Editing Curry (programming language) (section)

===Non-determinism===
Since Curry is able to solve equations containing function calls with unknown values, its execution mechanism is based on non-deterministic computations, similarly to logic programming. This mechanism supports also the definition of ''non-deterministic operations'', i.e., operations that delivers more than one result for a given input. The archetype of non-deterministic operations is the predefined infix operation {{Mono|?}}, called ''choice'' operator, that returns one of its arguments. This operator is defined by the following rules:

  x ? y = x
  x ? y = y

Thus, the evaluation of the expression {{Mono|0 ? 1}} returns {{Mono|0}} as well as {{Mono|1}}. Computing with non-deterministic operations and computing with free variables by narrowing has the same expressive power.<ref>{{cite book |last1=Sergio |first1=Antoy |last2=Hanus |first2=Michael |title=Logic Programming |chapter=Overlapping Rules and Logic Variables in Functional Logic Programs |series=Lecture Notes in Computer Science |year=2006 |doi=10.1007/11799573_9 |volume=4079 |pages=87–101 |isbn=978-3-540-36635-5}}</ref>

The rules defining {{Mono|?}} show an important feature of Curry: all rules are tried in order to evaluate some operation. Hence, one can define by
<syntaxhighlight lang="haskell">
 insert x ys     = x : ys
 insert x (y:ys) = y : insert x ys
</syntaxhighlight>
an operation to insert an element into a list at an indeterminate position so that the operation {{Mono|perm}} defined by
<syntaxhighlight lang="haskell">
 perm []     = []
 perm (x:xs) = insert x (perm xs)
</syntaxhighlight>
returns any permutation of a given input list.