:= to keep track of generated quantities

src/compiler.jl

@@ -371,9 +371,33 @@ function generate_mainbody!(mod, found, expr::Expr, warn)
         )
     end
 
+    # Modify the assignment operators.
+    args_assign = getargs_coloneq(expr)
+    if args_assign !== nothing
+        L, R = args_assign
+        return Base.remove_linenums!(
+            generate_assign(
+                generate_mainbody!(mod, found, L, warn),
+                generate_mainbody!(mod, found, R, warn),
+            ),
+        )
+    end
+
     return Expr(expr.head, map(x -> generate_mainbody!(mod, found, x, warn), expr.args)...)
 end
 
+function generate_assign(left, right)
+    right_expr = :($(TrackedValue)($right))
+    tilde_expr = generate_tilde(left, right_expr)
+    return quote
+        if $(is_extracting_values)(__context__)
+            $tilde_expr
+        else
+            $left = $right
+        end
+    end
+end
+
 function generate_tilde_literal(left, right)
     # If the LHS is a literal, it is always an observation
     @gensym value

src/utils.jl

@@ -163,6 +163,20 @@ function getargs_assignment(expr::Expr)
     end
 end
 
+"""
+    getargs_coloneq(x)
+
+Return the arguments `L` and `R`, if `x` is an expression of the form `L := R`, or `nothing`
+otherwise.
+"""
+getargs_coloneq(x) = nothing
+function getargs_coloneq(expr::Expr)
+    return MacroTools.@match expr begin
+        (L_ := R_) => (L, R)
+        x_ => nothing
+    end
+end
+
 function to_namedtuple_expr(syms)
     length(syms) == 0 && return :(NamedTuple())
 

src/values_as_in_model.jl

@@ -1,3 +1,11 @@
+struct TrackedValue{T}
+    value::T
+end
+
+is_tracked_value(::TrackedValue) = true
+is_tracked_value(::Any) = false
+
+check_tilde_rhs(x::TrackedValue) = x
 
 """
     ValuesAsInModelContext
@@ -29,6 +37,13 @@ function setchildcontext(context::ValuesAsInModelContext, child)
     return ValuesAsInModelContext(context.values, child)
 end
 
+is_extracting_values(context::ValuesAsInModelContext) = true
+function is_extracting_values(context::AbstractContext)
+    return is_extracting_values(NodeTrait(context), context)
+end
+is_extracting_values(::IsParent, ::AbstractContext) = false
+is_extracting_values(::IsLeaf, ::AbstractContext) = false
+
 function Base.push!(context::ValuesAsInModelContext, vn::VarName, value)
     return setindex!(context.values, copy(value), vn)
 end
@@ -48,7 +63,12 @@ end
 
 # `tilde_asssume`
 function tilde_assume(context::ValuesAsInModelContext, right, vn, vi)
-    value, logp, vi = tilde_assume(childcontext(context), right, vn, vi)
+    if is_tracked_value(right)
+        value = right.value
+        logp = zero(getlogp(vi))
+    else
+        value, logp, vi = tilde_assume(childcontext(context), right, vn, vi)
+    end
     # Save the value.
     push!(context, vn, value)
     # Save the value.
@@ -58,7 +78,12 @@ end
 function tilde_assume(
     rng::Random.AbstractRNG, context::ValuesAsInModelContext, sampler, right, vn, vi
 )
-    value, logp, vi = tilde_assume(rng, childcontext(context), sampler, right, vn, vi)
+    if is_tracked_value(right)
+        value = right.value
+        logp = zero(getlogp(vi))
+    else
+        value, logp, vi = tilde_assume(rng, childcontext(context), sampler, right, vn, vi)
+    end
     # Save the value.
     push!(context, vn, value)
     # Pass on.

test/compiler.jl

@@ -687,6 +687,34 @@ module Issue537 end
         # And one explicit test for logging so know that is working.
         @model demo_with_logging() = @info "hi"
         model = demo_with_logging()
-        @test model() == nothing
+        @test model() === nothing
+        # Make sure that the log message is present.
+        @test_logs (:info, "hi") model()
+    end
+
+    @testset ":= (tracked values)" begin
+        @model function demo_tracked()
+            x ~ Normal()
+            y := 100 + x
+            return (; x, y)
+        end
+        @model function demo_tracked_submodel()
+            @submodel (x, y) = demo_tracked()
+            return (; x, y)
+        end
+        for model in [demo_tracked(), demo_tracked_submodel()]
+            # Make sure it's runnable and `y` is present in the return-value.
+            @test model() isa NamedTuple{(:x, :y)}
+
+            # `VarInfo` should only contain `x`.
+            varinfo = VarInfo(model)
+            @test haskey(varinfo, @varname(x))
+            @test !haskey(varinfo, @varname(y))
+
+            # While `values_as_in_model` should contain both `x` and `y`.
+            values = values_as_in_model(model, deepcopy(varinfo))
+            @test haskey(values, @varname(x))
+            @test haskey(values, @varname(y))
+        end
     end
 end