haskell

monday, 5 january 2015

there are expressive programming languages. And then there are beautiful ones. Haskell, a pure functional programming language, is such a language with its mathematical eloquence.

Transitioning from imperative languages to Haskell can entail more than a brain cramp – the purity of the language lacks the common control structures that give imperative languages their familiar linear organization: the learning curve is steep. Instead, the syntax is equational, the mathematical roots of the language being very apparent. It’s a difficult journey across the landscape of lazy evaluation, static typing, functors, monoids and monads, but worth it just for the insightful possibilities and the exercise of seeing in terms of equations versus sequential operations.

The functional pattern matching is reminiscent of object oriented message dispatching which, with the power of its type declaratives, can create solutions that read very much like a domain specific language. It takes awhile to grok the power of static typing, type classes and instances, which lie at the heart of the language and facilitate such eloquent solutions.

Haskell is way beyond my ken – though, I have been fine tuning my Xmonad desktop environment which is written in Haskell as a means of immersing myself in its delightful power. It is used in academia for esoteric problem domains. I love the Ruby programming language. But there is a hypnotically alluring quality to Haskell’s mathematical form, which sings the language of the universe.

type class hiearchy

class subclass of prelude instances
Show   All Prelude types
Read   All except IO, (->)
Bounded   Int, Char, Bool, (), Ordering, tuples
Eq   All except IO, (->)
Enum   (), Bool, Char, Ordering, Int, Integer, Float, Double
Ord Eq All except (->), IO, IOError
Ix Ord Int, Integer, Char, Bool, Enum, tuples
Num Eq Int, Integer, Float, Double
Real Ord, Num Int, Integer, Float, Double
Fractional Num Float, Double
Integral Enum, Real Int, Integer
RealFrac Real Float, Double
Floating Fractional Float, Double
RealFloat RealFrac, Floating Float, Double
Functor   IO, [], Maybe
Monad   IO, [], Maybe
MonadPlus Monad IO, [], Maybe

Note: Haskell type classes are not to be confused with object oriented class specifications

functors

class Functor f where fmap :: (a -> b) -> f a -> f b

prelude

instance Functor [] where fmap = map instance Functor Maybe where fmap f (Just x) = Just (f x) fmap f Nothing = Nothing instance Functor (Either a) where fmap f (Right x) = Right (f x) fmap f (Left x) = Left x

functor laws

fmap id = id fmap (f . g) = fmap f . fmap g

applicative functors

class (Functor f) => Applicative f where pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b (<$>) :: (a -> b) -> f a -> f b f <$> x = fmap f x

control.applicative

instance Applicative Maybe where pure = Just Nothing <> _ = Nothing (Just f) <> something = fmap f something instance Applicative [] where pure x = [x] fs <> xs = [f x | f <- fs, x <- xs] instance Applicative IO where pure = return a <> b = do f <- a x <- b return (f x) instance Applicative ((->) r) where pure x = (_ -> x) f <*> g = \x -> f x (g x)

applicative functor laws

pure id <*> v = v pure (.) <*> u <*> v <*> w = u <*> (v <*> w) pure f <*> pure x = pure (f x) u <*> pure y = pure ($ y) <*> u

monoids

class Monoid m where mempty :: m mappend :: m -> m -> m mconcat :: [m] -> m mconcat = foldr mappend mempty

data.monoid

instance Monoid [a] where mempty = [] mappend = (++) newtype Any = Any { getAny :: Bool } deriving (Eq, Ord, Read, Show, Bounded) instance Monoid Any where mempty = Any False Any x `mappend` Any y = Any (x || y) newtype All = All { getAll :: Bool } deriving (Eq, Ord, Read, Show, Bounded) instance Monoid All where mempty = All True All x `mappend` All y = All (x && y) instance Monoid Ordering where mempty = EQ LT `mappend` _ = LT EQ `mappend` y = y GT `mappend` _ = GT instance Monoid a => Monoid (Maybe a) where mempty = Nothing Nothing `mappend` m = m m `mappend` Nothing = m Just m1 `mappend` Just m2 = Just (m1 `mappend` m2)

monoid laws

mempty `mappend` x = x x `mappend` mempty = x (x `mappend` y) `mappend` z = x `mappend` (y `mappend` z)

monads

class (Applicative m) => Monad m where return :: a -> m a (»=) :: m a -> (a -> m b) -> m b (») :: m a -> m b -> m b x » y = x »= _ -> y fail :: String -> m a fail msg = error msg

Note: the class constraint Applicative m is not part of the formal definition of the monad class – it is inferred. return is equivalent to pure of applicative functors.

control.monad

instance Monad Maybe where return x = Just x Nothing »= f = Nothing Just x »= f = f x fail _ = Nothing instance Monad [] where return x = [x] xs »= f = concat (map f xs) fail _ = []

monad laws

Left Identity: return x >>= f = f x Right Identity: m >>= return = m Associativity: (m >>= f) >>= g = m >>= (\x -> f x >>= g)

do notation

foo :: IO () foo = ... >>= (\x -> … » … »= (\y -> ... (.. x .. y)))

can be expressed with Haskell’s syntactic sugar as..

foo :: IO () foo = do x <- … y <- … ... (.. x .. y)

which is just different syntax for chaining monadic values. In effect, monads sequence operations to simulate stateful or imperative computations.

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