All Haskell values have a type:

char = 'a'    :: Char
int = 123     :: Int
fun = isDigit :: Char -> Bool

You have to pass the right type of values to functions, or the compiler will reject the program:

isDigit 1

You can decode bytes into text:

bytes = Crypto.Hash.SHA1.hash "hello" :: ByteString
text = decodeUtf8 bytes               :: Text

But you cannot decode Text, which is already a vector of Unicode points:

doubleDecode = decodeUtf8 (decodeUtf8 bytes)

The following function takes an integer and returns an integer. By the type it cannot do any side-effects whatsoever, it cannot mutate any of its arguments.

square :: Int -> Int
square x = x * x

The following string concatenation is okay:

"Hello: " ++ "World!" 

The following string concatenation is a type error:

"Name: " ++ getLine

Because getLine has type IO String and not String, like "Name: " is. So by the type system you cannot mix and match purity with impurity.

This example has a type signature for every binding:

main :: IO ()
main = do line :: String <- getLine
          print (parseDigit line)
  where parseDigit :: String -> Maybe Int
        parseDigit ((c :: Char) : _) =
          if isDigit c
             then Just (ord c - ord '0')
             else Nothing

But you can just write:

main = do line <- getLine
          print (parseDigit line)
  where parseDigit (c : _) =
          if isDigit c
             then Just (ord c - ord '0')
             else Nothing

You can also use inference to avoid wasting time explaining what you want:

do ss <- decode "[\"Hello!\",\"World!\"]"
   is <- decode "[1,2,3]"
   return (zipWith (\s i -> s ++ " " ++ show (i + 5)) ss is)
 => Just ["Hello! 6","World! 7"]

Types give a parser specification for free, the following input is not accepted:

do ss <- decode "[1,2,3]"
   is <- decode "[null,null,null]"
   return (zipWith (\s i -> s ++ " " ++ show (i + 5)) ss is)
 => Nothing

Easily launch threads and communicate with the standard library:

main = do
  done <- newEmptyMVar
  forkIO (do putStrLn "I'm one thread!"
             putMVar done "Done!")
  second <- forkIO (do delayThread 100000
                       putStrLn "I'm another thread!")
  killThread second
  msg <- takeMVar done
  putStrLn msg

Use an asynchronous API for threads:

do a1 <- async (getURL url1)
  a2 <- async (getURL url2)
  page1 <- wait a1
  page2 <- wait a2
  ...

Atomic threading with software transactional memory:

transfer :: Account -> Account -> Int -> IO ()
transfer from to amount =
  atomically (do deposit to amount
                 withdraw from amount)

Atomic transactions must be repeatable, so arbitrary IO is disabled in the type system:

main = atomically (putStrLn "Hello!")

Define control structures easily:

when p m = if p then m else return ()
main = do args <- getArgs
          when (null args)
               (putStrLn "No args specified!") 

If you notice a repeated expression pattern, like

if c then t else False

you can give this a name, like

and c t = if c then t else False

and then use it with the same effect as the orginal expression.

Get code re-use by composing lazy functions. It's quite natural to express the any function by reusing the map and or functions:

any :: (a -> Bool) -> [a] -> Bool
any p = or . map p

Reuse the recursion patterns in map, filter, foldr, etc.

There are 6,954 packages freely available. Here is a sample of the most common ones: