15-Jan-15 More Haskell Functions Maybe, Either, List, Set, Map.

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15-Jan-15 More Haskell Functions Maybe, Either, List, Set, Map Slide 2 Maybe find (imported from Data.List ) takes a predicate and a list, and returns the first element that satisfies the predicate Example: find (> 4) [1..10] But what if there is no such element? find (> 40) [1..10] In such a case, Java would return null Any Java method that supposed to return an object might instead return null Thus, it is common to get NullPointerException In Haskell, find returns a Maybe find :: (a -> Bool) -> [a] -> Maybe a A Maybe can have the value Nothing or Just something find (>4) [1..10] Just 5 find (> 40) [1..10] Nothing This works well when combined with pattern matching Slide 3"No, sorry!" GHCi> tell (Just "bananas") "Yes, we have bananas" GHCi> tell Nothing "No, sorry!""> Using Maybe Maybe is a monad For now, you can just think of it as a wrapper The usual way to deal with monads like this is with a case expression tell :: Maybe String -> String tell arg = case arg of Just s -> "Yes, we have " ++ s Nothing -> "No, sorry!" GHCi> tell (Just "bananas") "Yes, we have bananas" GHCi> tell Nothing "No, sorry!" Slide 4buy "lamp" 15 Left "Purchased lamp" *Main> buy "sofa" 300 Right 300">Int -> Either String Int buy item cost = if cost < 20 then Left ("Purchased " ++ item) else"> Either Either takes two types: Either a b buy :: String -> Int -> Either String Int buy item cost = if cost < 20 then Left ("Purchased " ++ item) else Right cost *Main> buy "lamp" 15 Left "Purchased lamp" *Main> buy "sofa" 300 Right 300 Slide 5 Int -> String tell_if_bought ite">Int -> String tell_if_bought item price = case buy item price of Left s -> "Yes, " ++ s Right i -> "$" ++ (show price) ++ " was too expensive." GHCi> tell_if_bought "Sofa" 300 "$300 was too expensive." GHCi> tell_if_bought "lamp" 16 "Yes, Purchased lamp"">Int -> String tell_if_bought ite" title="Using Either buy item cost = if cost < 20 then Left("Purchased " ++ item) else Right cost tell_if_bought :: String -> Int -> String tell_if_bought ite"> Using Either buy item cost = if cost < 20 then Left("Purchased " ++ item) else Right cost tell_if_bought :: String -> Int -> String tell_if_bought item price = case buy item price of Left s -> "Yes, " ++ s Right i -> "$" ++ (show price) ++ " was too expensive." GHCi> tell_if_bought "Sofa" 300 "$300 was too expensive." GHCi> tell_if_bought "lamp" 16 "Yes, Purchased lamp" Slide 6 Modules A Haskell module is like a Java package A module contains functions, types, and typeclasses Unlike Java, there are a lot of name collisions, so modules often have to be imported in a qualified way To import into GHCi, use :m + module... module To import into a program, use import module import module ( f1,..., fn ) will import only the named functions import module hiding ( f1,..., fn ) will import all but the named functions import qualified module imports the module; we call an imported function fn with module. fn import qualified module as M imports the module; we call an imported function fn with M. fn Slide 7 Typeclasses A Haskell typeclass is like a Java interface--it tells what functions an object can support Some typeclasses and what they support: Eq -- == and /= Ord -- = > Num -- + - * / and others Show -- show (enables printing as a string) Read -- read (conversion from a string to something else) Functor -- fmap (enables mapping over things) List s belong to the Functor typeclass Monad -- >>= >> return fail Slide 8[[a]] -> [a] intercalate " and " ["one", "two", "three"] "one and two and three" transpose :: [[a]] -> [[a]] transpose [[1,2,3],[4,5,6]] [[1,4],[2,5],[3,6]] take 5 ( iterate (* 2) 1) [1,2,4,8,16] take 5 ( drop 5 (iterate (* 2) 1)) [32,64,128,256,512] take 5 $ drop 5 $ iterate (* 2) 1 [32,64,128,256,512] takeWhile (/= ' ') "Hello there" "Hello" dropWhile (/= ' ') "Hello there" " there"">[a] -> [a] intersperse ' ' "hel"> Data.List I The standard Prelude imports many Data.List functions for us: map, filter, foldl, etc. intersperse :: a -> [a] -> [a] intersperse ' ' "hello" "h e l l o intercalate :: [a] -> [[a]] -> [a] intercalate " and " ["one", "two", "three"] "one and two and three" transpose :: [[a]] -> [[a]] transpose [[1,2,3],[4,5,6]] [[1,4],[2,5],[3,6]] take 5 ( iterate (* 2) 1) [1,2,4,8,16] take 5 ( drop 5 (iterate (* 2) 1)) [32,64,128,256,512] take 5 $ drop 5 $ iterate (* 2) 1 [32,64,128,256,512] takeWhile (/= ' ') "Hello there" "Hello" dropWhile (/= ' ') "Hello there" " there" Slide 9 Data.List II The following are especially helpful when dealing with text: span isLetter "one two three" ("one"," two three") break isSpace "one two three" ("one"," two three") words "Here are some words." ["Here","are","some","words."] unwords $ words "Here are some words." "Here are some words." lines "Roses are red\nViolets are blue" ["Roses are red","Violets are blue"] unlines $ lines "Roses are red\nViolets are blue" "Roses are red\nViolets are blue\n" Slide 10 Data.Char Predicates: isControl isSpace (any whitespace) isLower, isUpper isAlpha, isAlphaNum, isDigit isPunctuation and others Conversions: toUpper, toLower, toTitle digitToInt, intToDigit ord, chr Slide 11lookup 3 nums Just "three" Using a Map: *Main> let dict = Map.fromList nums *Main> dict fromList [(1,"one"),(2,"two"),(3,"three"),(4,"four"),(5,"five")] *Main> :t Map.fromList Map.fromList :: (Ord k) => [(k, a)] -> Map.Map k a *Main> Map.lookup 3 dict Just "three" *Main> Map.lookup 7 dict Nothing">let nums = [(1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five""> Data.Map Maps are constructed from lists of 2-tuples Not using a Map: *Main> let nums = [(1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five")] *Main> lookup 3 nums Just "three" Using a Map: *Main> let dict = Map.fromList nums *Main> dict fromList [(1,"one"),(2,"two"),(3,"three"),(4,"four"),(5,"five")] *Main> :t Map.fromList Map.fromList :: (Ord k) => [(k, a)] -> Map.Map k a *Main> Map.lookup 3 dict Just "three" *Main> Map.lookup 7 dict Nothing Slide 12 Map operations I Maps in Haskell are implemented with binary trees, not with hash tables Hence, keys must belong to the Ord typeclass Map.empty -- returns an empty map Map.null map -- tests if a map is empty Map.singleton key value -- returns a map with one key/value pair Map.fromList list -- given a list of 2-tuples, returns a map Note: Only the last value is kept if a key is repeated Map.insert key value map -- inserts a key/value pair Map.size map -- returns the number of key/value pairs Map.member key -- tests if the key is in the map Map.lookup key -- returns Just value or Nothing Slide 13 Map operations II Map.map f map -- returns a map in which f has been applied to each value Map.filter f map -- returns a map containing only those key/value pairs for which f value is True Map.keys map -- returns a list of keys Map.elems map -- returns a list of values Map.toList map -- returns a list of (key, value) 2-tuples Map.fromListWith f list -- given a list of 2-tuples, returns a map; f is applied to combine duplicate values for the same key Map.insertWith f key value -- inserts the key/value pair into the map, using the function f to combine duplicate values for the same key Slide 14 Sets in Haskell Sets, like Maps, are constructed from lists The import should be qualified to avoid name collisions: import qualified Data.Set as Set This is also true for Maps: import qualified Data.Map as Map Set.fromList list -- returns a set created from a list (duplicates are removed) Set.toList set -- returns an ordered list from a set Slide 15 Set operations Set.empty Set.null set Set.member value set Set.union set1 set2 Set.intersection set1 set2 Set.difference set1 set2 Set.size set Set.singleton value Set.insert value set Set.delete value set Set.map f set Set.filter f set Slide 16 Compiling a Haskell program On UNIX (including Linux and Mac OS): Compile with ghc --make filename (omit the.hs ) Run with./filename On Windows: Set the PATH environment variable to something like C:\ghc\ghc- 6.6\bin Compile with ghc inputfile -o outputfile Also works on a Mac compiling hello.hs results in hello.hi, hello.o, and main.exe Run with outputfile.exe Running as an interpreted program, without compiling: runhaskell filename.hs Slide 17 The End