Editing Logic programming (section)

=== Relationship with functional programming ===
{{See also|Functional programming#Comparison to logic programming}}
Logic programming can be viewed as a generalisation of functional programming, in which functions are a special case of relations.<ref name=dis>{{cite book|author1=Daniel Friedman|author2=William Byrd|author3=Oleg Kiselyov|author4=Jason Hemann|title=The Reasoned Schemer, Second Edition|year=2018|publisher=The MIT Press}}</ref>
For example, the function, mother(X) = Y, (every X has only one mother Y) can be represented by the relation mother(X, Y). In this respect, logic programs are similar to [[relational databases]], which also represent functions as relations.

Compared with relational syntax, functional syntax is more compact for nested functions. For example, in functional syntax the definition of maternal grandmother can be written in the nested form:
<syntaxhighlight lang="prolog">
maternal_grandmother(X) = mother(mother(X)).
</syntaxhighlight>
The same definition in relational notation needs to be written in the unnested, flattened form:
<syntaxhighlight lang="prolog">
maternal_grandmother(X, Y) :- mother(X, Z), mother(Z, Y).
</syntaxhighlight>

However, nested syntax can be regarded as syntactic sugar for unnested syntax. [[Ciao (programming language)|Ciao]] Prolog, for example, transforms functional syntax into relational form and executes the resulting logic program using the standard Prolog execution strategy.<ref>A. Casas, D. Cabeza, M. V. Hermenegildo. A Syntactic Approach to
  Combining Functional Notation, Lazy Evaluation and Higher-Order in
  LP Systems. The 8th International Symposium on Functional and Logic   Programming (FLOPS'06), pages 142-162, April 2006.</ref> Moreover, the same transformation can be used to execute nested relations that are not functional. For example:
  
  <syntaxhighlight lang="prolog">
grandparent(X) := parent(parent(X)).
parent(X) := mother(X).
parent(X) := father(X).

mother(charles) := elizabeth.
father(charles) := phillip.
mother(harry) := diana.
father(harry) := charles.

?- grandparent(X,Y).
X = harry,
Y = elizabeth.
X = harry,
Y = phillip.
</syntaxhighlight>