Haskell IO 1

1
Haskell IO 1
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Haskell Review

• Environment uses commands prefixed by colon
• ? -- for help
• load to load up a file
• quit
• edit
• Types
• Int, Float, Char, Bool, Integer, Rational
• Lists and Tuples
• 1,2,3 all elements of the same type
• 1..5 gives consecutive numbers
• Strings
• String Char
• 12,3 and 1,2,3 4,5,6

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Defining your own functions

• sq Int -gt Int -- First you define the type
• sq x x x -- Then the definition of the
  function
• mystrlen String -gt Int
• mystrlen 0
• mystrlen (ht) 1 mystrlen t
• -- If you dont want to worry about the type!
• mystrlen 0
• mystrlen (ht) 1 mystrlen t
• To find the type simply type
• gt type function, eg
• gt type mystrlen

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Review Continued

• Problem 1 write a Haskell function to perform
  palindrome checks, for example
• gtpalindrome abc
• False
• gtpalindrome abba
• True
• gtpalindrome abba1
• False
• First write a function mylast that returns the
  last character in a string.

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Palindrome code

• mylast String -gt Char
• mylast (x ) x
• mylast (ht) mylast t
• palindrome String -gt Bool
• palindrome True
• palindrome (h ) False
• palindrome (ht) (h mylast t)
• palindrome (ht) palindrome t

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More on Functions

• Functions can be written to work with options
• hcf Int -gt Int -gt Int
• hcf n m
• m n n
• m gt n hcf m n
• otherwise hcf (n-m) m
• gt hcf 24 32
• 8

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Rewrite Palindrome with guards

• palindrome1 String -gt Bool
• palindrome1 a
• a True
• tail a False
• (head a mylast (tail a)) True
• otherwise palindrome1 (tail a)

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Useful String Operations

• gt lines ABC\nDEF\nGHI
• ABC, DEF, GHI
• gt unlines "ABC", "DEF", "GHI"
• "ABC\nDEF\nGHI\n"
• gt words " The quick \n brown fox.."
• "The", "quick", "brown", "fox.."
• gt unwords "The","quick","brown","fox.."
• "The quick brown fox..
• gt show 345
• gt 345 -- so show converts anything to a
  string

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Word Count Example

• wc String -gt (Int,Int)
• wc s (length(lines s), length(words s))
• prettywc s "Lines " show l
• "\nWords " show w "\n"
• where (l,w) wc s
• Note that where uses indentation!
• gt wc "This is\na test"
• (2,4)
• gt prettywc "This is\na b c d e f g h"
• Lines 1\nWords 10\n

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Alternative to indentation

• getw (Int,Int) -gt Int
• getw (l,w) w
• getl (Int,Int) -gt Int
• getl (l,w) l
• prettywc1 s "Lines " show (getl(wc s))
• "\nWords " show (getw(wc s))
  "\n"

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Output options

• Still got the show problems
• gt putStr(prettywc This is a\ntest)
• Lines 1
• Words 4
• putStr is an IO function it has an IO type!
• gt type putStr
• putStr String -gt IO ( )

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Haskell IO

• Haskell IO is purely functional
• Monad is from "Category Theory" (Maths)
• Basically, we need to thread a state argument
  through all function calls, for which each call
  also has to return the new state.
• In Haskell/Hugs, each I/O action returns a value
  so has a type
• IO a (where a is the return type).
• Eg getLine IO String
• getChar IO Char

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Haskell IO

• Some actions have no interesting values, like
  putStr and
• putChar Char -gt IO ( )
• This is similar to a void type.

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Output Actions

• These write to standard output (cout)
• putChar Char -gt IO ()
• putStr String -gt IO ()
• putStrLn String -gt IO () -- adds \n
• These write to (or create) output files
• type FilePath String
• writeFile FilePath -gt String -gt IO()
• appendFile FilePath -gt String -gt IO()
• Eg gt putStr "Hello World!"
• Hello World!

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Input Actions

• These read from standard input (cin)
• getChar IO Char -- gets 1 char
• getLine IO String -- strips \n
• getContents IO String
• -- returns ALL the rest of cin!
• This reads the entire contents of a file!
• readFile FilePath -gt IO String

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Combining Actions

• The "do" notation is the nicest way of combining
  I/O actions into a program (209IO1.txt)
• main
• do putStr "Input file? "
• ifile lt- getLine
• putStr "Output file? "
• ofile lt- getLine
• s lt- readFile ifile
• writeFile ofile (s s)
• Note that main is a special function of type IO
  ( )

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Syntax for the do statement

• do keyword, then several "statements".
• Each statement must be one of
• ioaction (where ioaction IO ( )
  )
• patt lt- ioaction (where ioaction IO b)
• (variables in patt are created, and
  initialized
• from the b value returned by ioaction)
• let patt expr (for local definitions)
• if pred then ioaction else ioaction
• return expr (just like in C!)

