Involution DSL appears unable to read ":continuous" and ":discrete" annotation through variables

[fsaad]

julia> @transform f (model_in, aux_in) to (model_out, aux_out) begin
           z = :continuous
           @write(model_out[:x], 1, z)
       end

ERROR: LoadError: MethodError: no method matching var"@write"(::LineNumberNode, ::Module, ::Expr, ::Int64, ::Symbol)
Closest candidates are:
  var"@write"(::LineNumberNode, ::Module, ::Any, ::Any, ::QuoteNode) at ~/.julia/dev/Gen/src/inference/trace_translators.jl:202

[ztangent]

Seems like the issue is that we're hardcoding how the :continuous annotation gets parsed by the macro:

Gen.jl/src/inference/trace_translators.jl

Lines 182 to 194 in b959223

	"""
	@read(<source>, <annotation>)
	Macro for reading the value of a random choice from an input trace in the [Trace Transform DSL](@ref).
	<source> is of the form <trace>[<addr>] where <trace> is an input trace, and <annotation>
	is either :discrete or :continuous.
	"""
	macro read(src, ann::QuoteNode)
	return quote read($(esc(bij_state)), $(esc(src)), $(esc(typed(ann.value)))) end
	end
	"""

Gen.jl/src/inference/trace_translators.jl

Lines 169 to 180 in b959223

	const DISCRETE = [:discrete, :disc]
	const CONTINUOUS = [:continuous, :cont]
	function typed(annotation::Symbol)
	if annotation in DISCRETE
	return DiscreteAnn()
	elseif annotation in CONTINUOUS
	return ContinuousAnn()
	else
	error("error")
	end
	end

The fixed definition would be something like:

macro read(src, ann)
    _typed = GlobalRef(Gen, :typed)
    return quote read($(esc(bij_state)), $(esc(src)), $(_typed)($(esc(ann.value)))) end
end

The downside is that it would lead to a somewhat slower dynamic call to the typed function at run-time, instead of handling that logic at compile time. I'm not sure how much slower, but perhaps it's worth not making the switch and just clearly documenting that the user has to directly specify whether they want something to be :continuous or :discrete.