Add rand for sampling from prior of Models (#381)

A simple+intuitively-named way to generate a prior sample as a way to initialize a sampler.

Co-authored-by: Hong Ge <[email protected]>
Co-authored-by: David Widmann <[email protected]>

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.17.7"
+version = "0.17.8"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"

src/model.jl

@@ -517,6 +517,27 @@ Get the name of the `model` as `Symbol`.
 """
 Base.nameof(model::Model) = model.name
 
+"""
+    rand([rng=Random.GLOBAL_RNG], [T=NamedTuple], model::Model)
+
+Generate a sample of type `T` from the prior distribution of the `model`.
+"""
+function Base.rand(rng::Random.AbstractRNG, ::Type{T}, model::Model) where {T}
+    x = last(
+        evaluate!!(
+            model,
+            SimpleVarInfo{Float64}(),
+            SamplingContext(rng, SampleFromPrior(), DefaultContext()),
+        ),
+    )
+    return DynamicPPL.values_as(x, T)
+end
+
+# Default RNG and type
+Base.rand(rng::Random.AbstractRNG, model::Model) = rand(rng, NamedTuple, model)
+Base.rand(::Type{T}, model::Model) where {T} = rand(Random.GLOBAL_RNG, T, model)
+Base.rand(model::Model) = rand(Random.GLOBAL_RNG, NamedTuple, model)
+
 """
     logjoint(model::Model, varinfo::AbstractVarInfo)
 

test/model.jl

@@ -81,4 +81,26 @@
         call_retval = model()
         @test !any(map(x -> x isa DynamicPPL.AbstractVarInfo, call_retval))
     end
+
+    @testset "rand" begin
+        model = gdemo_default
+
+        Random.seed!(1776)
+        s, m = model()
+        sample_namedtuple = (; s=s, m=m)
+        sample_dict = Dict(:s => s, :m => m)
+
+        # With explicit RNG
+        @test rand(Random.seed!(1776), model) == sample_namedtuple
+        @test rand(Random.seed!(1776), NamedTuple, model) == sample_namedtuple
+        @test rand(Random.seed!(1776), Dict, model) == sample_dict
+
+        # Without explicit RNG
+        Random.seed!(1776)
+        @test rand(model) == sample_namedtuple
+        Random.seed!(1776)
+        @test rand(NamedTuple, model) == sample_namedtuple
+        Random.seed!(1776)
+        @test rand(Dict, model) == sample_dict
+    end
 end