Interactive Programming in Haskell: Chapter 10 Overview
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Chapter 10 - Interactive Programming
1
Introduction
To date, we have seen how Haskell can be used to
write
batch
programs that take all their inputs at
the start and give all their outputs at the end.
2
However, we would also like to use Haskell to write
interactive
programs that read from the keyboard
and write to the screen, as they are running.
3
The Problem
Haskell programs are pure mathematical functions:
However, reading from the keyboard and writing
to the screen are side effects:
z
Haskell programs
have no side effects
.
z
Interactive programs
have side effects
.
4
The Solution
Interactive programs can be written in Haskell by
using types to distinguish pure expressions from
impure
actions
that may involve side effects.
IO a
The type of actions that
return a value of type a.
5
For example:
IO Char
IO ()
The type of actions that
return a character.
The type of purely side
effecting actions that
return
no
result value.
z
() is the type of tuples with no components.
Note:
6
Basic Actions
The standard library provides a number of actions,
including the following three primitives:
getChar :: IO Char
z
The action
getChar
reads a character from the
keyboard, echoes it to the screen, and returns
the character as its result value:
7
z
The action
putChar c
writes the character c to
the screen, and returns no result value:
putChar :: Char
IO ()
z
The action
return v
simply returns the value v,
without performing any interaction:
return :: a
IO a
8
A sequence of actions can be combined as a single
composite action using the keyword
do
.
For example:
Sequencing
act :: IO (Char,Char)
act = do x
getChar
getChar
y
getChar
return (x,y)
9
Derived Primitives
getLine :: IO String
getLine = do x
getChar
if x == '\n' then
return []
else
do xs
getLine
return (x:xs)
z
Reading a string from the keyboard:
10
putStr :: String
IO ()
putStr [] = return ()
putStr (x:xs) = do putChar x
putStr xs
z
Writing a string to the screen:
z
Writing a string and moving to a new line:
putStrLn :: String
IO ()
putStrLn xs = do putStr xs
putChar '\n'
11
Example
We can now define an action that prompts for a
string to be entered and displays its length:
strlen :: IO ()
strlen = do putStr "Enter a string: "
xs
getLine
putStr "The string has "
putStr (show (length xs))
putStrLn " characters"
12
For example:
> strlen
Enter a string: Haskell
The string has 7 characters
z
Evaluating an action
executes
its side effects,
with the final result value being discarded.
Note:
13
Hangman
Consider the following version of
hangman
:
z
One player secretly types in a word.
z
The other player tries to deduce the word, by
entering a sequence of guesses.
z
For each guess, the computer indicates which
letters in the secret word occur in the guess.
14
z
The game ends when the guess is correct.
hangman :: IO ()
hangman = do putStrLn "Think of a word: "
word
sgetLine
putStrLn "Try to guess it:"
play word
We adopt a
top down
approach to implementing
hangman in Haskell, starting as follows:
15
The action
sgetLine
reads a line of text from the
keyboard, echoing each character as a dash:
sgetLine :: IO String
sgetLine = do x
getCh
if x == '\n' then
do putChar x
return []
else
do putChar '-'
xs
sgetLine
return (x:xs)
16
import System.IO
getCh :: IO Char
getCh = do hSetEcho stdin False
x
getChar
hSetEcho stdin True
return x
The action
getCh
reads a single character from the
keyboard, without echoing it to the screen:
17
The function
play
is the main loop, which requests
and processes guesses until the game ends.
play :: String
IO ()
play word =
do putStr "? "
guess
getLine
if guess == word then
putStrLn "You got it!"
else
do putStrLn (match word guess)
play word
18
The function
match
indicates which characters in
one string occur in a second string:
For example:
> match "haskell" "pascal"
"-as--ll"
match :: String
String
String
match xs ys =
[if elem x ys then x else '-' | x
xs]
19
Exercise
Implement the game of
nim
in Haskell, where the
rules of the game are as follows:
z
The board comprises five rows of stars:
1: * * * * *
2: * * * *
3: * * *
4: * *
5: *
20
z
Two players take it turn about to remove one
or more stars from the end of a single row.
z
The winner is the player who removes the last
star or stars from the board.
Hint:
Represent the board as a list of five integers that
give the number of stars remaining on each row.
