Introduction of SamplingContext: keeping it simple (#259)

This is #253 but the only motivation here is to get `SamplingContext` in, nothing relating to interactions with other contexts, etc.

Co-authored-by: Hong Ge <hg344@cam.ac.uk>

Author: torfjelde

src/DynamicPPL.jl

@@ -75,6 +75,7 @@ export AbstractVarInfo,
     SampleFromPrior,
     SampleFromUniform,
     # Contexts
+    SamplingContext,
     DefaultContext,
     LikelihoodContext,
     PriorContext,

src/compiler.jl

@@ -286,11 +286,7 @@ function generate_tilde(left, right)
     if !(left isa Symbol || left isa Expr)
         return quote
             $(DynamicPPL.tilde_observe!)(
-                __context__,
-                __sampler__,
-                $(DynamicPPL.check_tilde_rhs)($right),
-                $left,
-                __varinfo__,
+                __context__, $(DynamicPPL.check_tilde_rhs)($right), $left, __varinfo__
             )
         end
     end
@@ -304,9 +300,7 @@ function generate_tilde(left, right)
         $isassumption = $(DynamicPPL.isassumption(left))
         if $isassumption
             $left = $(DynamicPPL.tilde_assume!)(
-                __rng__,
                 __context__,
-                __sampler__,
                 $(DynamicPPL.unwrap_right_vn)(
                     $(DynamicPPL.check_tilde_rhs)($right), $vn
                 )...,
@@ -316,7 +310,6 @@ function generate_tilde(left, right)
         else
             $(DynamicPPL.tilde_observe!)(
                 __context__,
-                __sampler__,
                 $(DynamicPPL.check_tilde_rhs)($right),
                 $left,
                 $vn,
@@ -337,11 +330,7 @@ function generate_dot_tilde(left, right)
     if !(left isa Symbol || left isa Expr)
         return quote
             $(DynamicPPL.dot_tilde_observe!)(
-                __context__,
-                __sampler__,
-                $(DynamicPPL.check_tilde_rhs)($right),
-                $left,
-                __varinfo__,
+                __context__, $(DynamicPPL.check_tilde_rhs)($right), $left, __varinfo__
             )
         end
     end
@@ -355,9 +344,7 @@ function generate_dot_tilde(left, right)
         $isassumption = $(DynamicPPL.isassumption(left))
         if $isassumption
             $left .= $(DynamicPPL.dot_tilde_assume!)(
-                __rng__,
                 __context__,
-                __sampler__,
                 $(DynamicPPL.unwrap_right_left_vns)(
                     $(DynamicPPL.check_tilde_rhs)($right), $left, $vn
                 )...,
@@ -367,7 +354,6 @@ function generate_dot_tilde(left, right)
         else
             $(DynamicPPL.dot_tilde_observe!)(
                 __context__,
-                __sampler__,
                 $(DynamicPPL.check_tilde_rhs)($right),
                 $left,
                 $vn,
@@ -398,10 +384,8 @@ function build_output(modelinfo, linenumbernode)
     # Add the internal arguments to the user-specified arguments (positional + keywords).
     evaluatordef[:args] = vcat(
         [
-            :(__rng__::$(Random.AbstractRNG)),
             :(__model__::$(DynamicPPL.Model)),
             :(__varinfo__::$(DynamicPPL.AbstractVarInfo)),
-            :(__sampler__::$(DynamicPPL.AbstractSampler)),
             :(__context__::$(DynamicPPL.AbstractContext)),
         ],
         modelinfo[:allargs_exprs],
@@ -449,8 +433,12 @@ end
 
 """
     matchingvalue(sampler, vi, value)
+    matchingvalue(context::AbstractContext, vi, value)
+
+Convert the `value` to the correct type for the `sampler` or `context` and the `vi` object.
 
-Convert the `value` to the correct type for the `sampler` and the `vi` object.
+For a `context` that is _not_ a `SamplingContext`, we fall back to
+`matchingvalue(SampleFromPrior(), vi, value)`.
 """
 function matchingvalue(sampler, vi, value)
     T = typeof(value)
@@ -465,7 +453,16 @@ function matchingvalue(sampler, vi, value)
         return value
     end
 end
-matchingvalue(sampler, vi, value::FloatOrArrayType) = get_matching_type(sampler, vi, value)
+function matchingvalue(sampler::AbstractSampler, vi, value::FloatOrArrayType)
+    return get_matching_type(sampler, vi, value)
+end
+
+function matchingvalue(context::AbstractContext, vi, value)
+    return matchingvalue(SampleFromPrior(), vi, value)
+end
+function matchingvalue(context::SamplingContext, vi, value)
+    return matchingvalue(context.sampler, vi, value)
+end
 
 """
     get_matching_type(spl::AbstractSampler, vi, ::Type{T}) where {T}