Fix potential undefined variable in model compiler output (#1110)

Author: penelopeysm

HISTORY.md

@@ -1,5 +1,9 @@
 # DynamicPPL Changelog
 
+## 0.38.7
+
+Made a small tweak to DynamicPPL's compiler output to avoid potential undefined variables when resuming model functions midway through (e.g. with Libtask in Turing's SMC/PG samplers).
+
 ## 0.38.6
 
 Renamed keyword argument `only_ddpl` to `only_dppl` for `Experimental.is_suitable_varinfo`.

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.38.6"
+version = "0.38.7"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"

src/compiler.jl

@@ -461,9 +461,16 @@ function generate_tilde(left, right)
         elseif $isassumption
             $(generate_tilde_assume(left, dist, vn))
         else
-            # If `vn` is not in `argnames`, we need to make sure that the variable is defined.
-            if !$(DynamicPPL.inargnames)($vn, __model__)
-                $left = $(DynamicPPL.getconditioned_nested)(
+            # If `vn` is not in `argnames`, then it's definitely been conditioned on (if
+            # it's not in `argnames` and wasn't conditioned on, then `isassumption` would
+            # be true).
+            $left = if $(DynamicPPL.inargnames)($vn, __model__)
+                # This is a no-op and looks redundant, but defining the compiler output this
+                # way ensures that the variable `$left` is always defined. See
+                # https://github.com/TuringLang/DynamicPPL.jl/pull/1110.
+                $left
+            else
+                $(DynamicPPL.getconditioned_nested)(
                     __model__.context, $(DynamicPPL.prefix)(__model__.context, $vn)
                 )
             end