Input and Output Actions in Haskell - Lecture Slides | CS 457, Study notes of Computer Science

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CS 457/557: Functional

Languages

I/O Actions in Haskell Mark P Jones Portland State University

Question:

If functional programs don’t

have any side-effects, then

how can we ever do anything

useful?

Demo:

… of Mac OS X Automator …

IO Actions:

An IO action is a value of type IO T T is the type of values that it produces

:: IO a

action

The New Haskell Logo:

Building Blocks:

(>>) :: IO a -> IO b -> IO b p >> q is an I/O action in which the output of p is ignored by q p >> q == p >>= \x -> q (where x does not appear in q)

Building Blocks:

inIO :: (a -> b) -> a -> IO b An action inIO f applies the function f to each input of type a and produces outputs of type b as its results

Building Blocks:

mapM :: (a -> IO b) -> [a] -> IO [b] An action mapM f takes a list of inputs of type [a] as its input, runs the action f on each element in turn, and produces a list of outputs of type [b]

Terminal Output:

putStr :: String -> IO () putStrLn :: String -> IO () An action putStr s takes a String input and outputs it on the terminal producing a result of type () putStrLn s does the same thing but adds a trailing new line

Terminal Output:

print :: Show a => a -> IO () A print action takes a value whose type is in Show and outputs a corresponding String on the terminal

Web Actions:

The WebActions module provides the following I/O actions: getText :: URL -> IO String getByteString :: URL -> IO ByteString writeByteString :: String -> ByteString -> IO () downloadTo :: FilePath -> URL -> IO () getTags :: URL -> IO [Tag] getHrefs :: URL -> IO [URL] getHTML :: URL -> IO [TagTree] getXML :: URL -> IO [Content]

Viewing a Webpage:

return url

= getText = putStr

Counting Lines:

return url

= getText = inIO (length. lines) = print

Viewing a Webpage as Tags:

return url

= getTags = inIO (unlines. map show) = putStr