Editing Functional programming (section)

== 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>{{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). Whereas functions have a strict input-output pattern of arguments, relations can be queried with any pattern of inputs and outputs. Consider the following logic program:
<syntaxhighlight lang="prolog">
mother(charles, elizabeth).
mother(harry, diana).
</syntaxhighlight>
The program can be queried, like a functional program, to generate mothers from children:
<syntaxhighlight lang="prolog">
?- mother(harry, X).
X = diana.
?- mother(charles, X).
X = elizabeth.
</syntaxhighlight>
But it can also be queried ''backwards'', to generate children:
<syntaxhighlight lang="prolog">
?- mother(X, elizabeth).
X = charles.
?- mother(X, diana).
X = harry.
</syntaxhighlight>
It can even be used to generate all instances of the mother relation:
<syntaxhighlight lang="prolog">
?- mother(X, Y).
X = charles,
Y = elizabeth.
X = harry,
Y = diana.
</syntaxhighlight>
Compared with relational syntax, functional syntax is a more compact notation for nested functions. For example, the definition of maternal grandmother in functional syntax 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 form:
<syntaxhighlight lang="prolog">
maternal_grandmother(X, Y) :- mother(X, Z), mother(Z, Y).
</syntaxhighlight>
Here <code>:-</code> means ''if'' and <code> , </code>means ''and''.

However, the difference between the two representations is simply syntactic. In [[Ciao (programming language)|Ciao]] Prolog, relations can be nested, like functions in functional programming:<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>
<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>
Ciao transforms the function-like notation into relational form and executes the resulting logic program using the standard Prolog execution strategy.