SLIDE 1
1
2
1 2 3 4 printChar 5 printChar 6 printChar xs = [printChar - - PowerPoint PPT Presentation
1 2 3 4 printChar 5 printChar 6 printChar xs = [printChar - - PowerPoint PPT Presentation
1 2 3 4 printChar 5 printChar 6 printChar xs = [printChar a, printChar b] printChar 7 8 9 type IO = World -> World type IO a = World -> (a, World) b -> World -> (a, World) 10 IO a IO Char IO ()
SLIDE 2
SLIDE 3
3
SLIDE 4
4
SLIDE 5
5
printChar
…
SLIDE 6
6
printChar
SLIDE 7
7
printChar xs = [printChar ’a’, printChar ’b’]
…
printChar
SLIDE 8
8
SLIDE 9
9
SLIDE 10
10
type IO = World -> World type IO a = World -> (a, World) b -> World -> (a, World)
SLIDE 11
11
IO a IO Char IO ()
SLIDE 12
12
getChar :: IO Char putChar :: Char -> IO () return :: a -> IO a
SLIDE 13
13
(>>=) :: IO a -> (a -> IO b) -> IO b (act1 >>= act2) = \world -> case act1 world of (v, world’) -> act2 v world’ (>>) :: IO a -> IO b -> IO b (act1 >> act2) = \world -> case act1 world of (v, world’) -> act2 world’
SLIDE 14
14
getLine :: IO String getLine = getChar >>= \x -> if x == ‘\n’ then return [] else (getLine >>= \xs -> return (x:xs))
- putStr :: String → IO ()
putStr [] = return () putStr (x:xs) = putChar x >> putStr xs
- putStrLn :: String → IO ()
putStrLn xs = putStr xs >> putChar '\n'
SLIDE 15
15
getLine :: IO String getLine = do x <- getChar if x == ‘\n’ then return [] else do xs <- getLine return (x:xs)
- putStr :: String → IO ()
putStr [] = return () putStr (x:xs) = do putChar x putStr xs
- putStrLn :: String → IO ()
putStrLn xs = do putStr xs putChar ‘\n’
SLIDE 16
16
strlen :: IO () strlen = putStr "Enter a string: ” >> getLine >>= \xs -> putStr "The string has ” >> putStr (show (length xs)) >> putStrLn " characters."
SLIDE 17
17
strlen :: IO () strlen = do putStr “Enter a string: ” xs <- getLine putStr “The string has ” putStr (show (length xs)) putStrLn “ characters.”
SLIDE 18
18
return :: a -> IO a (>>=) :: IO a -> (a -> IO b) -> IO b getChar :: IO Char putChar :: Char -> IO ()
- penFile :: [Char] -> IOMode -> IO Handle
- return
<-
SLIDE 19
19
main :: IO () main = getLine >>= \cs -> putLine (reverse cs)
SLIDE 20
20
SLIDE 21
21
SLIDE 22
22
SLIDE 23
23
f :: a -> Maybe b -- returns Just b or Nothing data Maybe a = Nothing | Just a
SLIDE 24
24
doQuery :: Query -> DB -> Maybe Record r :: Maybe Record r = case doQuery q1 db of Nothing -> Nothing Just r1 -> case doQuery (q2 r1) db of Nothing -> Nothing Just r2 -> case doQuery (q3 r2) db of Nothing -> Nothing Just r3 -> . . .
SLIDE 25
25
thenMB :: Maybe a -> (a -> Maybe b) -> Maybe b mB `thenMB` f = case mB of Nothing -> Nothing Just a -> f a r :: Maybe Record r = doQuery q1 db `thenMB` \r1 -> doQuery (q2 r1) db `thenMB` \r2 -> doQuery (q3 r2) db `thenMB` . . .
SLIDE 26
26
(>>=) :: [a] -> (a -> [b]) -> [b] concatMap :: (a -> [b]) -> [a] -> [b] instance Monad [] where m >>= f = concatMap f m return x = [x]
SLIDE 27
27
SLIDE 28
28
return :: a -> M a (>>=) :: M a -> (a -> M b) -> M b return x >>= f == f x -- left identity m >>= return == m -- right identity (m >>= f) >>= g == m >>= (\x -> f x >>= g) -- associativity
SLIDE 29
29
do { x’ <- return x; f x’ == do { f x } }
SLIDE 30
30
do { x <- m; return x == do { m } }
SLIDE 31
31
do { y <- do { x <- m; == do { x <- m; f x y <- f x; } g y g y } }
SLIDE 32
32
class Monad m where >>= :: m a -> (a -> m b) -> m b >> :: m a -> m b -> m b return :: a -> m a m >> k = m >>= \_ -> k data Maybe a = Just a | Nothing thenMB :: Maybe a -> (a -> Maybe b) -> Maybe b instance Monad Maybe where (>>=) = thenMB return a = Just a
SLIDE 33
33
class Monad m where >>= :: m a -> (a -> m b) -> m b >> :: m a -> m b -> m b return :: a -> m a m >> k = m >>= \_ -> k sequence :: Monad m => [m a] -> m [a] sequence [] = return [] sequence (c:cs) = c >>= \x -> sequence cs >>= \xs -> return (x:xs)
SLIDE 34
34