teaching machines

CS 330 Lecture 23 – Guards, Tail Recursion, and Higher-order Functions

April 1, 2013 by Chris Johnson. Filed under cs330, lectures, spring 2013.

Agenda

• what ?s
• replicate
• program this
• guards
• tail recursion
• higher-order functions
• map
• filter
• lambda

TODO

• Read http://learnyouahaskell.com/higher-order-functions. Quarter sheet.

Program This

Code

today.hs

import Data.Char

replicate' item 0 = []
replicate' item n = item:replicate' item (n - 1)

isIn [] needle = False
-- isIn haystack needle = if head haystack = needle then True else isIn (tail haystack) needle 
-- isIn (first:rest) needle = if first = needle then True else isIn rest needle 
isIn (first:rest) needle
  | first == needle = True
  | otherwise = isIn rest needle
-- isIn (first:rest) needle = first == needle || isIn rest needle 

eachLower [] = []
eachLower (first:rest) = toLower first:eachLower rest

eachMult10 [] = []
eachMult10 (first:rest) = first * 10:eachMult10 rest

eachDecapitate [] = []
eachDecapitate ((first:_):rest) = first:eachDecapitate rest

map' f [] = []
map' f (first:rest) = f first:map' f rest

eachLower' list = map' toLower list

-- mult10 a = 10 * a 
-- eachMult10' list = map' mult10 list 
eachMult10' list = map' (\a -> 10 * a) list
-- eachMult10' list = map' (*10) list

eachDecapitate' list = map' head list

filter' f [] = []
filter' f (first:rest)
  | f first = first:filter' f rest
  | otherwise = filter' f rest

Haiku