Curry: An Integrated Functional Logic Language

do line do stmts
expr >>= \p -> do stmts
let decls
in do stmts
Global search, possibly implemented by backtracking, must be avoided in some situations (usercontrol
of efficiency, concurrent computations, non-backtrackable I/O). Hence it is sometimes necessary
to encapsulate search, i.e., non-deterministic computations in parts of larger programs.
Non-deterministic computations might occur in Curry whenever a function must be evaluated with
a free variable at a flexible argument position. In this case, the computation must follow different
branches with different bindings applied to the current expression which has the effect that the
entire expression is split into (at least) two independent disjunctions. To give the programmer control
on the actions taken in this situation, Curry provides a primitive search operator which is the
basis to implement sophisticated search strategies. This section sketches the idea of encapsulating
search in Curry and describes some predefined search strategies.
8.1 Search Goals and Search Operators
Since search is used to find solutions to some constraint, search is always initiated by a constraint
containing a search variable for which a solution should be computed. 11 Since the search variable
may be bound to different solutions in different disjunctive branches, it must be abstracted.
Therefore, a search goal has the type a->Success where a is the type of the values which we are
searching for. In particular, if c is a constraint containing a variable x and we are interested in
solutions for x, i.e., values for x such that c is satisfied, then the corresponding search goal has the
form \x->c. However, any other expression of the same type can also be used as a search goal.
To control the search performed to find solutions to search goals, Curry has a predefined operator
try :: (a->Success) -> [a->Success]
11 The generalization to more than one search variable is straightforward by the use of tuples.
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