yabetancourt / HigherOrderPredicatesInProlog

Higher order predicates are a very useful tool in Prolog for building generic and reusable predicates

Higher Order Predicates in Prolog

By Yadier Betancourt

Introduction

Higher order predicates are a very useful tool in Prolog for building generic and reusable predicates. They allow encapsulating common goals and applying them in a parametric way over lists. In this work, their application is studied for frequent operations on lists, which allows simplifying and improving the resulting Prolog code.

Higher Order Predicates in Prolog

Concept and Advantages

A higher order predicate is a predicate that takes another predicate as an argument. They allow treating predicates as data, passing them as arguments.

The advantages are:

• Code reuse: generic parametrizable predicates can be defined.
• Abstraction: common tasks are encapsulated in higher order predicates.
• Readability: frequent use predicates are simplified.
• Modularity: generic parts are separated from specific ones.

Typical Applications

Some typical uses of higher order predicates in Prolog:

• Operations on data structures (lists, trees, etc)
• Search with parametrizable strategies.
• Generic traversals of data structures.
• Data filtering and transformation.

Higher Order Predicates on Lists

map/2 for property checking

map/2 predicate is defined to apply a single argument predicate to each element of a list and verify if all elements satisfy a given condition.

See prolog/map.pl

Examples of use:

• Check if all elements of a list are natural numbers
• Check if all elements of a list are variables
• Check if all words in a list are capitalized

See examples/map_examples.pl

map/3 for predicate application

map/3 predicate is defined to apply a two argument predicate to each pair of elements from two lists at the same position and store the results in a third list.

See prolog/map.pl

Examples of use:

• Square each element of a list
• Translate numbers to text

See examples/map_examples.pl

filter/3 and exclude/3 for filtering

filter/3 and exclude/3 predicates are defined for filtering operations. filter/3 takes a predicate and a list, and returns a list with elements that satisfy the predicate. exclude/3 takes a predicate and a list, and returns a list excluding elements that satisfy the predicate.

See prolog/filter.pl

Examples of use:

• Get even and odd numbers from a list

See examples/filter_examples.pl

reduce/4

reduce/4 predicate is defined to apply a three argument predicate cumulatively to the elements of a list, starting with an initial accumulator value.

See prolog/reduce.pl

Examples of use:

• Sum and product of a list of numbers

See examples/reduce_examples.pl

Other Application Examples

Additional examples:

• printlist/1: Print elements of a list
• allpairs/1: Check if all elements of a list are even
• translateRests/2: Get remainder of dividing list elements by 10 in Spanish
• opposites/2: Get opposite of each number in a list
• separateMultiplesN/4: Separate multiples and non-multiples of N in a list

See examples/other_examples.pl