An introduction to logic programming through Prolog

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Logical programming is a programming paradigm which has its foundations in mathematical logic. In contrast to languages like Java or C, programs written in a logic programming languages such as Prolog are not composed of sequences of instructions but of a set of axioms, or rules which define relationships between objects.

Logic Programming and Prolog

They also follow a declarative rather than an imperative approach. However, what does that mean? Imagine you walk into your favourite coffee place and that you like to order some coffee. So rather than providing a step by step instruction to archive x or y imperative , you tell the system what you need and let it try to come up with a solution declarative. Prolog is based on Horn clauses a subset of first-order logic and it is probably the most famous language in the logic programming family.

Its first version appeared, like Smalltalk and C, in It also significantly influenced the development of Erlang programming language. One thing in particular which appealed to me is its ridiculous simple execution model. Prolog has four building block, logical or , logical and , term rewriting and unification.

By combining these four blocks, we can perform any computation we care about. Prolog, like SQL, has two main aspects, one to express the data and another to query it. The basic constructs of logic programming, terms and statements, are inherited from logic. There are three basic statements:. Facts and rules are stored in a knowledge base, which the Prolog compiler transpiles into a form which is more efficient to query against.

Prolog Versus You

Prolog has a straightforward syntax. You will pick up the few rules very quickly.

Let us transform the Socrates example from earlier into an actual Prolog program and analyse what is happing inside the compiler. Rules are a key concept of the language and allow us to make general statements about objects and their relationships. If you have read those three lines of code thoughtfully, you may have observed the case-sensitivity. In contrast to most other languages capitalisation matters in Prolog.

Quintus Prolog Homepage

Strings starting with a lowercase character are immutable and called atoms , you can compare them with the symbol type in Ruby. Strings starting with an uppercase character or an underscore are variables and can change their value. In our example socrates is an atom and the uppercase X a variable.

Also, note that the full stop after each clause is mandatory.

Prolog Tutorial

Prolog will, of course, respond false , because we do not have Plato defined in our knowledge base. What about the next question:. Have you noticed the tremendous difference between imperative and declarative style? In contrast to the imperative way of programming, we have not provided any instructions to the program how exactly the variable should be defined. We have just asked the engine a question, and it automatically bound a value to our variable X!

This process of matching variables with items is called unification and is precisely where logic programming has its strengths.

Tutorial Material

The assignment statement is fundamental to most imperative programming languages. Hence we are using the term unification : the process of being united or made into a whole.

We can find applications of unification also in imperative languages where it is typically used to enable type inference. Hence both arguments in the first example are the same atom Prolog returns true and false in the second case. Hint: Most constructs in Prolog use prefix syntax by default. Well, that is an easy one. Both terms are variables, of course, they unify with each other depending on your Prolog implementation you get true back. When Prolog resolves queries to come up with a conclusion, it tries to unify terms. Therefore a possible definition could be:.

Two terms unify if they are the same term or if they contain variables that can be uniformly instantiated with terms in such a way that the resulting terms are equal. Hitherto we have only considered basic terms such as atoms, variables, integers and floating point numbers as arguments for our programs. However, in Prolog, like in other logic programming languages, the most common data-structure is the list. Lists in Prolog are a special case of terms. The syntax is identical to Python, they start and end with square brackets, and a comma separates each list item. Download or read it online for free here: Download link 1. The solutions could have been constructed during the contest under time pressure, and so you will find many solutions using the generate and test strategy.

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Applications of Prolog by Attila Csenki - BookBoon In this volume the author discusses some areas where Prolog can be fruitfully employed. There are 54 exercises in this book. Prolog Programming by Roman Bartak Prolog is a programming language with precise operational meaning that borrows its basic concepts from logic programming.