Adds @returned_quantities macro

docs/src/api.md

@@ -14,10 +14,11 @@ These statements are rewritten by `@model` as calls of [internal functions](@ref
 @model
 ```
 
-One can nest models and call another model inside the model function with [`@submodel`](@ref).
+One can nest models and call another model inside the model function with [`@submodel`](@ref) and [`@returned_quantities`](@ref).
 
 ```@docs
 @submodel
+@returned_quantities
 ```
 
 ### Type

src/DynamicPPL.jl

@@ -125,6 +125,7 @@ export AbstractVarInfo,
     # Convenience macros
     @addlogprob!,
     @submodel,
+    @returned_quantities,
     value_iterator_from_chain,
     check_model,
     check_model_and_trace,

src/submodel_macro.jl

@@ -248,3 +248,185 @@
         end
     end
 end
+
+"""
+    @returned_quantities [prefix=...] model
+
+Run `model` nested inside of another model and return the return-values of the `model`.
+
+Valid expressions for `prefix=...` are:
+- `prefix=false`: no prefix is used. This is the default.
+- `prefix=expression`: results in the prefix `Symbol(expression)`.
+
+Prefixing makes it possible to run the same model multiple times while keeping track of
+all random variables correctly, i.e. without name clashes.
+
+# Examples
+
+## Simple example
+```jldoctest submodel-returned-quantities; setup=:(using Distributions)
+julia> @model function demo1(x)
+           x ~ Normal()
+           return 1 + abs(x)
+       end;
+
+julia> @model function demo2(x, y)
+            a = @returned_quantities(demo1(x))
+            return y ~ Uniform(0, a)
+       end;
+```
+
+When we sample from the model `demo2(missing, 0.4)` random variable `x` will be sampled:
+```jldoctest submodel-returned-quantities
+julia> vi = VarInfo(demo2(missing, 0.4));
+
+julia> @varname(x) in keys(vi)
+true
+```
+
+Variable `a` is not tracked since it can be computed from the random variable `x` that was
+tracked when running `demo1`:
+```jldoctest submodel-returned-quantities
+julia> @varname(a) in keys(vi)
+false
+```
+
+We can check that the log joint probability of the model accumulated in `vi` is correct:
+
+```jldoctest submodel-returned-quantities
+julia> x = vi[@varname(x)];
+
+julia> getlogp(vi) ≈ logpdf(Normal(), x) + logpdf(Uniform(0, 1 + abs(x)), 0.4)
+true
+```
+
+## With prefixing
+```jldoctest submodel-returned-quantities-prefix; setup=:(using Distributions)
+julia> @model function demo1(x)
+           x ~ Normal()
+           return 1 + abs(x)
+       end;
+
+julia> @model function demo2(x, y, z)
+            a = @returned_quantities prefix="sub1" demo1(x)
+            b = @returned_quantities prefix="sub2" demo1(y)
+            return z ~ Uniform(-a, b)
+       end;
+```
+
+When we sample from the model `demo2(missing, missing, 0.4)` random variables `sub1.x` and
+`sub2.x` will be sampled:
+```jldoctest submodel-returned-quantities-prefix
+julia> vi = VarInfo(demo2(missing, missing, 0.4));
+
+julia> @varname(var"sub1.x") in keys(vi)
+true
+
+julia> @varname(var"sub2.x") in keys(vi)
+true
+```
+
+Variables `a` and `b` are not tracked since they can be computed from the random variables `sub1.x` and
+`sub2.x` that were tracked when running `demo1`:
+```jldoctest submodel-returned-quantities-prefix
+julia> @varname(a) in keys(vi)
+false
+
+julia> @varname(b) in keys(vi)
+false
+```
+
+We can check that the log joint probability of the model accumulated in `vi` is correct:
+
+```jldoctest submodel-returned-quantities-prefix
+julia> sub1_x = vi[@varname(var"sub1.x")];
+
+julia> sub2_x = vi[@varname(var"sub2.x")];
+
+julia> logprior = logpdf(Normal(), sub1_x) + logpdf(Normal(), sub2_x);
+
+julia> loglikelihood = logpdf(Uniform(-1 - abs(sub1_x), 1 + abs(sub2_x)), 0.4);
+
+julia> getlogp(vi) ≈ logprior + loglikelihood
+true
+```
+
+## Different ways of setting the prefix
+```jldoctest submodel-returned-quantities-prefix-alts; setup=:(using DynamicPPL, Distributions)
+julia> @model inner() = x ~ Normal()
+inner (generic function with 2 methods)
+
+julia> # When `prefix` is unspecified, no prefix is used.
+       @model submodel_noprefix() = a = @returned_quantities inner()
+submodel_noprefix (generic function with 2 methods)
+
+julia> @varname(x) in keys(VarInfo(submodel_noprefix()))
+true
+
+julia> # Explicitely don't use any prefix.
+       @model submodel_prefix_false() = a = @returned_quantities prefix=false inner()
+submodel_prefix_false (generic function with 2 methods)
+
+julia> @varname(x) in keys(VarInfo(submodel_prefix_false()))
+true
+
+julia> # Using a static string.
+       @model submodel_prefix_string() = a = @returned_quantities prefix="my prefix" inner()
+submodel_prefix_string (generic function with 2 methods)
+
+julia> @varname(var"my prefix.x") in keys(VarInfo(submodel_prefix_string()))
+true
+
+julia> # Using string interpolation.
+       @model submodel_prefix_interpolation() = a = @returned_quantities prefix="\$(nameof(inner()))" inner()
+submodel_prefix_interpolation (generic function with 2 methods)
+
+julia> @varname(var"inner.x") in keys(VarInfo(submodel_prefix_interpolation()))
+true
+
+julia> # Or using some arbitrary expression.
+       @model submodel_prefix_expr() = a = @returned_quantities prefix=1 + 2 inner()
+submodel_prefix_expr (generic function with 2 methods)
+
+julia> @varname(var"3.x") in keys(VarInfo(submodel_prefix_expr()))
+true
+```
+"""
+macro returned_quantities(expr)
+    return returned_quantities_expr(:(prefix = false), expr)
+end
+
+macro returned_quantities(prefix_expr, expr)
+    return returned_quantities_expr(prefix_expr, expr)
+end
+
+"""
+    @returned_quantities_expr model
+
+Returns an expression that captures the return-values of a model in addition to the varinfo.
+"""
+function returned_quantities_expr(prefix_expr, expr, ctx=esc(:__context__))
+    prefix_left, prefix = getargs_assignment(prefix_expr)
+    if prefix_left !== :prefix
+        error("$(prefix_left) is not a valid kwarg")
+    end
+
+    # The user expects `@submodel ...` to return the
+    # return-value of the `...`, hence we need to capture
+    # the return-value and handle it correctly.
+    @gensym retval
+
+    # Prefix.
+    if prefix !== nothing
+        ctx = prefix_submodel_context(prefix, ctx)
+    end
+    return quote
+        # Evaluate the model and capture the return values + varinfo.
+        $retval, $(esc(:__varinfo__)) = $(_evaluate!!)(
+            $(esc(expr)), $(esc(:__varinfo__)), $(ctx)
+        )
+
+        # Return the return-value of the model.
+        $retval
+    end
+end