, var"##observations#283" = EmptyChoiceMap())
        var"##check#282" && Gen.check_is_kernel(Gen.reversal(mh))
        tr = ((Gen.reversal(mh))(tr, select(:a), var"##check#282" = var"##check#282", var"##observations#283" = var"##observations#283"))[1]
        var"##check#282" && Gen.check_observations(get_choices(tr), var"##observations#283")
        metadata = nothing
        (tr, metadata)
    end
    Gen.check_is_kernel(::typeof(var"##reverse_kernel#284")) = true
    Gen.reversal(::typeof(myk)) = var"##reverse_kernel#284"
    Gen.reversal(::typeof(var"##reverse_kernel#284")) = myk
end

[femtomc]

Ah, @ztangent -- we can't gensym the keyword arguments, because we assume check and observations throughout the Gen inference library (for MCMC kernels) :(

[femtomc]

Okay, I removed outermost esc -- now only escaping kernel + reversal names, and macro expansion takes care of gensyming the argument names.

[ztangent]

Looks great to me! Thanks for doing this :)