More Haskell Functions Maybe, Either, List, Set, Map 4-Apr-19

Maybe find 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 There’s nothing in the syntax that warns you this might happen Thus, you can get a 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

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

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

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) Lists belong to the Functor typeclass Monad -- >>= >> return fail

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"

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"

Data.Char Predicates: Conversions: isControl isSpace (any whitespace) isLower, isUpper isAlpha, isAlphaNum, isDigit isPunctuation and others Conversions: toUpper, toLower, toTitle digitToInt, intToDigit ord, chr

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

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

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

Sets in Haskell Sets, like Maps, are constructed from lists Like Maps, the import should be qualified to avoid name collisions: import qualified Data.Set as Set Set.fromList list -- returns a set created from a list (duplicates are removed) Set.toList set -- returns an ordered list from a set

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

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

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