Haskell Chapter 8

Input and Output  What are I/O actions?  How do I/O actions enable us to do I/O?  When are I/O actions actually performed?

Review of Functional Language characteristics  Haskell is purely functional  Provide definitions of what things are, Haskell figures out the steps to execute  Functions are not allowed to have side effects  If a function is called two times with the same parameters, it must return the same result (referential transparency)  Helps us reason about our programs (e.g., don’t have to trace entire program to see if some other function is messing with your variables!)

I/O  But what about showing the result? That changes the state of the screen… a side effect!  What to do???  Separate the “impure” parts of the program from the “pure” parts. Like FP on steroids

Our first program main = putStrLn "Hello World!" At the command prompt  ghc --make helloworld  helloworld (windows) or./helloworld (nix)

What’s putStrLn?  Prelude> :t putStrLn  putStrLn :: String -> IO ()  Prelude> :t putStrLn "hello, world"  putStrLn "hello, world" :: IO ()  putStr take a String, returns an IO action  An IO action does some action with a side effect (e.g., read input or write output) and also yields a result  The value of putStrLn is just ()

How does this work in GHCi?  *Main> putStrLn "howdy pardner"  howdy pardner  In GHCi, it performs the IO action and then prints it to the terminal using putStrLn

When are I/O actions performed?  Must give it a name (e.g., main) and then run the program  Can glue several I/O actions into one.  Don’t usually specify a type declaration for main – it has signature of main :: IO something where something is a concrete type, for example:  *Main> :t main  main :: IO ()  getLine is an IO action that yields a String  getLine is impure  *Main> :t getLine  getLine :: IO String

So what exactly is an IO action??  You can think of it as a box with feet.  Goes out into the world and does something  Maybe brings something back  Use <- to “open” the box and get the value  We can only take it out when we’re inside another IO action  This won’t work!!  nameTag = "Hello, my name is " ++ getLine

How do the pure and impure parts interact? main2 = do putStrLn "Hello, what's your name?" name <- getLine putStrLn $ "Zis is your future: " ++ tellFortune name tellFortune name | name == "Lucky" = name ++ ",you will win a million dollars" | otherwise = name ++ ", you will have no money but be happy"  *Main> main2  Hello, what's your name?  Lucky  Zis is your future: Lucky,you will win a million dollars  tellFortune is just a regular function!  *Main> tellFortune "Cyndi"  "Cyndi, you will have no money but be happy"

Bindings import Data.Char main3 = do putStrLn "What's your first name? " firstName <- getLine putStrLn "What's your last name? " lastName <- getLine let bigFirstName = map toUpper firstName let bigLastName = map toUpper lastName putStrLn $ "hey " ++ bigFirstName ++ " " ++ bigLastName ++ ", how are you?“  <- binds values from IO actions to names  let binds pure expressions to names

More details - return  return() is not like other languages.   return makes an I/O action out of a pure value (i.e., takes a value and “wraps” it in an IO action box). Return does not end execution. main4 = do line <- getLine if null line then return () else do putStrLn $ reverseWords line main4 reverseWords :: String -> String reverseWords = unwords. map reverse. words remember: if must always have else

Understanding return main5 = do return () return "HAHAHA" line <- getLine return "BLAH BLAH" return 4 putStrLn line  Creates lots of I/O actions that are not used

More on return main6 = do a <- return "Hello" b <- return "World" putStrLn $ a ++ " " ++ b  More likely to use let in this scenario… but it illustrates how return works  Common to use return when last statement of do doesn’t have desired result. So create a return that yields the desired result, put it at the end  Also used when we need an IO action that doesn’t do anything (like the return () in main4) Quick trace: what is this doing?

Other useful functions  putStr – like putStrLn, but no newline  putChar – takes a character and returns an IO action that will print it to the terminal - putChar 'a'  print – takes a value that’s an instance of Show (i.e., we know how to represent as string), applies show to stringify it, outputs string to terminal - print "haha"

when  when – syntactic sugar. Takes Bool & I/O, returns I/O  when :: Monad m => Bool -> m () -> m () import Control.Monad main7 = do input <- getLine if (input == "SWORDFISH") then putStrLn input else return () main8 = do input <- getLine when (input == "SWORDFISH") $ do putStrLn input

sequence  sequence takes a list of I/O actions, returns an I/O action that performs those actions one after the other main9 = do a <- getLine b <- getLine c <- getLine print [a,b,c] main10 = do rs <- sequence [getLine, getLine, getLine] print rs

mapM  Common scenario: map a function that returns an I/O action (e.g., print) over a list and then sequence it. mapM does this for us, mapM_ does it but throws away the result.  *Main> mapM print [1,2,3]  1  2  3  [(),(),()]  *Main> mapM_ print [1,2,3]  1  2  3

Other useful functions in book  forever  forM

Play and Share?  Let’s start on the homework