Why Haskell?

Bob Ippolito

January 8, 2014

Hello world!

main = print "hello world"

With types

main :: IO ()
main = putStrLn "hello world"

You'll need a local Haskell interpreter for this due to input. You can replace getLine with return "some input" to try here.

Run with runhaskell WOPR.hs.

-- WOPR.hs

main :: IO ()
main = do
  putStrLn "WHAT GAME WOULD YOU LIKE TO PLAY?"
  game <- getLine
  if game == "GLOBAL THERMONUCLEAR WAR"
    then putStrLn "WOULDN'T YOU PREFER A NICE GAME OF CHESS?"
    else putStrLn "GOOD CHOICE."

Functions

circleArea :: Float -> Float
circleArea r = pi * r ^ 2

main :: IO ()
main = print (circleArea 10)

Functions

circleArea :: Float -> Float
circleArea = \r -> pi * r ^ 2

main :: IO ()
main = print (circleArea 10)

Multiple arguments

rectangleArea :: Float -> Float -> Float
rectangleArea w h = w * h

main :: IO ()
main = print (rectangleArea 10 40)

Multiple arguments

rectangleArea :: Float -> Float -> Float
rectangleArea = \w -> \h -> w * h

main :: IO ()
main = print (rectangleArea 10 40)

Pattern matching

myAnd :: Bool -> Bool -> Bool
myAnd True b = b
myAnd _    _ = False

main :: IO ()
main = print (myAnd (10 > 5) (100 * 2 == 200))

Pattern matching

myAnd :: Bool -> Bool -> Bool
myAnd = \a ->
  case a of
    True  -> \b -> b
    False -> \_ -> False

main :: IO ()
main = print (myAnd (10 > 5) (100 * 2 == 200))

Can you define xor?

myXor :: Bool -> Bool -> Bool
myXor a b = undefined

main :: IO ()
main = do
  -- these should all print True
  print (myXor True True == False)
  print (myXor True False == True)
  print (myXor False True == True)
  print (myXor False False == False)

Guards

fac :: Integer -> Integer
fac n
  | n > 1     = n * fac (n - 1)
  | otherwise = n

main :: IO ()
main = print (fac 26)

Operators

(.^) :: Bool -> Bool -> Bool
True .^ True = False
x    .^ y    = x || y

main :: IO ()
main = print (True .^ True)

Operators

(.^) :: Bool -> Bool -> Bool
(.^) True True = False
(.^) x    y    = x || y

main :: IO ()
main = print (True .^ True)

Operators

(.^) :: Bool -> Bool -> Bool
True .^ True = False
x    .^ y    = x || y

main :: IO ()
main = print ((.^) True True)

Operators

xor :: Bool -> Bool -> Bool
True `xor` True = False
x    `xor` y    = x || y

main :: IO ()
main = print (xor True True)

Operators

(.^) :: Bool -> Bool -> Bool
xor True True = False
xor x    y    = x || y

main :: IO ()
main = print (True `xor` True)

Haskell

Pre-History

Early History

Evolution

Fun Quotes

Domain

General Purpose

• Very effective for parsing and compilation
• Great for DSEL (Domain Specific Embedded Languages)
• Has been popular in academia for some time
• Becoming more popular in industry

Commercial Use

Consumer Apps

Commercial Services

Compilers

Standalone Apps

Haskell Platform

Haskell: Batteries Included

• http://www.haskell.org/platform
• GHC compiler and GHCi interpreter
• Robust and stable set of vetted packages
• Cabal; easily fetch more packages from Hackage

Editor Support

Abstract Data Types

-- sum type, 3 possible values
data Choice = Definitely
            | Possibly
            | NoWay

-- product type, 9 possible values (3 * 3)
data Choices = Choices Choice Choice

-- record syntax defines accessors automatically
data Choices = Choices { fstChoice :: Choice
                       , sndChoice :: Choice
                       }

Time to model Rock Paper Scissors

data Move = Rock
          | Paper
          | Scissors
          deriving (Show, Eq)

data Outcome = Lose
             | Draw
             | Win
             deriving (Show, Eq)

score :: Move -> Move -> Outcome
score a b = undefined

main :: IO ()
main = do
  print (score Rock Paper)
  print (score Rock Scissors)

