Curry: An Integrated Functional Logic Language

Then the expression
best (\x -> let y free in append x y =:= [1,2,3]) shorter
computes the shortest list which can be obtained by splitting the list [1,2,3] into this list
and some other list, i.e., it reduces to [\x->x=:=[]]. Similarly, the expression
best (\x -> let y free in append x y =:= [1,2,3])
(\l1 l2 -> length l1 > length l2)
reduces to [\x->x=:=[1,2,3]].
one :: (a->Success) -> [a->Success]
Compute one solution via a fair strategy. If there is no solution and the search space is
finite, the result is the empty list, otherwise the list contains one element (the first solution
represented as a search goal).
condSearch :: (a->Success) -> (a->Bool) -> [a->Success]
Compute the first solution (via a fair strategy) that meets a specified condition. The condition
(second argument) must be a unary Boolean function.
browse :: (a->Success) -> IO ()
Show the solution of a solved constraint on the standard output, i.e., a call browse g, where
g is a solved search goal, is evaluated to an I/O action which prints the solution. If g is not
an abstraction of a solved constraint, a call browse g produces a runtime error.
browseList :: [a->Success] -> IO ()
Similar to browse but shows a list of solutions on the standard output. The browse operators
are useful for testing search strategies. For instance, the evaluation of the expression
browseList (solveAll \x -> let y free in append x y =:= [0,1,2])
produces the following result on the standard output:
[]
[0]
[0,1]
[0,1,2]
Due to the laziness of the evaluation strategy, one can also browse the solutions of a goal
with infinitely many solutions which are printed on the standard output until the process is
stopped.
8.4 Choice
In order to provide a “don’t care” selection of an element out of different alternatives, which is
necessary in concurrent computation structures to support many-to-one communication, Curry
provides the special evaluation annotation choice:
f eval choice
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