haskell-course

Compiler Warnings

It’s a good thing to have compiler warnings enabled.

Add {-# OPTIONS_GHC -Wall #-} to the top of your file. Or add -Wall in the package.yaml file of your stack config.

Lists

• Pattern matching on lists
  • map, filter
  • Types of map and filter
  • Other functions take, drop, length, (!!), zip, zipWith
• Using currying
  • map (+1) [1,2,3] = [2,3,4]
• List comprehensions
  • triangles = [ (a,b,c) | c <- [1..10], b <- [1..10], a <- [1..10] ]
  • rightTriangles = [ (a,b,c) | c <- [1..10], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2]
  • We will revisit this concept again, as well.
• dotProduct

dotProduct xs ys = go 0 xs ys
  where
    go n [] ys = n
    go n xs [] = n
    go n (x : xs) (y : ys) = go (n + (x * y)) xs ys

dotProduct xs ys = sum $ zipWith (*) xs ys

• quicksort

quicksort (x : xs) = lesser ++ [x] ++ greater
  where
    lesser = [y | y <- xs, y <= x]
    greater = [y | y <- xs, y >= x]

Laziness

• Consider

myMult 0 a = 0
myMult b a = (a * b)

• (1/0) is undefined
  • But myMult 0 (1/0) is defined.

• [1..] does not terminate but take 3 [1..] works perfectly

• Sieve of Eratosthenes

primes = filterPrime [2..]
  where filterPrime (p:xs) =
          p : filterPrime [x | x <- xs, x `mod` p /= 0]

• repeat, iterate
  • cycle

foldl, foldr, foldl'

• foldr (\x y -> "( " ++ show x ++ " * " ++ y ++ ")") "init" [1..5]
• foldl (\x y -> "( " ++ x ++ " * " ++ show y ++ ")") "init" [1..5]
• foldr1, foldl1
• Definition of foldr

  foldr op init [] = init
  foldr op init (x:xs) = x `op` (foldr op init xs)
  

• Definition of foldl

  foldl op init [] = init
  foldl op init (x:xs) = foldl op (init `op` x) xs
  

• foldr lends itself to laziness. foldr (&&) False (repeat False)