From f618024134b10764471113e5626716b406fe5ef9 Mon Sep 17 00:00:00 2001
From: Penelope Yong <penelopeysm@gmail.com>
Date: Thu, 22 May 2025 11:39:18 +0100
Subject: [PATCH 1/3] Fix getparams on empty varinfo

---
 HISTORY.md             |  4 ++++
 Project.toml           |  2 +-
 src/mcmc/Inference.jl  |  3 +++
 test/mcmc/Inference.jl | 24 ++++++++++++++++++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/HISTORY.md b/HISTORY.md
index 42415d7efc..3e9b784608 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,3 +1,7 @@
+# Release 0.38.2
+
+`getparams(::Model, ::AbstractVarInfo)` now returns an empty `Float64` if the VarInfo contains no parameters.
+
 # Release 0.38.1
 
 The method `Bijectors.bijector(::DynamicPPL.Model)` was moved to DynamicPPL.jl.
diff --git a/Project.toml b/Project.toml
index acffce201e..522d352471 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "Turing"
 uuid = "fce5fe82-541a-59a6-adf8-730c64b5f9a0"
-version = "0.38.1"
+version = "0.38.2"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
diff --git a/src/mcmc/Inference.jl b/src/mcmc/Inference.jl
index 0cbb45b48f..cdc9e2570c 100644
--- a/src/mcmc/Inference.jl
+++ b/src/mcmc/Inference.jl
@@ -358,6 +358,9 @@ function getparams(model::DynamicPPL.Model, vi::DynamicPPL.VarInfo)
     # Materialize the iterators and concatenate.
     return mapreduce(collect, vcat, iters)
 end
+function getparams(::DynamicPPL.Model, ::DynamicPPL.VarInfo{NamedTuple{(),Tuple{}}})
+    return float(Real)[]
+end
 
 function _params_to_array(model::DynamicPPL.Model, ts::Vector)
     names_set = OrderedSet{VarName}()
diff --git a/test/mcmc/Inference.jl b/test/mcmc/Inference.jl
index 5966589090..f994a36af3 100644
--- a/test/mcmc/Inference.jl
+++ b/test/mcmc/Inference.jl
@@ -631,6 +631,30 @@ using Turing
             StableRNG(seed), demo_incorrect_missing([missing]), NUTS(), 10; check_model=true
         )
     end
+
+    @testset "getparams" begin
+        @model function e(x=1.0)
+            return x ~ Normal()
+        end
+        evi = Turing.VarInfo(e())
+        @test isempty(Turing.Inference.getparams(e(), evi))
+
+        @model function f()
+            return x ~ Normal()
+        end
+        fvi = Turing.VarInfo(f())
+        @test only(Turing.Inference.getparams(f(), fvi)) == (@varname(x), fvi[@varname(x)])
+
+        @model function g()
+            x ~ Normal()
+            return y ~ Poisson()
+        end
+        gvi = Turing.VarInfo(g())
+        gparams = Turing.Inference.getparams(g(), gvi)
+        @test gparams[1] == (@varname(x), gvi[@varname(x)])
+        @test gparams[2] == (@varname(y), gvi[@varname(y)])
+        @test length(gparams) == 2
+    end
 end
 
 end

From 83a0ebf3750a5290c61475f5f2b6e7fc61fd0934 Mon Sep 17 00:00:00 2001
From: Penelope Yong <penelopeysm@gmail.com>
Date: Thu, 22 May 2025 11:57:46 +0100
Subject: [PATCH 2/3] Fix two methods in MH for empty tuples

---
 src/mcmc/mh.jl | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/mcmc/mh.jl b/src/mcmc/mh.jl
index 6a03f5359f..3450cfdc74 100644
--- a/src/mcmc/mh.jl
+++ b/src/mcmc/mh.jl
@@ -246,6 +246,7 @@ end
     ]
     return expr
 end
+_val_tuple(::VarInfo, ::Tuple{}) = ()
 
 @generated function _dist_tuple(
     props::NamedTuple{propnames}, vi::VarInfo, vns::NamedTuple{names}
@@ -267,6 +268,7 @@ end
     ]
     return expr
 end
+_dist_tuple(::@NamedTuple{}, ::VarInfo, ::Tuple{}) = ()
 
 # Utility functions to link
 should_link(varinfo, sampler, proposal) = false

From 632fef61993f7fca1218b713ccf5747e54e8b7aa Mon Sep 17 00:00:00 2001
From: Penelope Yong <penelopeysm@gmail.com>
Date: Thu, 22 May 2025 11:57:59 +0100
Subject: [PATCH 3/3] Add a test to sample an empty model

---
 test/mcmc/Inference.jl | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/test/mcmc/Inference.jl b/test/mcmc/Inference.jl
index f994a36af3..55d989b154 100644
--- a/test/mcmc/Inference.jl
+++ b/test/mcmc/Inference.jl
@@ -655,6 +655,15 @@ using Turing
         @test gparams[2] == (@varname(y), gvi[@varname(y)])
         @test length(gparams) == 2
     end
+
+    @testset "empty model" begin
+        @model function e(x=1.0)
+            return x ~ Normal()
+        end
+        # Can't test with HMC/NUTS because some AD backends error; see 
+        # https://github.com/JuliaDiff/DifferentiationInterface.jl/issues/802
+        @test sample(e(), IS(), 100) isa MCMCChains.Chains
+    end
 end
 
 end