Implement shapeArea

data Shape = Circle Float
           | Rectangle Float Float
           deriving (Show, Eq)

shapeArea :: Shape -> Float
shapeArea _ = undefined

main :: IO ()
main = do
  print (shapeArea (Circle 10) == pi * 100)
  print (shapeArea (Rectangle 2 12) == 24)

Using Types

-- Bindings can be annotated
success :: a -> Maybe a
-- Constructors are functions
success = Just

-- Constructors can be pattern matched
-- _ is a wildcard
case success True of
  Just True -> ()
  _         -> ()

-- Values can be annotated in-line
2 ^ (1 :: Int)

Typeclass Syntax

class Equals a where
  isEqual :: a -> a -> Bool

instance Equals Choice where
  isEqual Definitely Definitely = True
  isEqual Possibly   Possibly   = True
  isEqual NoWay      NoWay      = True
  isEqual _          _          = False

instance (Equals a) => Equals [a] where
  isEqual (a:as) (b:bs) = isEqual a b &&
                          isEqual as bs
  isEqual as     bs     = null as && null bs

Bindings & Functions

greeting :: String
greeting = "Hello, "

sayHello :: String -> String
sayHello name = greeting ++ name
-- desugars to:
sayHello = \name -> (++) greeting name

sayHello name = result
  where result = greeting ++ name
-- desugars to:
sayHello = \name ->
  let result = (++) greeting name
  in result

Pattern Matching

-- Unlike Erlang, pattern matching is only on
-- constructors, never variables
isJust (Just _) = True
isJust Nothing  = False
-- desugars to:
isJust = \x ->
  case x of
    (Just _) -> True
    Nothing  -> False

Guards

isNegative :: (Num a) => a -> Bool
isNegative x
  | x < 0     = True
  | otherwise = False
-- desugars to:
isNegative = \x ->
  if (<) x 0
  then True
  else False

`Infix` and (Prefix)

-- Symbolic operators can be used
-- prefix when in (parentheses)
(+) a b

-- Named functions can be used
-- infix when in `backticks`
x `elem` xs

-- infixl, infixr define associativity
-- and precedence (0 lowest, 9 highest)
infixr 5 `append`
a `append` b = a ++ b

Do syntax

do m
-- desugars to:
m

do a <- m
   return a
-- desugars to:
m >>= \a -> return a

do m
   return ()
-- desugars to:
m >> return ()

GHCi

Interactive Haskell

$ runhaskell --help
Usage: runghc [runghc flags] [GHC flags] module [program args]

The runghc flags are
    -f /path/to/ghc       Tell runghc where GHC is
    --help                Print this usage information
    --version             Print version number

$ ghci
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
h> 

:t shows type information

h> :t map
map :: (a -> b) -> [a] -> [b]
h> :t map (+1)
map (+1) :: Num b => [b] -> [b]
h> :t (>>=)
(>>=) :: Monad m => m a -> (a -> m b) -> m b

:i shows typeclass info

h> :i Num
class Num a where
  (+) :: a -> a -> a
  (*) :: a -> a -> a
  (-) :: a -> a -> a
  negate :: a -> a
  abs :: a -> a
  signum :: a -> a
  fromInteger :: Integer -> a
    -- Defined in `GHC.Num'
instance Num Integer -- Defined in `GHC.Num'
instance Num Int -- Defined in `GHC.Num'
instance Num Float -- Defined in `GHC.Float'
instance Num Double -- Defined in `GHC.Float'

:i shows value info

h> :info map
map :: (a -> b) -> [a] -> [b]   
-- Defined in `GHC.Base'
h> :info (>>=)
class Monad m where
  (>>=) :: m a -> (a -> m b) -> m b
  ...
    -- Defined in `GHC.Base'
infixl 1 >>=

