Possible Improvements to FixedContext

src/compiler.jl

@@ -240,6 +240,17 @@ function unwrap_right_left_vns(
     left::AbstractArray,
     vn::VarName,
 )
+    # Need to check that we don't end up double-counting log-probabilities.
+    combined_axes = Broadcast.combine_axes(left, right)
+    if prod(length, combined_axes) > length(left)
+        throw(
+            ArgumentError(
+                "a `.~` statement cannot result in a broadcasted expression with more elements than the left-hand side",
+            ),
+        )
+    end
+
+    # Extract the sub-varnames.
     vns = map(CartesianIndices(left)) do i
         return Accessors.IndexLens(Tuple(i)) ∘ vn
     end

src/context_implementations.jl

@@ -331,6 +331,72 @@
     )
 end
 
+# `FixedContext`
+function dot_tilde_assume(context::FixedContext, right, left, vns, vi)
+    if !has_fixed_symbol(context, first(vns))
+        # Defer to `childcontext`.
+        return dot_tilde_assume(childcontext(context), right, left, vns, vi)
+    end
+
+    # If we're reached here, then we didn't hit the initial `getfixed` call in the model body.
+    # We _might_ also have some of the variables fixed, but not all.
+    logp = 0
+    # TODO(torfjelde): Add a check to see if the `Symbol` of `vns` exists in `FixedContext`.
+    # If the `Symbol` is not present, we can just skip this check completely. Such a check can
+    # then be compiled away in cases where the `Symbol` is not present.
+    left_bc = Broadcast.broadcastable(left)
+    right_bc = Broadcast.broadcastable(right)
+    for I_left in Iterators.product(Broadcast.broadcast_axes(left_bc)...)
+        for I_right in Iterators.product(Broadcast.broadcast_axes(right_bc)...)
+            vn = vns[I_left...]
+            if hasfixed(context, vn)
+                left[I_left...] = getfixed(context, vn)
+            else
+                # Defer to `tilde_assume`.
+                left[I_left...], logp_inner, vi = tilde_assume(
+                    childcontext(context), right_bc[I_right...], vn, vi
+                )
+                logp += logp_inner
+            end
+        end
+    end
+
+    return left, logp, vi
+end
+
+function dot_tilde_assume(
+    rng::Random.AbstractRNG, context::FixedContext, sampler, right, left, vns, vi
+)
+    if !has_fixed_symbol(context, first(vns))
+        # Defer to `childcontext`.
+        return dot_tilde_assume(rng, childcontext(context), sampler, right, left, vns, vi)
+    end
+    # If we're reached here, then we didn't hit the initial `getfixed` call in the model body.
+    # So we need to check each of the vns.
+    logp = 0
+    # TODO(torfjelde): Add a check to see if the `Symbol` of `vns` exists in `FixedContext`.
+    # If the `Symbol` is not present, we can just skip this check completely. Such a check can
+    # then be compiled away in cases where the `Symbol` is not present.
+    left_bc = Broadcast.broadcastable(left)
+    right_bc = Broadcast.broadcastable(right)
+    for I_left in Iterators.product(Broadcast.broadcast_axes(left_bc)...)
+        for I_right in Iterators.product(Broadcast.broadcast_axes(right_bc)...)
+            vn = vns[I_left...]
+            if hasfixed(context, vn)
+                left[I_left...] = getfixed(context, vn)
+            else
+                # Defer to `tilde_assume`.
+                left[I_left...], logp_inner, vi = tilde_assume(
+                    rng, childcontext(context), sampler, right_bc[I_right...], vn, vi
+                )
+                logp += logp_inner
+            end
+        end
+    end
+
+    return left, logp, vi
+end
+
 """
     dot_tilde_assume!!(context, right, left, vn, vi)
 

src/contexts.jl

@@ -529,6 +529,13 @@ NodeTrait(::FixedContext) = IsParent()
 childcontext(context::FixedContext) = context.context
 setchildcontext(parent::FixedContext, child) = FixedContext(parent.values, child)
 
+has_fixed_symbol(context::FixedContext, vn::VarName) = has_symbol(context.values, vn)
+
+has_symbol(d::AbstractDict, vn::VarName) = haskey(d, vn)
+@generated function has_symbol(::NamedTuple{names}, ::VarName{sym}) where {names,sym}
+    return sym in names
+end
+
 """
     hasfixed(context::AbstractContext, vn::VarName)
 

test/compiler.jl

@@ -720,4 +720,13 @@ module Issue537 end
         res = model()
         @test res == (a=1, b=1, c=2, d=2, t=DynamicPPL.TypeWrap{Int}())
     end
+
+    @testset "invalid .~ expressions" begin
+        @model function demo_with_invalid_dot_tilde()
+            m = Matrix{Float64}(undef, 1, 2)
+            return m .~ [Normal(); Normal()]
+        end
+
+        @test_throws ArgumentError demo_with_invalid_dot_tilde()()
+    end
 end