# What is SLD resolution?

by Stephen M. Walker II, Co-Founder / CEO

## What is SLD resolution?

SLD (Selective Linear Definite) resolution is a refined version of the standard linear definite clause resolution method used in automated theorem proving and logic programming, particularly in Prolog. It combines the benefits of linearity and selectivity to improve efficiency and reduce search space complexity.

In SLD resolution, the goal is to prove that a given query (a logical statement) follows from a set of facts (clauses). The algorithm proceeds by selecting one unifiable atom from the goal and applying resolution with a compatible clause from the knowledge base. This process continues recursively until either the query is proven true or a contradiction is found.

Selectivity in SLD resolution means that only those clauses are considered for resolution that can potentially contribute to proving the selected atom from the current goal. Linearity ensures that at most one atom is unified with each clause during any given iteration of the algorithm, which helps keep the search space manageable.

SLD resolution has proven to be a powerful and efficient method for proving theorems and solving problems in various domains of artificial intelligence. It forms the foundation for many advanced techniques in logic programming, such as backtracking, constraint propagation, and tabulation.

## What is the difference between sld resolution and resolution method?

Resolution and SLD (Selective Linear Definite) resolution are both methods of proving theorems in propositional and first-order logic, but they differ in their approach and specific use cases.

Resolution is a general inference rule used in logic. It is applied to two clauses in a sentence, and by unification, it eliminates a literal that occurs positive in one clause and negative in the other. This method is used to prove the unsatisfiability of a set of formulas, which is motivated by the desire to prove logical consequences of a set of axioms.

SLD resolution, on the other hand, is a refinement of the resolution principle for first-order logic. It is a theorem proving technique for automated deduction, used in automated theorem provers and inference systems. SLD resolution is the basic inference rule used in logic programming and is both sound and refutation complete for Horn clauses. The SLD inference rule is applied given a goal clause, represented as the negation of a problem to be solved, and an input definite clause. In the more general case, the unifying substitution is necessary to make the two literals identical.

The key difference between the two lies in the fact that SLD resolution is a goal-directed form of resolution. It uses a selection function to choose which literal to resolve against, which makes it more efficient for certain types of problems. This selection function is what makes SLD resolution "selective". Furthermore, SLD resolution is linear, meaning that the resolution proof can be restricted to a linear sequence of clauses.

SLD resolution is also used as a control strategy in logic programming languages to resolve issues of nondeterminism. A definite sentence has exactly one positive literal in each clause and this literal is selected to be replaced in the goal clause by the conjunction of negative literals which form the body of the clause.

## How does sld resolution control nondeterminism in logic programming?

SLD resolution, or Selective Linear Definite resolution, is a control strategy used in logic programming to manage nondeterminism. Nondeterminism arises when there are multiple ways to proceed with a computation, and a choice must be made. SLD resolution helps manage this by using a selection function to choose which literal to resolve against, thereby guiding the computation in a specific direction.

SLD resolution implicitly defines a search tree of alternative computations, where different branches represent alternative computations. The initial goal clause is associated with the root of the tree. For every node in the tree and for every definite clause, a new goal clause is constructed by selecting some renamed program clause whose head unifies with a subgoal.

In Prolog, a popular logic programming language, SLD resolution with backtracking is the basic control mechanism. When a computation fails to achieve the goal, Prolog backtracks to a previous computation point where a different choice can be made, and then proceeds from there. This backtracking mechanism allows Prolog to explore different branches of the computation tree, thereby resolving nondeterminism.

In some cases, SLD resolution can be extended to increase the "don't care" nondeterminism of computation rules, which can decrease the size of the search space. This means that the system can make arbitrary choices about which computation to perform next, without affecting the final outcome, thereby potentially simplifying the computation.

## What are the benefits of SLD resolution in AI?

SLD (Selective Linear Definite clause) resolution is a fundamental inference rule used in logic programming, particularly in Prolog, which is sound and refutation complete for Horn clauses. The benefits of SLD resolution include:

**Goal-Directed Reasoning**— SLD resolution is goal-directed, meaning it starts with a query and works backward to find the necessary facts and rules that satisfy the query.**Efficiency**— It only needs to find substitutions of variables, which can be done efficiently.**Soundness and Completeness**— SLD resolution is both sound (it only derives true conclusions from true premises) and complete (it can derive any conclusion that is logically implied by the premises) for Horn clauses.**Simple Implementation**— The implementation of SLD resolution in Prolog engines is relatively straightforward, making it a powerful tool for automatic or semi-automatic verification systems.

## What are the challenges of SLD resolution in AI?

Despite its benefits, SLD resolution faces several challenges:

**Infinite Loops**— SLD resolution is susceptible to infinite loops, which can occur if the resolution process enters a cycle.**Redundant Subcomputations**— There can be redundant subcomputations, which affect the efficiency of the resolution process.**Non-Monotonic Reasoning**— SLD resolution, especially when combined with negation-as-failure (SLDNF), has limitations for non-monotonic reasoning, which is important for many AI applications.**Memory and Time Resources**— Effective construction of extended or abstract programs for SLD resolution can require significant memory and time resources.

## How can SLD resolution in AI be improved?

Improvements to SLD resolution can be made by:

**Parallelism**— Introducing parallelism into the resolution process, such as tabled evaluations or multi-SLD resolution, can help overcome some of the deficiencies by allowing multiple threads to handle different parts of the computation.**Similarity Relations**— Exploiting similarity relations between predicates and constants can help overcome failures in the unification process.**Trusted Theorem Proving**— Integrating SLD resolution into verification systems in a disciplined manner, where the Prolog engine justifies its results with simple natural deduction reasoning, can enhance the credibility of the results.

## What are the future directions for SLD resolution in AI?

Future directions for SLD resolution may include:

**Deep Learning Integration**— Combining SLD resolution with deep learning techniques to learn from successful resolution processes and guide new ones.**Enhanced Search Heuristics**— Developing more sophisticated search heuristics to avoid infinite loops and redundant computations.**Advanced Parallel Computing Models**— Exploring data or parallelism models for logic programming to improve the efficiency of SLD resolution on parallel computing architectures.**Meta-Interpretive Learning**— Investigating meta-interpretive learning, which uses SLD resolution to construct hypotheses consistent with examples, to push the boundaries of inductive logic programming.