More Haskell Functions Maybe Either List Set Map

More Haskell Functions Maybe, Either, List, Set, Map 19 -Feb-21

Maybe n find (imported from Data. List) takes a predicate and a list, and returns the first element that satisfies the predicate n n But what if there is no such element? n n n Any Java method that supposed to return an object might instead return null Thus, it is common to get Null. Pointer. Exception In Haskell, find returns a Maybe n n find (> 40) [1. . 10] In such a case, Java would return null n n Example: find (> 4) [1. . 10] find : : (a -> Bool) -> [a] -> Maybe a A Maybe can have the value Nothing or Just something n n n find (>4) [1. . 10] Just 5 find (> 40) [1. . 10] Nothing This works well when combined with pattern matching

Using Maybe n Maybe is a monad n n n 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!"

Either n n 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

Using Either n n 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"

Modules n A Haskell module is like a Java package n n A module contains functions, types, and typeclasses Unlike Java, there 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 n n import module (f 1, . . . , fn) will import only the named functions import module hiding (f 1, . . . , 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 n n A Haskell typeclass is like a Java interface--it tells what functions an object can support Some typeclasses and what they support: n n n 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) n n Lists belong to the Functor typeclass Monad -- >>= >> return fail

Data. List I n n The standard Prelude imports many Data. List functions for us: map, filter, foldl, etc. intersperse : : a -> [a] n n intercalate : : [a] -> [[a]] -> [a] n n n intercalate " and " ["one", "two", "three"] "one and two and three" transpose : : [[a]] -> [[a]] n n intersperse ' ' "hello" "h e l l o” 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] take. While (/= ' ') "Hello there" "Hello" drop. While (/= ' ') "Hello there" " there"

Data. List II n n n n The following are especially helpful when dealing with text: span is. Letter "one two three" ("one", " two three") break is. Space "one two three" ("one", " two three") words "Here are some words. " ["Here", "are", "some", "words. "] unwords $ words "Here are some words. " lines "Roses are redn. Violets are blue" ["Roses are red", "Violets are blue"] unlines $ lines "Roses are redn. Violets are blue" "Roses are redn. Violets are bluen"

Data. Char n Predicates: n n n n is. Control is. Space (any whitespace) is. Lower, is. Upper is. Alpha, is. Alpha. Num, is. Digit is. Punctuation and others Conversions: n n n to. Upper, to. Lower, to. Title digit. To. Int, int. To. Digit ord, chr

Data. Map n n Maps are constructed from lists of 2 -tuples Not using a Map: n n n *Main> let nums = [(1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five")] *Main> lookup 3 nums Just "three" Using a Map: n n n *Main> let dict = Map. from. List nums *Main> dict from. List [(1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five")] *Main> : t Map. from. List : : (Ord k) => [(k, a)] -> Map k a *Main> Map. lookup 3 dict Just "three" *Main> Map. lookup 7 dict Nothing

Map operations I n Maps in Haskell are implemented with binary trees, not with hash tables n n n 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. from. List list -- given a list of 2 -tuples, returns a map n n n Hence, keys must belong to the Ord typeclass 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 n n n n 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. to. List map -- returns a list of (key, value) 2 -tuples Map. from. List. With f list -- given a list of 2 -tuples, returns a map; f is applied to combine duplicate values for the same key Map. insert. With 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 n Sets, like Maps, are constructed from lists n n 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. from. List list -- returns a set created from a list (duplicates are removed) Set. to. List set -- returns an ordered list from a set

Set operations n n n Set. empty Set. null set Set. member value set Set. union set 1 set 2 Set. intersection set 1 set 2 Set. difference set 1 set 2 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 n On UNIX (including Linux and Mac OS): n n n Compile with ghc --make filename (omit the. hs) Run with. /filename On Windows: n n Set the PATH environment variable to something like C: ghc 6. 6bin Compile with ghc inputfile -o outputfile n n 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: n runhaskell filename. hs

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