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 programmain = 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 <- getLineputStrLn $ "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 returnmain5 = do

return ()return "HAHAHA"line <- getLinereturn "BLAH BLAH"return 4putStrLn line

Creates lots of I/O actions that are not used

More on returnmain6 = 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.Monadmain7 = do

input <- getLineif (input == "SWORDFISH")

then putStrLn inputelse return ()

main8 = do input <- getLinewhen (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 = doa <- getLineb <- getLinec <- getLineprint [a,b,c]

main10 = dors <- 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

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