:i shows type info

h> :info Int
data Int = ghc-prim:GHC.Types.I#
  ghc-prim:GHC.Prim.Int#
  -- Defined in `ghc-prim:GHC.Types'
instance Bounded Int -- Defined in `GHC.Enum'
instance Enum Int -- Defined in `GHC.Enum'
instance Eq Int -- Defined in `GHC.Classes'
instance Integral Int -- Defined in `GHC.Real'
instance Num Int -- Defined in `GHC.Num'
instance Ord Int -- Defined in `GHC.Classes'
instance Read Int -- Defined in `GHC.Read'
instance Real Int -- Defined in `GHC.Real'
instance Show Int -- Defined in `GHC.Show'

:l load a module

:r to reload

h> :! echo 'hello = print "hello"' > Hello.hs
h> :l Hello
[1 of 1] Compiling Main ( Hello.hs, interpreted )
Ok, modules loaded: Main.
h> hello
"hello"
h> :! echo 'hello = print "HELLO"' > Hello.hs
h> :r
[1 of 1] Compiling Main ( Hello.hs, interpreted )
Ok, modules loaded: Main.
h> hello
"HELLO"

Declarative

• "Describe the problem, not the solution"
• Great syntax for this (but not C-like)!
• Functional code is (often) easier to understand and test
• Pure, no side-effects?!

-- MergeSort1.hs
module MergeSort1 (mergeSort) where

-- | Bottom-up merge sort.
mergeSort :: Ord a => [a] -> [a]
mergeSort [] = []
mergeSort xs = mergeAll [[x] | x <- xs]

mergeAll :: Ord a => [[a]] -> [a]
mergeAll [xs] = xs
mergeAll xss  = mergeAll (mergePairs xss)

mergePairs :: Ord a => [[a]] -> [[a]]
mergePairs (a:b:xs) =
  merge a b : mergePairs xs
mergePairs xs = xs

merge :: Ord a => [a] -> [a] -> [a]
merge as@(a:as') bs@(b:bs')
 | a > b     = b : merge as bs'
 | otherwise = a : merge as' bs
merge [] bs = bs
merge as [] = as

-- MergeSort2.hs
module MergeSort2 (mergeSort) where

-- | Bottom-up merge sort.
mergeSort :: Ord a => [a] -> [a]
mergeSort = mergeAll . map (:[])
  where
    mergeAll []   = []
    mergeAll [xs] = xs
    mergeAll xss  = mergeAll (mergePairs xss)

    mergePairs (a:b:xs) =
      merge a b : mergePairs xs
    mergePairs xs = xs

    merge as@(a:as') bs@(b:bs')
      | a > b     = b : merge as bs'
      | otherwise = a : merge as' bs
    merge [] bs = bs
    merge as [] = as

# merge_sort.py
def merge_sort(lst):
    if not lst:
        return []
    lists = [[x] for x in lst]
    while len(lists) > 1:
        lists = merge_lists(lists)
    return lists[0]

def merge_lists(lists):
    result = []
    for i in range(0, len(lists) // 2):
        result.append(merge2(lists[i*2], lists[i*2 + 1]))
    if len(lists) % 2:
        result.append(lists[-1])
    return result

def merge2(xs, ys):
    i = 0
    j = 0
    result = []
    while i < len(xs) and j < len(ys):
        x = xs[i]
        y = ys[j]
        if x > y:
            result.append(y)
            j += 1
        else:
            result.append(x)
            i += 1
    result.extend(xs[i:])
    result.extend(ys[j:])
    return result

-- WordCount1.hs

main :: IO ()
main = do
  input <- getContents
  let wordCount = length (words input)
  print wordCount

-- WordCount2.hs

main :: IO ()
main =
  getContents >>= \input ->
    let wordCount = length (words input)
    in print wordCount

-- WordCount3.hs

main :: IO ()
main = getContents >>= print . length . words

what.the >>=?

• do is just syntax sugar for the >>= (bind) operator.
• IO is still purely functional, we are just building a graph of actions, not executing them in-place!
• Starting from main, the Haskell runtime will interpret these actions
• It works much like continuation passing style, with a state variable for the current world state (behind the scenes)
• There are ways to cheat and write code that is not pure, but you will have to go out of your way to do it

Common combinators

-- Function composition
(.) :: (b -> c) -> (a -> b) -> a -> c
f . g = \x -> f (g x)

-- Function application (with a lower precedence)
($) :: (a -> b) -> a -> b
f $ x =  f x

Pure

• Haskell's purity implies referential transparency
• This means that function invocation can be freely replaced with its return value without changing semantics
• Fantastic for optimizations
• Also enables equational reasoning, which makes it easier to prove code correct

Optimizations

• Common sub-expression elimination
• Inlining (cross-module too!)
• Specialize
• Float out
• Float inwards
• Demand analysis
• Worker/Wrapper binds
• Liberate case
• Call-pattern specialization (SpecConstr)

GHC RULES!