For example, the initial board is [5,4,3,2,1].
Haskell programs are inherently pure functions without side effects. However, to create interactive programs in Haskell, we use types to distinguish between pure expressions and impure actions involving side effects. Actions like getChar for reading from the keyboard and putChar for writing to the screen are examples of side-effecting actions in Haskell.
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Presentation Transcript
PROGRAMMING IN HASKELL PROGRAMMING IN HASKELL Chapter 10 - Interactive Programming 0
Introduction To date, we have seen how Haskell can be used to write batch programs that take all their inputs at the start and give all their outputs at the end. batch program inputs outputs 1
However, we would also like to use Haskell to write interactive programs that read from the keyboard and write to the screen, as they are running. keyboard interactive program inputs outputs screen 2
The Problem Haskell programs are pure mathematical functions: z Haskell programs have no side effects. However, reading from the keyboard and writing to the screen are side effects: z Interactive programs have side effects. 3
The Solution Interactive programs can be written in Haskell by using types to distinguish pure expressions from impure actions that may involve side effects. IO a The type of actions that return a value of type a. 4
For example: The type of actions that return a character. IO Char The type of purely side effecting actions that return no result value. IO () Note: z () is the type of tuples with no components. 5
Basic Actions The standard library provides a number of actions, including the following three primitives: z The action getChar reads a character from the keyboard, echoes it to the screen, and returns the character as its result value: getChar :: IO Char 6
z The action putChar c writes the character c to the screen, and returns no result value: putChar :: Char IO () z The action return v simply returns the value v, without performing any interaction: return :: a IO a 7
Sequencing A sequence of actions can be combined as a single composite action using the keyword do. For example: act :: IO (Char,Char) act = do x getChar getChar y getChar return (x,y) 8
Derived Primitives z Reading a string from the keyboard: getLine :: IO String getLine = do x getChar if x == '\n' then return [] else do xs getLine return (x:xs) 9
z Writing a string to the screen: putStr :: String IO () putStr [] = return () putStr (x:xs) = do putChar x putStr xs z Writing a string and moving to a new line: putStrLn :: String IO () putStrLn xs = do putStr xs putChar '\n' 10
Example We can now define an action that prompts for a string to be entered and displays its length: strlen :: IO () strlen = do putStr "Enter a string: " xs getLine putStr "The string has " putStr (show (length xs)) putStrLn " characters" 11
For example: > strlen Enter a string: Haskell The string has 7 characters Note: z Evaluating an action executes its side effects, with the final result value being discarded. 12
Hangman Consider the following version of hangman: z One player secretly types in a word. z The other player tries to deduce the word, by entering a sequence of guesses. z For each guess, the computer indicates which letters in the secret word occur in the guess. 13
z The game ends when the guess is correct. We adopt a top down approach to implementing hangman in Haskell, starting as follows: hangman :: IO () hangman = do putStrLn "Think of a word: " word sgetLine putStrLn "Try to guess it:" play word 14
The action sgetLine reads a line of text from the keyboard, echoing each character as a dash: sgetLine :: IO String sgetLine = do x getCh if x == '\n' then do putChar x return [] else do putChar '-' xs sgetLine return (x:xs) 15
The action getCh reads a single character from the keyboard, without echoing it to the screen: import System.IO getCh :: IO Char getCh = do hSetEcho stdin False x getChar hSetEcho stdin True return x 16
The function play is the main loop, which requests and processes guesses until the game ends. play :: String IO () play word = do putStr "? " guess getLine if guess == word then putStrLn "You got it!" else do putStrLn (match word guess) play word 17
The function match indicates which characters in one string occur in a second string: match :: String String String match xs ys = [if elem x ys then x else '-' | x xs] For example: > match "haskell" "pascal" "-as--ll" 18
Exercise Implement the game of nim in Haskell, where the rules of the game are as follows: z The board comprises five rows of stars: 1: * * * * * 2: * * * * 3: * * * 4: * * 5: * 19
z Two players take it turn about to remove one or more stars from the end of a single row. z The winner is the player who removes the last star or stars from the board. Hint: Represent the board as a list of five integers that give the number of stars remaining on each row. For example, the initial board is [5,4,3,2,1]. 20
