Curry: An Integrated Functional Logic Language

❀ {x=1,y=3}
(The empty constraint is omitted in the final answer.)
4 Types
4.1 Built-in Types
This section describes the types that are predefined in Curry. For each type, some important
operations are discussed. The complete definition of all operations can be found in the standard
prelude (see Appendix B).
4.1.1 Boolean Values
Boolean values are predefined by the datatype declaration
data Bool = True | False
The (sequential) conjunction is predefined as the left-associative infix operator &&:
(&&) :: Bool -> Bool -> Bool
True && x = x
False && x = False
Similarly, the (sequential) disjunction || and the negation not are defined as usual (see Appendix
B). Furthermore, the function otherwise is predefined as True to write rules with multiple
guards more nicely.
Boolean values are mainly used in conditionals, i.e., the conditional function if_then_else is
predefined as
if_then_else :: Bool -> a -> a -> a
if True then x else y = x
if False then x else y = y
where “if b then x else y” is syntactic sugar for the application (if_then_else b x y).
A function with result type Bool is often called a predicate (in contrast to constraints which
have result type Success, see below). 7 There are a number of built-in predicates for comparing
objects, like the predicate “