• Term rewriting engine
• RULES pragma allows library defined optimizations
• Used to great effect for short cut fusion
• Example: map f (map g xs) = map (f . g) xs
• Prevent building of intermediate data structures
• Commonly used for lists, Text, ByteString, etc.
• Great incentive to write high-level code!
• ANY LIBRARY CAN USE THIS!

{-# RULES
"ByteString specialise break (x==)" forall x.
    break ((==) x) = breakByte x
"ByteString specialise break (==x)" forall x.
    break (==x) = breakByte x
  #-}

Lazy

• Call by need (outside in), not call by value (inside out)
• Non-strict evaluation separates equation from execution
• No need for special forms for control flow, no value restriction
• Enables infinite or cyclic data structures
• Can skip unused computation (better minimum bounds)

Call by need

• Expressions are translated into a graph (not a tree!)
• Evaluated with STG (Spineless Tagless G-Machine)
• Pattern matching forces evaluation

-- [1..] is an infinite list, [1, 2, 3, ...]
print (head (map (*2) [1..]))
-- Outside in, print x = putStrLn (show x)
putStrLn (show (head (map (*2) [1..]))
-- head (x:_) = x
-- map f (x:xs) = f x : map f xs
-- desugar [1..] syntax
putStrLn (show (head (map (*2) (enumFrom 1))))
-- enumFrom n = n : enumFrom (succ n)
putStrLn (show (head (map (*2) (1 : enumFrom (succ 1)))))
-- apply map
putStrLn (show (head ((1*2) : map (*2) (enumFrom (succ 1)))))
-- apply head
putStrLn (show (1*2))
-- show pattern matches on its argument
putStrLn (show 2)
-- apply show
putStrLn "2"

if' :: Bool -> a -> a -> a
if' cond a b = case cond of
  True  -> a
  False -> b

(&&) :: Bool -> Bool -> Bool
a && b = case a of
  True  -> b
  False -> False

const :: a -> b -> a
const x = \_ -> x

fib :: [Integer]
fib = 0 : 1 : zipWith (+) fib (tail fib)

cycle :: [a] -> [a]
cycle xs = xs ++ cycle xs

iterate :: (a -> a) -> a -> [a]
iterate f x = x : iterate f (f x)

takeWhile :: (a -> Bool) -> [a] -> [a]
takeWhile _ [] = []
takeWhile p (x:xs)
  | p x       = x : takeWhile p xs
  | otherwise = []

Types

• Enforce constraints at compile time
• No NULL
• Can have parametric polymorphism and/or recursion
• Built-in types are not special (other than syntax)
• Typeclasses for ad hoc polymorphism (overloading)

h> let f x = head True

<interactive>:23:16:
    Couldn't match expected type `[a0]' with actual type `Bool'
    In the first argument of `head', namely `True'
    In the expression: head True
    In an equation for `f': f x = head True

h> let f x = heads True

<interactive>:24:11:
    Not in scope: `heads'
    Perhaps you meant one of these:
      `reads' (imported from Prelude), `head' (imported from Prelude)

No Null References

• Haskell doesn't repeat Hoare's "Billion Dollar Mistake"
• Instead of NULL, wrap the value in a sum type such as Maybe or Either
• Bottom ⊥ can be expressed in any type, as it's always possible to express a computation that does not terminate

data Maybe a = Just a
             | Nothing

data Either a b = Left a
                | Right b

parseBit :: Char -> Maybe Int
parseBit '0' = Just 0
parseBit '1' = Just 1
parseBit _ = Nothing

h> let x = x in x
-- Infinite recursion, not a fun case to deal with!

