teaching machines

CS 330 Lecture 26 – More Folding, Composing, Custom Data Types

April 8, 2013 by Chris Johnson. Filed under cs330, lectures, spring 2013.

Agenda

• more folding: join, length, map, find, filter
• folding on 1+ lists: min, max
• composing: countOvers, any, all
• custom data types: an expression hierarchy
• making something an Ord

TODO

• Read chapter 9 through section 9.2.2. Quarter sheet.

Code

today.hs

find' [] needle = Nothing
find' (first:rest) needle
  | (fst first) == needle = Just (snd first)
  | otherwise = find' rest needle

foldr' mix accum [] = accum
foldr' mix accum (first:rest) = mix first (foldr' mix accum rest)

and' = foldr' (&&) True
or' = foldr' (||) False
sum' = foldr' (+) 0
product' = foldr' (*) 1
join' = foldr' (++) []
join'' = foldr' (++) ""
length' = foldr' (\item accum -> 1 + accum) 0
average' l = sum' l / length' l
map' xform = foldr' (\item accum -> xform item:accum) []
filter' predicate = foldr' (\item accum -> if predicate item then item:accum else accum) []

-- countOvers threshold los = length' (filter' (\s -> length' s > threshold) los) 
countOvers threshold = length' . filter' (\s -> length' s > threshold)


data Expr = Add Expr Expr
          | Subtract Expr Expr
          | Negate Expr
          | Literal Double
          | Variable String
          deriving (Show)

evaluate (Add e1 e2) = evaluate e1 + evaluate e2
evaluate (Subtract e1 e2) = evaluate e1 - evaluate e2
evaluate (Negate e) = -1 * evaluate e
evaluate (Literal d) = d
evaluate (Variable name) = error "I'm not there yet. :("
 
data LineSegment = LineSegment Double Double Double Double deriving (Show, Eq)

data Vowel = A | E | I | O | U deriving (Bounded, Show)

seglen (LineSegment x1 y1 x2 y2) = sqrt ((x2 - x1) ^ 2 + (y2 - y1) ^ 2)

instance Ord LineSegment where 
  compare s1 s2 = compare (seglen s1) (seglen s2)

Haiku