Curry: An Integrated Functional Logic Language

-- (flip f) is identical to f but with the order of arguments reversed
flip :: (a -> b -> c) -> b -> a -> c
flip f x y = f y x
-- Repeat application of a function until a predicate holds
until :: (a -> Bool) -> (a -> a) -> a -> a
until p f x = if p x then x else until p f (f x)
-- Right-associative application
($) :: (a -> b) -> a -> b
f $ x = f x
-- Evaluate the first argument to head normal form and return the
-- second argument. Suspend if the first argument evaluates to a
-- free variable.
seq :: a -> b -> b
-- Right-associative application with strict evaluation of its argument.
($!) :: (a -> b) -> a -> b
f $! x = x ‘seq‘ f x
-- Abort the execution with an error message
error :: String -> _
-- failed is a non-reducible polymorphic function.
-- It is useful to express a failure in a search branch of the execution.
failed :: _
failed | 1=:=2 = x where x free
-- Boolean values
data Bool = True | False
-- Sequential conjunction
(&&) :: Bool -> Bool -> Bool
True && x = x
False && _ = False
-- Sequential disjunction
(||) :: Bool -> Bool -> Bool
True || _ = True
False || x = x
-- Negation
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