h> case False of True -> ()
*** Exception: <interactive>:29:1-24: Non-exhaustive patterns in case

h> head []
*** Exception: Prelude.head: empty list

h> error "this throws an exception"
*** Exception: this throws an exception

h> undefined
*** Exception: Prelude.undefined

-- Polymorphic and recursive
data List a = Cons a (List a)
            | Nil
            deriving (Show)

data Tree a = Leaf a
            | Branch (Tree a) (Tree a)
            deriving (Show)

listMap :: (a -> b) -> List a -> List b
listMap _ Nil         = Nil
listMap f (Cons x xs) = Cons (f x) (listMap f xs)

treeToList :: Tree a -> List a
treeToList root = go root Nil
  where
    -- Note that `go` returns a function!
    go (Leaf x)     = Cons x
    go (Branch l r) = go l . go r

-- (), pronounced "unit"
unit :: ()
unit = ()

-- Char
someChar :: Char
someChar = 'x'

-- Instances of Num typeclass
someDouble :: Double
someDouble = 1

-- Instances of Fractional typeclass
someRatio :: Rational
someRatio = 1.2345

-- [a], type can be written prefix as `[] a`
someList, someOtherList :: [Int]
someList = [1, 2, 3]
someOtherList = (:) 4 (5 : (:) 6 [])

-- (a, b), can be written prefix as `(,) a b`
someTuple, someOtherTuple :: (Int, Char)
someTuple = (10, '4')
someOtherTuple = (,) 4 '2'

-- [Char], also known as String
-- (also see the OverloadedStrings extension)
someString :: String
someString = "foo"

Typeclasses

• Used for many of the Prelude operators and numeric literals
• Ad hoc polymorphism (overloading)
• Many built-in typeclasses can be automatically derived (Eq, Ord, Enum, Bounded, Show, and Read)!

module List where

data List a = Cons a (List a)
            | Nil

instance (Eq a) => Eq (List a) where
  (Cons a as) == (Cons b bs) = a == b && as == bs
  Nil         == Nil         = True
  _           == _           = False

instance Functor List where
  fmap _ Nil         = Nil
  fmap f (Cons x xs) = Cons (f x) (fmap f xs)

{-# LANGUAGE DeriveFunctor #-}

module List where

data List a = Cons a (List a)
            | Nil
            deriving (Eq, Functor)

import Data.List (sort)

newtype Down a = Down { unDown :: a }
                 deriving (Eq)

instance (Ord a) => Ord (Down a) where
  compare (Down a) (Down b) = case compare a b of
    LT -> GT
    EQ -> EQ
    GT -> LT

reverseSort :: Ord a => [a] -> [a]
reverseSort = map unDown . sort . map Down

Abstractions

Monoid

class Monoid a where
  mempty :: a
  mappend :: a -> a -> a

instance Monoid [a] where
  mempty = []
  mappend = (++)

infixr 6 <>
(<>) :: (Monoid a) => a -> a -> a
(<>) = mappend

Functor

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

instance Functor [] where
  fmap = map

instance Functor Maybe where
  fmap f (Just x) = Just (f x)
  fmap _ Nothing  = Nothing

infixl 4 <$>
(<$>) :: Functor f => (a -> b) -> f a -> f b
(<$>) = fmap

Applicative

class (Functor f) => Applicative f where
  pure :: a -> f a
  infixl 4 <*>
  (<*>) :: f (a -> b) -> f a -> f b

instance Applicative [] where
  pure x = [x]
  fs <*> xs = concatMap (\f -> map f xs) fs

instance Applicative Maybe where
  pure = Just
  Just f <*> Just x = Just (f x)
  _      <*> _      = Nothing

Monad

class Monad m where
  return :: a -> m a
  (>>=) :: m a -> (a -> m b) -> m b
  (>>)  :: m a -> m b -> m b
  ma >> mb = ma >>= \_ -> mb

instance Monad [] where
  return = pure
  m >>= f = concatMap f m

instance Monad Maybe where
  return = pure
  Just x  >>= f = f x
  Nothing >>= _ = Nothing

