Haskell Workshop: Basic Haskell

Haskell Workshop teaches you how to read types and typeclasses.

Presented by Ryan Trinkle
Slides by Kat Chuang

Transcript

1. Getting Started with Haskell Haskell Beginners’ Workshop NY Haskell Meetup November 12, 2013
2. Why Haskell? • Expressive • Declarative • Strongly typed • Easy to refactor • Referentially transparent
3. Project Euler #1 Multiples of 3 and 5 Haskell sum [x | x <- [1..999] , x `mod` 3 == 0 || x `mod` 5 == 0] C int main(void) { int total=0; int i=0; while(i<1000) { if(i%3==0 || i%5==0) total+=i; i++; } printf("%d", total); } Python def multiples_of_3_or_5(): for number in xrange(1000): if not number % 3 or not number % 5: yield number print sum(multiples_of_3_or_5()) Or, you can use list comprehension print sum( number for number in xrange(1000) if not (number % 3 and number % 5) )
4. Reading types "Hello, world!" :: String value type
5. Basic types Text • 'a' :: Char • "asdf" :: String Numbers • 1 :: Integer • 0.5 :: Double Booleans • True :: Bool • False :: Bool Functions • square :: Integer -> Integer
6. Containers Lists • [1, 2, 3] :: [Integer] • ["Hello", "Goodbye"] :: [String] • [True, False, True] :: [Bool] Tuples • (1, 2) :: (Integer, Integer) • (1, "Hello", True) :: (Integer, String, Bool)
7. Containers Tuples of lists and vice versa • [(1, "Hello"), (2, "Goodbye")] :: [(Integer, String)] • ([1, 2, 3], ["Hello", "Goodbye"]) :: ([Integer], [String])
8. GHCi Exercise - Type these commands Prelude> :t 'a' 'a' :: Char Prelude> :t "Hello" "Hello" :: [Char] Why is it [Char], not String? type String = [Char]
9. GHCi Exercise - Type these commands Prelude> :t 1 1 :: Num a => a Prelude> :t 0.5 0.5 :: Fractional a => a Why? 1 can be used with any numeric type; 0.5 can be used with any numeric type that supports fractions
10. Reading types 1 :: Num a => a "1 can have any type a, where a is a numeric type" The context comes before the =>, and tells us about the type that follows. value type context
11. Reading types 1 :: Num a => a Num is a typeclass, not a type. A typeclass tells you about a type. Num a tells you that the type a must support numeric operations like +, *, and - value type context
12. Functions Prelude> let square x = x^2 Prelude> square 5 25 Prelude> :t square square :: Num a => a -> a
13. Reading types square :: Num a => a -> a "square is a function that takes an a and returns an a, where a is a numeric type." Note: the input and output type must match: square cannot return a Float if you give it an Int. value input output context type
14. GHCi Prelude> :i Num You can use :i to get information about something in GHCi. For a typeclass, :i will give you its operations and a list of types that are its instances. 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'
15. Books learnyouahaskell.com book.realworldhaskell.org