LYAH-ch-4-5.md

LYAH Chapters 5 & 6

Homework

• How did it go?
• Head/Tail/Init/Last: https://gist.github.com/gabebw/a6284db3d96612aae5a5
• Pattern Matching: https://gist.github.com/gabebw/a837caca1869ab4daafa
• Bonus: Fill in the Types: https://gist.github.com/gabebw/039ca8c2199372b811ba

Chapter 5: Recursion

http://learnyouahaskell.com/recursion

• Recursive base case

• Infinite recursion (take 5 $ repeat 3) - works because Haskell is lazy

• Pattern matching goes really well with recursion because the base case (or, in the book, edge condition) is more obvious

maximum' :: (Ord a) => [a] -> a
-- Error :(
maximum' [] = error "maximum of empty list"
-- Base case
maximum' [x] = x
-- Do the work
maximum' (x:xs) = max x (maximum' xs)

• With pattern matching:

zip' :: [a] -> [b] -> [(a,b)]
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y):zip' xs ys

Chapter 6: Higher Order Functions

http://learnyouahaskell.com/higher-order-functions

• Currying - (a -> a -> a) = a -> (a -> a)
• Higher order functions, map, filter
• Sections: (+ 4)
• Lambdas
• Folds (http://forum.upcase.com/t/reading-group-learn-you-a-haskell-part-two/1946/6)
• Function application with $
• Function composition with .
  • comparison: http://stackoverflow.com/questions/22793956/difference-between-composition-and-function-application
• Pointfree style
  • f xs = head xs == f = head, because laziness and a -> b -> c is really a -> (b -> c)

Next Week

• Just 1 chapter: "Modules"
• HW: https://exercises.upcase.com/exercises/pointfree-style

Exercism.io

• Really hard to navigate the website
• brew install exercism
• exercism configure --key=API_KEY
• exercism fetch haskell
• README tells you how to run tests
• exercism submit MyCoolThing.hs