Use typed VarInfo for Prior sampler

Author: penelopeysm

src/mcmc/prior.jl

@@ -26,5 +26,36 @@ function AbstractMCMC.step(
         ),
     )
     _, vi = DynamicPPL.evaluate!!(sampling_model, vi)
-    return Transition(model, vi, nothing; reevaluate=false), nothing
+    vi = DynamicPPL.typed_varinfo(vi)
+    return Transition(model, vi, nothing; reevaluate=false), vi
+end
+
+function AbstractMCMC.step(
+    rng::Random.AbstractRNG,
+    model::DynamicPPL.Model,
+    sampler::DynamicPPL.Sampler{<:Prior},
+    vi::AbstractVarInfo;
+    kwargs...,
+)
+    # TODO(DPPL0.38/penelopeysm): replace this entire thing with init!!
+    #
+    # `vi` is a VarInfo from the previous step so already has all the
+    # right accumulators and stuff. The only thing we need to change is to make
+    # sure that the old values are overwritten when we resample.
+    #
+    # Note also that the values in the Transition (and hence the chain) are not
+    # obtained from the VarInfo's metadata itself, but are instead obtained
+    # from the ValuesAsInModelAccumulator, which is cleared in the evaluate!!
+    # call. Thus, the actual values in the VarInfo's metadata don't matter:
+    # we only set the del flag here to make sure that new values are sampled
+    # (and thus new values enter VAIMAcc), rather than the old ones being
+    # reused during the evaluation. Yes, SampleFromPrior really sucks.
+    for vn in keys(vi)
+        DynamicPPL.set_flag!(vi, vn, "del")
+    end
+    sampling_model = DynamicPPL.contextualize(
+        model, DynamicPPL.SamplingContext(rng, DynamicPPL.SampleFromPrior(), model.context)
+    )
+    _, vi = DynamicPPL.evaluate!!(sampling_model, vi)
+    return Transition(model, vi, nothing; reevaluate=false), vi
 end