Add rewrites for measurable negation and subtraction

aeppl/opt.py

@@ -9,7 +9,7 @@
 from aesara.graph.op import compute_test_value
 from aesara.graph.opt import local_optimizer
 from aesara.graph.optdb import EquilibriumDB, OptimizationQuery, SequenceDB
-from aesara.scalar import TrueDiv
+from aesara.scalar import Neg, Sub, TrueDiv
 from aesara.tensor.basic_opt import (
     ShapeFeature,
     register_canonicalize,
@@ -318,6 +318,34 @@ def measurable_div_to_reciprocal_product(fgraph, node):
         return [at.mul(numerator, at.reciprocal(denominator))]
 
 
+@local_optimizer([Elemwise])
+def measurable_neg_to_product(fgraph, node):
+    """Convert negation of `MeasurableVariable`s to product with `-1`."""
+    if isinstance(node.op.scalar_op, Neg):
+        inp = node.inputs[0]
+        if not (inp.owner and isinstance(inp.owner.op, MeasurableVariable)):
+            return None
+
+        return [at.mul(inp, -1.0)]
+
+
+@local_optimizer([Elemwise])
+def measurable_sub_to_neg(fgraph, node):
+    """Convert subtraction involving `MeasurableVariable`s to addition with neg"""
+    if isinstance(node.op.scalar_op, Sub):
+        minuend, subtrahend = node.inputs
+
+        if not (
+            (minuend.owner and isinstance(minuend.owner.op, MeasurableVariable))
+            or (
+                subtrahend.owner and isinstance(subtrahend.owner.op, MeasurableVariable)
+            )
+        ):
+            return None
+
+        return [at.add(minuend, at.neg(subtrahend))]
+
+
 logprob_rewrites_db = SequenceDB()
 logprob_rewrites_db.name = "logprob_rewrites_db"
 logprob_rewrites_db.register(
@@ -357,6 +385,14 @@ def measurable_div_to_reciprocal_product(fgraph, node):
     "basic",
 )
 
+measurable_ir_rewrites_db.register(
+    "measurable_neg_to_product", measurable_neg_to_product, -5, "basic"
+)
+
+measurable_ir_rewrites_db.register(
+    "measurable_sub_to_neg", measurable_sub_to_neg, -5, "basic"
+)
+
 logprob_rewrites_db.register(
     "post-canonicalize", optdb.query("+canonicalize"), 10, "basic"
 )

tests/test_transforms.py

@@ -589,17 +589,20 @@ def test_log_transform_rv():
 
 
 @pytest.mark.parametrize(
-    "rv_size, loc_type",
+    "rv_size, loc_type, addition",
     [
-        (None, at.scalar),
-        (2, at.vector),
-        ((2, 1), at.col),
+        (None, at.scalar, True),
+        (2, at.vector, False),
+        ((2, 1), at.col, True),
     ],
 )
-def test_loc_transform_rv(rv_size, loc_type):
+def test_loc_transform_rv(rv_size, loc_type, addition):
 
     loc = loc_type("loc")
-    y_rv = loc + at.random.normal(0, 1, size=rv_size, name="base_rv")
+    if addition:
+        y_rv = loc + at.random.normal(0, 1, size=rv_size, name="base_rv")
+    else:
+        y_rv = at.random.normal(0, 1, size=rv_size, name="base_rv") - at.neg(loc)
     y_rv.name = "y"
     y_vv = y_rv.clone()
 
@@ -725,3 +728,24 @@ def test_inverse_rv_transform(numerator):
         x_logp_fn(x_test_val),
         sp.stats.invgamma(shape, scale=scale * numerator).logpdf(x_test_val),
     )
+
+
+def test_negated_rv_transform():
+    x_rv = -at.random.halfnormal()
+    x_rv.name = "x"
+
+    x_vv = x_rv.clone()
+    x_logp_fn = aesara.function([x_vv], joint_logprob({x_rv: x_vv}))
+
+    assert np.isclose(x_logp_fn(-1.5), sp.stats.halfnorm.logpdf(1.5))
+
+
+def test_subtracted_rv_transform():
+    # Choose base RV that is assymetric around zero
+    x_rv = 5.0 - at.random.normal(1.0)
+    x_rv.name = "x"
+
+    x_vv = x_rv.clone()
+    x_logp_fn = aesara.function([x_vv], joint_logprob({x_rv: x_vv}))
+
+    assert np.isclose(x_logp_fn(7.3), sp.stats.norm.logpdf(5.0 - 7.3, 1.0))