Haskell Chapter 6

Modules  A module defines some functions, types, and type classes  A program is a collection of modules  Module used in GHCi is Prelude (you can import into your own files, to get the same environment)  Programming Language issues:  How well does the language help programmers organize their code (to support larger projects)  How does the language support/encourage reuse?  Accessing functionality: challenging for novice programmers (remember getting linker errors?)

Importing  imports must be before function definitions, normally at the top of the file  Each import statement goes on a separate line  Ex:  import Data.List  import Data.List (nub, sort)  What if you have your own function?  import Data.List hiding (nub)  What if same function (e.g., filter) in 2 modules?  import qualified Data.Map  then must specify Data.Map.filter  Must do for every function from module – tedious! so:  import qualified Data.Map as M  now do M.filter

More on modules and imports  To find useful functions:   In GHCi, add imports  :m + Data.List  :m + Data.List Data.Map Data.Set  After load file  :browse to get a list of functions

Some Examples  Use Data.List functions to count how many times each word appears in a string  Use Data.List functions to see if one list is wholly contained in another  Use Data.Char functions to encode using a Caesar cipher  Book has more

Count Words  Uses words, group, and sort from Data.List import Data.List -- function composition (.) in chapter 5 wordNums :: String -> [(String, Int)] wordNums = map (\ws -> (head ws, length ws)). group. sort. Words wordNums' :: String -> [(String, Int)] wordNums' str = map (\ws -> (head ws, length ws)) (group (sort (words str))) Quick Ex: Play with the individual functions on this slide and the next two… Figure out what words, group, sort, isPrefixOf, any, tails, or and chr do Example: type words "hi ho hi ho to work I go" nothing to submit

Needle in Haystack  Uses tails, isPrefixOf and any from Data.List isIn :: (Eq a) => [a] -> [a] -> Bool needle `isIn` haystack = any (needle `isPrefixOf`) (tails haystack)

Caesar Cipher  Uses ord and chr from Data.Char encode :: Int -> String -> String encode offset msg = map (\c -> chr $ ord c + offset) msg  Or use composition encode' :: Int -> String -> String encode' offset msg = map (chr. (+ offset). ord) msg  Can decode as: decode :: Int -> String -> String decode shift msg = encode (negate shift) msg

Create your own Modules  module moduleName  ( list of functions to export)  may also include “helper” functions that are not exported  must be in the same folder as the module that’s importing it

Example module Geometry ( sphereVolume, sphereArea, cubeVolume, cubeArea, cuboidArea, cuboidVolume,) where sphereVolume :: Float -> Float sphereVolume radius = (4.0 / 3.0) * pi * (radius ^3) sphereArea :: Float -> Float sphereArea radius = 4 * pi * (radius ^ 2) cubeVolume :: Float -> Float cubeVolume side = cuboidVolume side side side cubeArea :: Float -> Float cubeArea side = cuboidArea side side side cuboidVolume :: Float -> Float -> Float -> Float cuboidVolume a b c = rectArea a b * c cuboidArea :: Float -> Float -> Float -> Float cuboidArea a b c = rectArea a b * 2 + rectArea a c * 2 + rectArea c b * 2 rectArea :: Float -> Float -> Float rectArea a b = a * b

Modules can also be hierarchical Sphere.hs Cuboid.hs module Geometry.Sphere ( volume, area ) where volume :: Float -> Float volume radius = (4.0 / 3.0) * pi * (radius ^3) area :: Float -> Float area radius = 4 * pi * (radius ^ 2) module Geometry.Cuboid ( volume, area ) where volume :: Float -> Float -> Float -> Float volume a b c = rectArea a b * c area :: Float -> Float -> Float -> Float area a b c = rectArea a b * 2 + rectArea a c * 2 + rectArea c b * 2 rectArea :: Float -> Float -> Float rectArea a b = a * b

Hierarchical modules continued Cube.hs Using module Geometry.Cube ( volume, area ) where import qualified Geometry.Cuboid as Cuboid volume :: Float -> Float volume side = Cuboid.volume side side side area :: Float -> Float area side = Cuboid.area side side side import qualified Geometry.Sphere as Sphere import qualified Geometry.Cuboid as Cuboid import qualified Geometry.Cube as Cube sphereDemo radius = "The area of a sphere with radius " ++ show radius ++ " is " ++ show (Sphere.area radius)