-- Hoogle documentation, generated by Haddock
-- See Hoogle, http://www.haskell.org/hoogle/


-- | A library for writing predicates and transformations over predicates in Haskell
--   
--   This package provides ways to write predicates such that they compose
--   nicely and are easy to debug.
@package predicate-transformers
@version 0.7.0.2


-- | This library is based on the notion of a predicate transformer, the
--   below type <tt>PT a b</tt>, which is a function from <tt>a</tt> to
--   predicates on <tt>b</tt>. They act as a sort of compositional "matcher
--   language". Composing these predicate transformers is meant to be
--   analogous to composing optics and there are utilities for using
--   predicate transformers with (<a>lens</a>-style) optics.
--   
--   Some predicate transformers provided by other libraries: <a>all</a>,
--   <a>any</a> (base) <a>either</a> (base) <a>allOf</a> (lens)
module PredicateTransformers

-- | A convenient alias for predicates.
type Pred a = a -> Bool

-- | Predicate transformers form a category where composition is ordinary
--   function composition. Forms a category with <a>.</a> and <a>id</a>.
--   Multiple are already provided by the standard library, for instance
--   <a>all</a> and <a>any</a>.
type PT a b = Pred a -> Pred b

-- | Operate on the target of a prism, or fail.
match :: APrism s t a b -> PT a s

-- | Invert a predicate.
nay :: PT a a

-- | Operate on the <a>Just</a> branch of a <a>Maybe</a>, or fail.
just :: PT a (Maybe a)

-- | Operate on the <a>Left</a> branch of an <a>Either</a>, or fail.
left :: PT e (Either e a)

-- | Operate on the <a>Right</a> branch of an <a>Either</a>, or fail.
right :: PT a (Either e a)

-- | Operate on the last value in a foldable, or fail if it's not present.
endingWith :: Foldable f => PT a (f a)

-- | Operate on the first value in a foldable, or fail if it's not present.
startingWith :: Foldable f => PT a (f a)

-- | Require that a foldable has a single element, and operate on that
--   element.
only :: Foldable f => PT a (f a)

-- | Only test the <tt>k</tt>th element of a foldable.
kth :: Foldable f => Int -> PT a (f a)

-- | Given a list of predicates and a list of values, ensure that each
--   predicate holds for each respective value. Fails if the two lists have
--   different lengths.
list :: [Pred a] -> Pred [a]

-- | Given a functor-full of predicates, and a functor-full of values,
--   ensure that the structures of the two functors match and apply all of
--   the predicates to all of the values. Generalized version of
--   <a>list</a>.
dist :: (Eq (f ()), Functor f, Foldable f) => f (Pred a) -> Pred (f a)

-- | Given a representable functor-full of predicates, and a functor-full
--   of values, yield a representable functor-full of booleans. Similar to
--   <a>dist</a>.
distRep :: Representable f => f (a -> Bool) -> f a -> f Bool

-- | Test all predicates against one value.
allTrue :: [Pred a] -> Pred a

-- | Check that a predicate is true for all values behind a generalized
--   getter and that there's at least one value for which it's true.
allOf1 :: Getting (All, Any) s a -> PT a s

-- | Sugar for tupling.
(==>) :: a -> b -> (a, b)
pair :: Pred a -> Pred b -> Pred (a, b)

-- | Flipped function composition; <tt>f !</tt> for a function <tt>f</tt>
--   is a predicate transformer.
(!) :: (b -> a) -> (a -> c) -> b -> c

-- | Prints the input of a predicate, for debugging.
traced :: Show a => (a -> c) -> a -> c

-- | Prints the input of a predicate, if the predicate fails.
traceFail :: (a -> String) -> PT a a
traceFailShow :: Show a => PT a a

-- | Predicate which always succeeds.
something :: Pred a

-- | Predicate which triggers full evaluation of its input. Useful for
--   testing that an exception isn't thrown.
forced :: NFData a => a -> Bool