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Examples

• There are many options, below is the simplest
  style
• getTwoChars IO (Char,Char)
• getTwoChars do
• c1 lt- getChar
• c2 lt- getChar
• return (c1,c2)
• The return is vital here because do requires an
  IO action as result. The return allows a value to
  become an IO action.
• return a -gt IO a

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Testing IO functions

• gt do (c1, c2) lt- getTwoChars putChar c1
  putChar c2
• To convert from String to a type
• read Read a gt String -gt a
• Print is another name for show!
• print Show a gt a -gt IO ()
• getInt IO Int
• getInt do
• line lt- getLine
• return (read line Int)
• gt do x lt- getInt print x

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More useful IO functions

• getLine1 IO String
• getLine1 do
• c lt- getChar
• if c '\n' then return ""
• else do
• cs lt- getLine1
• return (ccs)
• gt do line lt- getLine1 print line

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Getting ints with prompt

• getIntPrompt p do
• putStr p
• line lt- getLine
• return (read line Int)
• gt do x lt- getIntPrompt Give me an intgt
  print x
• Give me an intgt 345
• 345

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Getting lots of ints

• getInts do
• n lt- getIntPrompt Enter Int (0 to stop)
• if n 0 then return
• else do
• rest lt- getInts
• return (nrest)

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Summing ints

• sumInts do
• ints lt- getInts
• return (sum ints)
• gt do x lt- sumInts print x
• Enter Int (0 to stop) 1
• Enter Int (0 to stop) 2
• Enter Int (0 to stop) 3
• Enter Int (0 to stop) 0
• 6

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IO Problem 1 (iotest.txt)

• Write a Haskell function iotest that prompts
  for the user for two integers a and b. If a
  is larger than or equal to b, the function will
  then countdown from a to b, outputting all these
  numbers. Should a be smaller than b, then the
  function should count up from a to b, and output
  all numbers too.
• gt iotest gtiotest gtiotest
• Input a 9 Input a 2 Input a 4
• Input b 7 Input b 5 Input b 4
• 9 2 4
• 8 3
• 7 4
• 5

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Solution

• iotest do
• a lt- getIntPrompt "Input a "
• b lt- getIntPrompt "Input b "
• iotest1 a b
• iotest1 a b
• a lt b output_upto a b
• otherwise output_downto a b

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Solution Continued

• output_upto a b
• a gt b return ()
• otherwise do print a output_upto (a1) b
• output_downto a b
• a lt b return ()
• otherwise do print a output_downto (a-1)
  b
• getIntPrompt p do
• putStr p
• line lt- getLine
• return (read line Int)

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IO Problem 2 (iotest2.txt)

• Write a Haskell function that acts like a
  simple-minded desktop calculator.
• Your program should handle addition, subtraction,
  multiplication, and division. Always prompt the
  user for an operation first. User input should be
  a single character indicating the operation
  wanted (, -, , /) or q to quit the
  calculator.
• gt dc
• Op(,-,,/,q) lt is user inputgt
• Arg1 12 lt12 is user inputgt
• Arg2 9 lt9 is user inputgt
• Result 21

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Solution

• dc IO ()
• dc
• do
• putStr "Op (,-,,/,q) "
• c lt- getChar
• if c 'q' then return ()
• else do
• perform c
• dc

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Solution Continued

• getInt IO Int
• getInt do
• line lt- getLine
• return (read line Int)
• getFloat IO Float
• getFloat do
• line lt- getLine
• return (read line Float)

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Calculator Operations

• perform Char -gt IO()
• perform '' do (arg1,arg2) lt- prompt putStr
  ("Result " show(arg1arg2) "\n")
• perform '-' do (arg1,arg2) lt- prompt putStr
  ("Result " show(arg1-arg2) "\n")
• perform '' do (arg1,arg2) lt- prompt putStr
  ("Result " show(arg1arg2) "\n")
• perform '/' do (arg1,arg2) lt- promptf
  process (arg1,arg2)

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Still more

• prompt IO (Int,Int)
• prompt do
• putStr "\nArg1 "
• arg1 lt- getInt
• putStr "Arg2 "
• arg2 lt- getInt
• return (arg1,arg2)

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Finally

• promptf IO (Float,Float)
• promptf do
• putStr "\nArg1 "
• arg1 lt- getFloat
• putStr "Arg2 "
• arg2 lt- getFloat
• return (arg1,arg2)
• process (arg1, arg2)
• arg2 0 putStr "ERROR Attempt to divide
  by zero\n"
• otherwise putStr ("Result "
  show(arg1/arg2) "\n")