Editing Curry (programming language) (section)

===Narrowing===
Narrowing is a mechanism whereby a variable is [[name binding|bound]] to a value selected from among alternatives imposed by constraints. Each possible value is tried in some order, with the remainder of the program invoked in each case to determine the validity of the binding. Narrowing is an extension of logic programming, in that it performs a similar search, but can actually generate values as part of the search rather than just being limited to testing them.

Narrowing is useful because it allows a function to be treated as a relation: its value can be computed "in both directions". The Curry examples of the previous section illustrate this.

As noted in the prior section, narrowing can be thought of as reduction on a program term graph, and there are often many different ways (''strategies'') to reduce a given term graph. Antoy et al.<ref>{{cite journal
 |last1=Sergio |first1=Antoy |last2=Echahed |first2=Rachid |last3=Hanus |first3=Michael
 |title=A Needed Narrowing Strategy
 |journal=Journal of the ACM
 |volume=47
 |issue=4
 |pages=776–822
 |publisher=ACM
 |year=2007
 |issn=0004-5411
 |doi=10.1145/347476.347484
| s2cid=47275506
}}</ref> proved in the 1990s that a particular narrowing strategy, ''needed narrowing'', is optimal in the sense of doing a number of reductions to get to a "normal form" corresponding to a solution that is minimal among sound and complete strategies. Needed narrowing corresponds to a lazy strategy, in contrast to the [[SLD resolution|SLD-resolution]] strategy of [[Prolog]].