programming in haskell

35
1 PROGRAMMING IN HASKELL Based on lecture notes by Graham Hutton The book “Learn You a Haskell for Great Goo (and a few other sources) I/O and functors

Upload: lee

Post on 26-Jan-2016

66 views

Category:

Documents


1 download

DESCRIPTION

PROGRAMMING IN HASKELL. I/O and functors. Based on lecture notes by Graham Hutton The book “ Learn You a Haskell for Great Good ” (and a few other sources). File I/O. - PowerPoint PPT Presentation

TRANSCRIPT

Page 1: PROGRAMMING IN HASKELL

1

PROGRAMMING IN HASKELL

Based on lecture notes by Graham HuttonThe book “Learn You a Haskell for Great Good”

(and a few other sources)

I/O and functors

Page 2: PROGRAMMING IN HASKELL

2

File I/OSo far, we’ve worked mainly at the prompt, and done very little true input or output. This is logical in a functional language, since nothing has side effects!

However, this is a problem with I/O, since the whole point is to take input (and hence change some value) and then output something (which requires changing the state of the screen or other I/O device.

Luckily, Haskell offers work-arounds that separate the more imperative I/O.

Page 3: PROGRAMMING IN HASKELL

3

A simple example: save the following file as helloword.hs

main = putStrLn "hello, world"

$ ghc --make helloworld  [1 of 1] Compiling Main       

( helloworld.hs, helloworld.o )  Linking helloworld ...     $ ./helloworld  hello, world 

Now we actually compile a program:

Page 4: PROGRAMMING IN HASKELL

4

What are these functions?

ghci> :t putStrLn  putStrLn :: String -> IO ()  ghci> :t putStrLn "hello, world"  putStrLn "hello, world" :: IO () 

So putStrLn takes a string and returns an I/O action (which has a result type of (), the empty tuple).

In Haskell, an I/O action is one with a side effect - usually either reading or printing. Usually some kind of a return value, where () is a dummy value for no return.

Page 5: PROGRAMMING IN HASKELL

5

An I/O action will only be performed when you give it the name “main” and then run the program.

A more interesting example:

main = do      putStrLn "Hello, what's your name?”      name <- getLine      putStrLn ("Hey " ++ name ++ ", 

you rock!")   

Notice the do statement - more imperative style. Each step is an I/O action, and these glue together.

Page 6: PROGRAMMING IN HASKELL

6

More on getLine:

ghci> :t getLine  getLine :: IO String   This is the first I/O we’ve seen that doesn’t have an empty tuple type - it has a String.

Once the string is returned, we use the <- to bind the result to the specified identifier.

Notice this is the first non-functional action we’ve seen, since this function will NOT have the same value every time it is run! This is called “impure” code, and the value name is “tainted”.

Page 7: PROGRAMMING IN HASKELL

7

An invalid example:

nameTag = "Hello, my name is " ++ getLine 

What’s the problem? Well, ++ requires both parameters to have the same type.

What is the return type of getLine?

Another word of warning: what does the following do?

name = getLine   

Page 8: PROGRAMMING IN HASKELL

8

Just remember that I/O actions are only performed in a few possible places:- A main function- inside a bigger I/O block that we have composed with a do (and remember that the last action can’t be bound to a name, since that is the one that is the return type).-At the ghci prompt:

ghci> putStrLn "HEEY"  HEEY    

Page 9: PROGRAMMING IN HASKELL

9

You can use let statements inside do blocks, to call other functions (and with no “in” part required):import Data.Char 

main = do      putStrLn "What's your first name?"      firstName <- getLine      putStrLn "What's your last name?"      lastName <- getLine      let bigFirstName = map toUpper firstName          bigLastName = map toUpper lastName      putStrLn $ "hey " ++ bigFirstName ++ " " ++ 

bigLastName ++ ", how are you?"    

Note that <- is for I/O, and let for expressions.

Page 10: PROGRAMMING IN HASKELL

10

Return in haskell: NOT like other languages.

main = do       line <- getLine      if null line          then return ()          else do              putStrLn $ reverseWords line              

main    reverseWords :: String -> String  

reverseWords = unwords . map reverse . words Note: reverseWords = unwords . map reverse . words is the same as reverseWords st = nwords (map reverse (words st))

Page 11: PROGRAMMING IN HASKELL

11

What is return?

