diff --git a/SciLean/Core/Distribution/VarInference.lean b/SciLean/Core/Distribution/VarInference.lean
new file mode 100644
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+++ b/SciLean/Core/Distribution/VarInference.lean
@@ -0,0 +1,72 @@
+import SciLean.Core.Rand.Rand
+import SciLean.Core.Rand.Distributions.Normal
+import SciLean.Core.Distribution.Basic
+import SciLean.Core.Distribution.ParametricDistribDeriv
+namespace SciLean.Rand
+
+
+variable {R} [RealScalar R]
+
+
+def model : Rand (R×R) := do
+  let v ← normal R (0:R) (5:R)
+  if v > 0 then
+    let obs ← normal R 1 1 -- 1 1
+    return (v,obs)
+  else
+    let obs ← normal R (-2) 1
+    return (v,obs)
+
+def prior : Rand R := normal R 0 5
+
+def likelihood (v : R) : Rand R :=
+  if v > 0 then
+    normal R 1 1
+  else
+    normal R (-2) 1
+
+open Classical in
+
+noncomputable
+def Rand.condition [Inhabited X₂] (rx : Rand X) (mk : X₁ → X₂ → X) (x₁ : X₁) : Rand X₂ :=
+  if h : ∃ rx₂ : X₁ → Rand X₂, ∀ (rx₁ : Rand X₁), (do let x₁ ← rx₁; let x₂ ← rx₂ x₁; return mk x₁ x₂) = rx then
+    choose h x₁
+  else
+    return default
+
+
+variable [Inhabited X]
+noncomputable
+def posterior (prior : Rand X) (likelihood : X → Rand Y) (obs : Y) : Rand X := do
+  let joint := do
+    let x ← prior
+    let y ← likelihood x
+    return (x,y)
+
+  joint.condition (fun y x => (x,y)) obs
+
+
+def guide (θ : R) : Rand R := normal R θ 1
+
+open MeasureTheory
+variable {X} [MeasurableSpace X]
+
+noncomputable
+def KLDiv (P Q : Rand X) : R := P.E (fun x => Scalar.log (P.pdf R Q.ℙ x))
+
+
+
+noncomputable
+def loss (θ : R) := KLDiv (R:=R) (guide θ) (posterior prior likelihood (0 : R))
+
+variable
+  {W} [Vec R W]
+  [Vec R X]
+
+
+theorem KLDiv.arg_P.cderiv_rule (P : W → Rand X) (Q : Rand X) :
+    cderiv R (fun w => KLDiv (R:=R) (P w) Q)
+    =
+    fun w dw =>
+      let dP := parDistribDeriv (fun w => (P w).ℙ.toDistribution (R:=R)) w dw
+      dP.extAction' (fun x => Scalar.log ((P w).pdf R Q.ℙ x) - 1) := sorry_proof