Parser Combinators

{-# LANGUAGE OverloadedStrings #-}
module SJSON where
import Prelude hiding (concat)
import Data.Text (Text, concat)
import Data.Attoparsec.Text
import Control.Applicative

data JSON = JArray [JSON]
          | JObject [(Text, JSON)]
          | JText Text
          deriving (Show)

pJSON :: Parser JSON
pJSON = choice [ pText, pObject, pArray ]
  where
    pString = concat <$> "\"" .*> many pStringChunk <*. "\""
    pStringChunk = choice [ "\\\"" .*> pure "\""
                          , takeWhile1 (not . (`elem` "\\\""))
                          , "\\" ]
    pText = JText <$> pString
    pPair = (,) <$> pString <*. ":" <*> pJSON
    pObject = JObject <$> "{" .*> (pPair `sepBy` ",") <*. "}"
    pArray = JArray <$> "[" .*> (pJSON `sepBy` ",") <*. "]"

Foreign Function Interface

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign.C.Types
import Control.Monad

foreign import ccall unsafe "stdlib.h rand"
     c_rand :: IO CUInt

main :: IO ()
main = replicateM_ 20 (c_rand >>= print)

Parallel Programming

-- FlipImage.hs
import System.Environment
import Data.Word
import Data.Array.Repa hiding ((++))
import Data.Array.Repa.IO.DevIL
import Data.Array.Repa.Repr.ForeignPtr

main :: IO () 
main = do
  [f] <- getArgs
  (RGB v) <- runIL $ readImage f
  rotated <- (computeP $ rot180 v) :: IO (Array F DIM3 Word8)
  runIL $ writeImage ("flip-"++f) (RGB rotated)

rot180 :: (Source r e) => Array r DIM3 e -> Array D DIM3 e
rot180 g = backpermute e flop g
  where
    e@(Z :. x :. y :. _) = extent g
    flop (Z :. i         :. j         :. k) =
         (Z :. x - i - 1 :. y - j - 1 :. k)

Concurrency

RAM footprint per unit of concurrency (approx)

import Control.Concurrent
import Network.HTTP

getHTTP :: String -> IO String
getHTTP url = simpleHTTP (getRequest url) >>= getResponseBody

urls :: [String]
urls = map ("http://ifconfig.me/"++) ["ip", "host"]

startRequest :: String -> IO (MVar ())
startRequest url = do
  v <- newEmptyMVar
  forkIO (getHTTP url >>= putStr >> putMVar v ())
  return v

main :: IO ()
main = do
  mvars <- mapM startRequest urls
  mapM_ takeMVar mvars

Why not Haskell?

• Lots of new terminology
• Mutable state takes more effort
• Laziness changes how you need to reason about code
• Once you get used to it, these aren't problematic

A monad is just a monoid in the category of endofunctors, what's the problem?

Terminology from category theory can be intimidating (at first)!

function main() {
  var foo = {bar: 1, baz: 20};
  while (foo.baz > foo.bar) {
    foo.bar += 1;
  }
  console.log(foo);
}

import Control.Concurrent
data Foo = Foo {bar :: Int, baz :: Int}
         deriving (Show)

main :: IO ()
main = do
  fooVar <- newMVar (Foo { bar = 1, baz = 20 })
  let whileLoop = do
      foo <- takeMVar fooVar
      if baz foo > bar foo
      then do
        putMVar fooVar (foo { bar = 1 + bar foo })
        whileLoop
      else
        putMVar fooVar foo
  whileLoop
  withMVar fooVar print

sum :: Num a => [a] -> a
sum []     = 0
sum (x:xs) = x + sum xs

sum :: Num [a] => [a] -> a
sum = go 0
  where
    go acc (x:xs) = go (acc + x) (go xs)
    go acc []     = acc

sum :: Num [a] => [a] -> a
sum = go 0
  where
    go acc _
      | seq acc False = undefined
    go acc (x:xs)     = go (acc + x) (go xs)
    go acc []         = acc

{-# LANGUAGE BangPatterns #-}

sum :: Num [a] => [a] -> a
sum = go 0
  where
    go !acc (x:xs) = go (acc + x) (go xs)
    go  acc []     = acc

Notable Libraries

Web Frameworks

Publishing and docs

Parser Combinators

Dev Tools

Parallel / Distributed

Testing & Profiling