Does NOT signal the end of execution! Return instead makes an I/O action out of a pure value.

main = do      a <- return "hell"      b <- return "yeah!"      putStrLn $ a ++ " " ++ b  

In essence, return is the opposite of <-. Instead of “unwrapping” I/O Strings, it wraps them.

Page 12: PROGRAMMING IN HASKELL

12

Last example was a bit redundant, though – could use a let instead:

main = do let a = "hell" b = "yeah" putStrLn $ a ++ " " ++ b  

Usually, you’ll use return to create I/O actions that don’t do anything (but you have to have one anyway, like an if-then-else), or for the last line of a do block, so it returns some value we want.

Page 13: PROGRAMMING IN HASKELL

13

Other I/O functions: -print (works on any type in show, but calls show first)-putStr - And as putStrLn, but no newline-putChar and getCharmain = do  print True              print 2              print "haha"              print 3.2              print [3,4,3] 

main = do         c <- getChar      if c /= ' '          then do              putChar c              main  

        else return ()  

Page 14: PROGRAMMING IN HASKELL

14

More advanced functionality is available in Control.Monad:

import Control.Monad  import Data.Char    main = forever $ do      putStr "Give me some input: "      l <- getLine      putStrLn $ map toUpper l 

(Will indefinitely ask for input and print it back out capitalized.)

Page 15: PROGRAMMING IN HASKELL

15

Other functions:•sequence: takes list of I/O actions and does them one after the other•mapM: takes a function (which returns an I/O) and maps it over a list

Others available in Control.Monad:•when: takes boolean and I/O action. If bool is true, returns same I/O, and if false, does a return instead

Page 16: PROGRAMMING IN HASKELL

16

Scripting functionality deals with I/O as a necessity.

The module System.Environment has several to help with this:•getArgs: returns a list of the arguments that the program was run with•getProgName: returns the string which is the program name

System Level programming

(Note: I’ll be assuming you compile using “ghc –make myprogram” and then running “./myprogram”.But you could also do “runhaskell myprogram.hs”.)

Page 17: PROGRAMMING IN HASKELL

17

import System.Environment import Data.List main = do args <- getArgs progName <- getProgName putStrLn "The arguments are:" mapM putStrLn args putStrLn "The program name is:" putStrLn progName

An example:

Page 18: PROGRAMMING IN HASKELL

18

$ ./arg-test first second w00t "multi word arg" The arguments are: first second w00t multi word arg The program name is: arg-test

The output:

Page 19: PROGRAMMING IN HASKELL

19

Recap of TypeclassesWe have seen typeclasses, which describe classes of data where operations of a certain type make sense.Look more closely:

class Eq a where (==) :: a -> a -> Bool (/=) :: a -> a -> Bool x == y = not (x /= y) x /= y = not (x == y)

Page 20: PROGRAMMING IN HASKELL

20

Now – say we want to make a new type and make sure it belongs to a given typeclass. Here’s how:

data TrafficLight = Red | Yellow | Green

instance Eq TrafficLight where Red == Red = True Green == Green = True Yellow == Yellow = True _ == _ = False

Page 21: PROGRAMMING IN HASKELL

21

Now maybe we want to be able to display these at the prompt. To do this, we need to add this to the “show” class. (Remember those weird errors with the trees yesterday? We hadn’t added trees to this class!)

instance Show TrafficLight where show Red = "Red light" show Yellow = "Yellow light" show Green = "Green light"

Page 22: PROGRAMMING IN HASKELL

22

And finally, we can use these things:

ghci> Red == Red True ghci> Red == Yellow False ghci> Red `elem` [Red, Yellow, Green] True ghci> [Red, Yellow, Green] [Red light,Yellow light,Green light]

Page 23: PROGRAMMING IN HASKELL

23

FunctorsFunctors are a typeclass, just like Ord, Eq, Show, and all the others. This one is designed to hold things that can be mapped over; for example, lists are part of this typeclass.

class Functor f where      fmap :: (a -> b) -> f a -> f b

This type is interesting - not like previous exmaples, like in EQ, where (==) :: (Eq a) => a -> a -> Bool. Here, f is NOT a concrete type, but a type constructor that takes one parameter.

