Haskell is good for you!
Jens Petersen (juhp)
petersen@fedoraproject.org
27 June 2015 – FUDCon Pune 2015
Learning Haskell makes you a better programmer
Slides
What is Haskell?
Haskell is a
- Functional
- Pure
- Lazy
- Statically typed
programming language.
(Haskell Basics by Brent Yorgey)
Functional
- functions are first-class
- evaluation of expressions rather than
executing instructions
(functional vs imperative)
Pure
- variables and data structures are immutable
- expressions do not have “side effects”
- calling the same function with same args gives same result every time: deterministic
⇓
- Equational reasoning
- Easy refactoring
- Parallelism
Lazy
- evaluation by need
⇓
- can easily define control structures
- allows handling of infinite data structures
Statically typed
- Every Haskell expression has a type
- Types are type-checked at compile-time
- Programs with type errors or mismatches
will not compile
Haskell 25 years old
designed by a committee of CS academics
in the late 80’s
to create a standard lazy
functional programming language
Haskell uses indentation heavily like Python
GHC
Glasgow Haskell Compiler
- BSD license
- native compiler
- ghci interactive client
ghc
- https://www.haskell.org/ghc/
- Latest stable release is 7.10.1
- Fedora 22 has 7.8.4
- Native Code Generator for i686/x86_64
- ARM uses LLVM backend
- ghc is a large project
- many optional extensions to Haskell
Cabal and Hackage
Cabal
Haskell package build system
$ ghc Setup
$ ./Setup configure
$ ./Setup build
$ ./Setup installlike Python’s pip
Hackage
source repository of open source Haskell packages
cabal-install
User package manager for Hackage packages
eg to install pandoc
$ cabal install pandocresolves and builds dependencies
Haskell Basics
Basic types
:: = “has type”
True :: Bool
1 :: Int -- machine integers
2^2^2^2^2 -1 :: Integer -- arbitrary precision
'a' :: Char
"Hello!" :: String
1.0 :: Double
() :: ()Functions
add :: Integer -> Integer -> Integer
add m n = m + n
add 3 2 ⟹ 5 :: Integer
-- partial application
add 5 :: Integer -> IntegerFunctions vs operators
prefix vs infix
2 + 4 ⟹ 6
-- prefix
(+) 2 4 ⟹ 6
div 4 2 ⟹ 2
-- infix
4 `div` 2 ⟹ 2Lists
Lists
[a] is a list of elements of type a
The type variable a is an example of polymorphism.
List examples
-- empty list
[] :: [a]
[True, False, True] :: [Bool]
[1..5] :: [Int]
["Haskell", "is", "good"] :: [String]Note ['a', 1, True] is not a valid Haskell list!
List append
(++) :: [a] -> [a] -> [a]
[1, 2] ++ [3, 4] ⟹ [1, 2, 3, 4]Polymorphic list functions
head :: [a] -> a
tail :: [a] -> [a]
length :: [a] -> Integerhead [1, 2, 3] ⟹ 1
tail [1, 2, 3] ⟹ [2, 3]
length [1, 2, 3] ⟹ 3
length $ show $ 2^2^2^2^2 -1 ⟹ 19729Cons constructor
a : as
1 : [2, 3] ⟹ [1, 2, 3]map
map :: (a -> b) -> [a] -> [b]
map (add 3) [1, 2, 3] ⟹ [4, 5, 6]Laziness
Laziness
Lazy evaluation allows infinite lists:
naturals = [0..]Fibonacci series
fib :: Int -> Int
-- naive definition
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)Let and with
let vs with
let x = 2 in
x*x + 3*x + 1
sumsquare x y = square x + square y
where
square u = u * uComprehensions
Comprehensions
squares = [ x * x | x <- [0..] ]
-- "nested 'for' loop"
pairs = [(x,y) | x <- [1..10], y <- [1..10]]Python’s comprehensions come from Haskell
Primes example
primes = filterPrime [2..]
where filterPrime (p:xs) =
p : filterPrime [x | x <- xs, x `mod` p /= 0](from http://haskell.org)
Tuples
Tuples
(a, b)is a pair(a, b ,c)is a triple- etc
("Life", 42) :: (String, Int)
(False, 'a', 1.1) :: (Bool, Char, Float)Pattern matching
Pattern matching
take :: Int -> [a] -> [a]
take 0 _ = []
take _ [] = []
take n (x:xs) = x : take (n-1) xsCase statement
data Arch = X86 | X86_64 | ARMv7
case arch of
X86 -> "/usr/lib"
X86_64 -> "/usr/lib64"
otherwise -> "/usr/lib"Types
Static type safety
vs
- Python backtraces
- Java NullPointerException
Type inference
type annotations are generally optional
Data types
data Maybe a = Just a | Nothing
data Bool = False | True
-- record types
data Point = Pt Float Float
pt = Pt 1.0 2.0
-- recursive datatype
data Tree a = Leaf a | Branch (Tree a) (Tree a)Monads and IO
IO Monad
putStrLn :: String -> IO ()
getLine :: IO Stringdo blocks
do
let greet = "Hello"
name <- getLine
putStrLn $ greet ++ " Your name is " ++ nameHello World
$ cat > test.hs
main = putStrLn "hello"
$ ghc test.hs
$ ./test
hello
$ Concurrency
Lightweight Threads
in GHC runtime
forkIO :: IO () -> IO ThreadIdasync
https://hackage.haskell.org/package/async
(page1, page2) <- concurrently (getURL url1) (getURL url2)Software Transactional Memory
concurrency control mechanism for shared memory,
analogous to database translations
Shell
turtle shell
#!/usr/bin/env runhaskell
-- #!/bin/bash
import Turtle --
--
main = do --
dir <- pwd -- DIR=$(pwd)
time <- datefile dir -- TIME=$(date -r $DIR)
print time -- echo $TIMEProjects
pandoc
markup conversion tool
eg markdown to html, etc
These slides created with pandoc: http://github.com/juhp/presentations
xmonad
X tiling Window Manager
Stackage and stack
Stable consistent version subsets of Hackage packages
shake
Build system library to replace Make
hledger
accounting
git-annex
manage files across systems with git
without checking them in
gtk2hs
gtk stack bindings
Agda and idris
Dependently-typed programming languages
hakyll
Static website generator
yi
Extensible customizable editor (like Emacs)
Web frameworks
- Yesod
- Snap
- Happstack
Scotty
Warp webserver can
“provide performance on a par with nginx”
and “three times faster than Node”.
Fedora Haskell
Fedora Haskell SIG
- IRC: #fedora-haskell
Fedora Haskell Packaging
- > 300 source packages
- cabal-rpm packaging tool
- copr repos
Resources
Resources
- http://haskell.org
- https://wiki.haskell.org/Tutorials
- School of Haskell
- Haskell Communities and Activities Report
- StackOverflow
Haskell Reddit
- mailing-lists:
beginnersandhaskell-cafe irc: #haskell-beginners and #haskell
Thank you
Questions?
Contact: petersen@fedoraproject.org (juhp)