Merge pull request #20 from TuringLang/phg/model_abstraction

Add function stubs as sketched in interface description

Author: yebai

Project.toml

@@ -7,9 +7,7 @@ version = "0.1.4"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
-StatsBase = "2913bbd2-ae8a-5f71-8c99-4fb6c76f3a91"
 
 [compat]
 AbstractMCMC = "2, 3"
-StatsBase = "0.33.4"
 julia = "1"

src/AbstractPPL.jl

@@ -15,7 +15,10 @@ export VarName,
 
 
 # Abstract model functions
-export AbstractProbabilisticProgram
+export AbstractProbabilisticProgram,
+    condition,
+    decondition,
+    logdensity
 
 
 # Abstract traces

src/abstractprobprog.jl

@@ -7,3 +7,53 @@ using AbstractMCMC
 Common base type for models expressed as probabilistic programs.
 """
 abstract type AbstractProbabilisticProgram <: AbstractMCMC.AbstractModel end
+
+
+"""
+    logdensity(model, trace)
+
+Evaluate the (possibly unnormalized) density of the model specified by the probabilistic program
+in `model`, at specific values for the random variables given through `trace`.
+
+`trace` can be of any supported internal trace type, or a fixed probability expression.
+
+`logdensity` should interact with conditioning and deconditioning in the way required by probability
+theory.
+"""
+function logdensity end
+
+
+"""
+    decondition(conditioned_model)
+
+Remove the conditioning (i.e., observation data) from `conditioned_model`, turning it into a
+generative model over prior and observed variables.
+
+The invariant 
+
+```
+m == condition(decondition(m), obs)
+```
+
+should hold for models `m` with conditioned variables `obs`.
+"""
+function decondition end
+
+
+"""
+    condition(model, observations)
+
+Condition the generative model `model` on some observed data, creating a new model of the (possibly
+unnormalized) posterior distribution over them.
+
+`observations` can be of any supported internal trace type, or a fixed probability expression.
+
+The invariant 
+
+```
+m = decondition(condition(m, obs))
+```
+
+should hold for generative models `m` and arbitrary `obs`.
+"""
+function condition end