Efficient inference in discrete and continuous domains for PLP - PowerPoint PPT Presentation

Efficient inference in discrete and continuous domains for PLP languages under the Distribution Semantics Elena Bellodi Department of Engineering, University of Ferrara, Italy PLP 2019, September 21st 2019 E. Bellodi 1 / 91

Outline Introduction to PLP 1 Inference under the Distribution Semantics (DS) in Discrete Domains 2 Exact inference Approximate inference Inference under the Distribution Semantics in Hybrid Domains 3 cplint Hybrid Programs (HP) References 4 E. Bellodi 2 / 91

Introduction to PLP Handling real world’s uncertainty Problem: real world is uncertain and complex Logic does not handle well uncertainty Graphical models do not handle well relationships among entities Solution: combine the two Many approaches proposed in the areas of Logic Programming, Machine Learning, Databases, Knowledge Representation E. Bellodi 3 / 91

Introduction to PLP Probabilistic Logic Programming (PLP) Distribution Semantics [Sato ICLP 95] A probabilistic logic program defines a probability distribution over normal logic programs (called instances or possible worlds or simply textitworlds) The distribution is extended to a joint distribution over worlds and interpretations (or queries) The probability of a query is obtained from this distribution E. Bellodi 4 / 91

Introduction to PLP Probabilistic Logic Programming Languages under the Distribution Semantics Probabilistic Logic Programs [Dantsin RCLP91] Probabilistic Horn Abduction [Poole NGC93], Independent Choice Logic (ICL) [Poole AI97] PRISM [Sato ICLP95] Logic Programs with Annotated Disjunctions (LPADs) [Vennekens et al. ICLP04] ProbLog [De Raedt et al. IJCAI07] ... They differ in the way they define the distribution over logic programs E. Bellodi 5 / 91

Introduction to PLP Logic Programs with Annotated Disjunctions (LPADs) sneezing ( X ) : 0 . 7 ∨ null : 0 . 3 ← flu ( X ) . sneezing ( X ) : 0 . 8 ∨ null : 0 . 2 ← hay _ fever ( X ) . flu ( bob ) . hay _ fever ( bob ) . Distributions over the head of rules null does not appear in the body of any rule, and is usually omitted E. Bellodi 6 / 91

Introduction to PLP Worlds for the "sneezing domain" (LPAD syntax) sneezing ( bob ) ← flu ( bob ) . null ← flu ( bob ) . sneezing ( bob ) ← hay _ fever ( bob ) . sneezing ( bob ) ← hay _ fever ( bob ) . flu ( bob ) . flu ( bob ) . hay _ fever ( bob ) . hay _ fever ( bob ) . P ( w 1 ) = 0 . 7 × 0 . 8 P ( w 2 ) = 0 . 3 × 0 . 8 sneezing ( bob ) ← flu ( bob ) . null ← flu ( bob ) . null ← hay _ fever ( bob ) . null ← hay _ fever ( bob ) . flu ( bob ) . flu ( bob ) . hay _ fever ( bob ) . hay _ fever ( bob ) . P ( w 3 ) = 0 . 7 × 0 . 2 P ( w 4 ) = 0 . 3 × 0 . 2 sneezing ( bob ) is true in 3 worlds out of 4 P ( sneezing ( bob )) = 0 . 7 × 0 . 8 + 0 . 3 × 0 . 8 + 0 . 7 × 0 . 2 = 0 . 94 E. Bellodi 7 / 91

Inference under the DS in Discrete Domains Inference under the Distribution Semantics Let At be the set of all ground (probabilistic and derived) atoms in a given LPAD program Assume that we are given a set E ⊂ At of observed atoms ( evidence atoms) a vector e with their observed truth values means that the evidence is E = e We are also given a set Q ⊂ At of atoms of interest ( query atoms) q indicates a single ground atom of interest Different inference tasks are possible E. Bellodi 8 / 91

Inference under the DS in Discrete Domains Inference under the Distribution Semantics Unconditional inference: computing the unconditional probability P ( e ) , the probability of evidence Conditional or marginal inference: computing the conditional probability distribution of every query atom given the evidence, i.e., computing P ( q | E = e ) , for each q ∈ Q Computing the probability distribution or density of the non-ground arguments of a conjunction of literals Q = q E. Bellodi 9 / 91

Inference under the DS in Discrete Domains Inference under the Distribution Semantics Example heads(Coin):1/2; tails(Coin):1/2:- toss(Coin),\+biased(Coin). heads(Coin):0.6; tails(Coin):0.4:- toss(Coin),biased(Coin). fair(Coin):0.9; biased(Coin):0.1. toss(coin). Unconditional inference: P ( heads ( coin )) , or P ( tails ( coin )) Conditional inference: P ( heads ( coin ) | biased ( coin )) E. Bellodi 10 / 91