Page 24: PROGRAMMING IN HASKELL

24

Compare fmap to map:

fmap :: (a -> b) -> f a -> f b

map :: (a -> b) -> [a] -> [b]   

So map is a lot like a functor! Here, map takes a function and a list of type a, and returns a list of type b. In fact, can define map in terms of fmap:

instance Functor [] where      fmap = map   

Page 25: PROGRAMMING IN HASKELL

25

Notice what we wrote:

instance Functor [] where      fmap = map   

We did NOT write “instance Functor [a] where…”, since f has to be a type constructor that takes one type. Here, [a] is already a concrete type, while [] is a type constructor that takes one type and can produce many types, like [Int], [String], [[Int]], etc.

Page 26: PROGRAMMING IN HASKELL

26

Another example:

instance Functor Maybe where      fmap f (Just x) = Just (f x)      fmap f Nothing = Nothing   Again, we did NOT write “instance Functor (Maybe m) where…”, since functor wants a type constructor.

Mentally replace the f’s with Maybe, so fmap acts like (a -> b) -> Maybe a -> Maybe b.

If we put (Maybe m), would have (a -> b) -> (Maybe m) a -> (Maybe m) b, which looks wrong.

Page 27: PROGRAMMING IN HASKELL

27

Using it: ghci> fmap (++ " HEY GUYS IM INSIDE THE  JUST") (Just "Something serious.")  Just "Something serious. HEY GUYS IM INSIDE THE JUST"  ghci> fmap (++ " HEY GUYS IM INSIDE THE  JUST") Nothing  Nothing  ghci> fmap (*2) (Just 200)  Just 400  ghci> fmap (*2) Nothing  Nothing     

Page 28: PROGRAMMING IN HASKELL

28

Back to trees, as an example to put it all together:

Let’s make a binary search tree type. Need comparisons to make sense, so want the type to be in Eq.

Also going to have it be Show and Read, so anything in the tree can be converted to a string and printed (just to make displaying easier). data Tree a = EmptyTree | Node a (Tree a) (Tree a)

deriving (Show,Read,Eq)

Page 29: PROGRAMMING IN HASKELL

29

Note that this is slightly different than our last class.Node 5 (Node 3 (Node 1 EmptyTree EmptyTree) (Node 4 EmptyTree EmptyTree)) (Node 6 EmptyTree EmptyTree)This will let us code an insert that’s a bit easier to process, though!

First step – a function to make a single node tree:

singleton :: a -> Tree a  singleton x = Node x EmptyTree EmptyTree

Page 30: PROGRAMMING IN HASKELL

30

Now – go code insert!

treeInsert :: (Ord a) => a -> Tree a -> Tree a  treeInsert x EmptyTree =   treeInsert x (Node a left right)  =     

Page 31: PROGRAMMING IN HASKELL

31

My insert:

treeInsert :: (Ord a) => a -> Tree a -> Tree a  treeInsert x EmptyTree = singleton x  treeInsert x (Node a left right)       | x == a = Node x left right      | x < a  = Node a (treeInsert x left) right      | x > a  = Node a left (treeInsert x right)     

Page 32: PROGRAMMING IN HASKELL

32

Find:

findInTree :: (Ord a) => a -> Tree a -> Bool  findInTree x EmptyTree = False  findInTree x (Node a left right)      | x == a = True      | x < a  = findInTree x left      | x > a  = findInTree x right    

Note: If this is an “unordered” tree, would need to search both left and right subtrees.

Page 33: PROGRAMMING IN HASKELL

33

An example run:

ghci> let nums = [8,6,4,1,7,3,5]  ghci> let numsTree = foldr treeInsert EmptyTree nums  ghci> numsTree  Node 5 (Node 3 (Node 1 EmptyTree EmptyTree) (Node 4 EmptyTree EmptyTree)) (Node 7 (Node 6 EmptyTree EmptyTree) (Node 8 EmptyTree EmptyTree))   

Page 34: PROGRAMMING IN HASKELL

34

Back to functors:If we looked at fmap as though it were only for trees, it would look something like:(a -> b) -> Tree a -> Tree b

We can certainly phrase this as a functor, also:

instance Functor Tree where      fmap f EmptyTree = EmptyTree      fmap f (Node x leftsub rightsub) = 

Node (f x) (fmap f leftsub)  (fmap f rightsub)   

Page 35: PROGRAMMING IN HASKELL

35

Using the tree functor:

ghci> fmap (*2) EmptyTree  EmptyTree  ghci> fmap (*4) (foldr treeInsert 

EmptyTree [5,7,3,2,1,7])  

Node 28 (Node 4 EmptyTree (Node 8 EmptyTree (Node 12 EmptyTree (Node 20 EmptyTree EmptyTree)))) EmptyTree