Inference under the DS in Discrete Domains Inference under the Distribution Semantics The inference problem is #P-hard and for large models is intractable. Possible solutions : 1 Exact inference Knowledge Compilation : compile the probabilistic logic program to an intermediate representation and then compute the probability by Weighted Model Counting Bayesian Network (BN) based : convert the probabilistic logic program into a BN and apply BN inference algorithms (Meert et al. (2010)) Lifted inference (Poole (2003)): exploits symmetries in the model to speed up inference 2 Approximate inference E. Bellodi 11 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation A probabilistic logic program can be compiled into the following intermediate representations: Binary Decision Diagrams (BDD): De Raedt et al. (2007), cplint (Riguzzi (2007),Riguzzi (2009)), PITA (Riguzzi and Swift (2010a)) deterministic-Decomposable Negation Normal Form circuits (d-DNNF) : ProbLog2 , Fierens et al. (2015) Sentential Decision Diagrams (SDD), Darwiche (2011) E. Bellodi 12 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD 1 Assign Boolean random variables (r.v.) to the probabilistic rules 2 Given a query Q , compute its explanations , i.e. assignments to the random variables that are sufficient for entailing the query 3 Let K be the set of all possible explanations 4 Build a Boolean formula f K representing K 5 Build a BDD encoding f K 6 Compute the probability of evidence from the BDD (unconditional inference) E. Bellodi 13 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (2,3,4) Example For the (LPAD) program: sneezing ( X ) : 0 . 7 ← flu ( X ) . sneezing ( X ) : 0 . 8 ← hay _ fever ( X ) . flu ( bob ) . hay _ fever ( bob ) . a set of covering explanations for Q = sneezing ( bob ) is K = { κ 1 , κ 2 } κ 1 = { ( C 1 , { X / bob } , 1 ) } κ 2 = { ( C 2 , { X / bob } , 1 ) } A composite choice κ is an explanation for a query Q if Q is true in every world compatible with κ E. Bellodi 14 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (2,3,4) Example θ 1 = X / bob X C 1 θ 1 → X 11 From κ 1 , X 11 = 1 X C 2 θ 1 → X 21 From κ 2 , X 21 = 1 f K ( X ) = ( X 11 = 1 ) ∨ ( X 21 = 1 ) P ( Q ) = P ( f K ( X )) = P ( X 11 ∨ X 21 )= P ( X 11 )+ P ( X 21 ) − P ( X 11 ) P ( X 21 ) E. Bellodi 15 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD And Now What? Worlds are mutually exclusive, and in theory we could compute P ( Q ) as � = Q P ( w ) , w ∈ W T : w | BUT in practice, it is unfeasible to find all the worlds w where the query is true It’s easier to find explanations for the query BUT explanations might not be mutually exclusive they must be made mutually exclusive in order to sum up probabilities Binary Decision Diagrams (BDD): they split paths on the basis of the values of binary variables, so the branches are mutually disjoint E. Bellodi 16 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (3,4) Building a formula for the explanations In order to use BDD, a multi-valued random variable X ij with n i values must be converted into n i − 1 Boolean variables X ij 1 , ..., X ijn i − 1 X ij = k for k = 1 , ..., n i − 1 is represented by the conjunction X ij 1 ∧ . . . ∧ X ijk − 1 ∧ X ijk X ij = n i by X ij 1 ∧ . . . ∧ X ijn i − 1 E. Bellodi 17 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (3,4) Example For both C 1 and C 2 : n i = 2 (multi-valued) X 11 = 1 → (Boolean) X 111 (multi-valued) X 11 = 2 → (Boolean) X 111 (multi-valued) X 21 = 1 → (Boolean) X 211 (multi-valued) X 21 = 2 → (Boolean) X 211 f ′ K ( X ) = X 111 ∨ X 211 E. Bellodi 18 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (6) Computing the probability from a BDD E. Bellodi 19 / 91

Inference under the DS in Discrete Domains Exact inference Knowledge Compilation to BDD (6) Example P ( n 2 ) = 0 · 0 . 2 + 1 · 0 . 8 = 0 . 8 P ( n 1 ) = P ( root ) = 1 · 0 . 7 + 0 . 3 · 0 . 8 = 0 . 94 = P ( Q ) = P ( sneezing ( bob )) E. Bellodi 20 / 91

Inference under the DS in Discrete Domains Exact inference Tabling-based Inference Idea: maintain in a table both subgoals encountered in a query evaluation and answers to these subgoals If a subgoal is encountered more than once, the evaluation reuses information from the table rather than re-performing resolution against program clauses Tabling can be used to evaluate programs with negation Tabling integrates closely with Prolog: a predicate p/n is evaluated using SLDNF by default; the predicate can be made to use tabling by the directive :- table p/n that is added by the user or compiler E. Bellodi 21 / 91