Class2_2

May 2004 SEU Slide
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small :: Int -> Bool small n = if n<10 then True else False
myfilter :: (a -> Bool) -> [a] -> [a] myfilter p [] = [] myfilter p (x:xs) = if p x == True then x : myfilter p xs else myfilter p xs
May 2004 SEU Slide 9
Now both Prelude.hs and test.hs are loaded, and functions from both scripts can be used:
> quadruple 10 40 > double 2 4
increment :: Int -> Int increment n = (n+1)
In general: t1 t2 is the type of functions that map values of type t1 to values to type t2.
May 2004 SEU Slide 22
Useful Hugs Commands
Command :l name :r :e name :e :t expr :? :q
Hugs 98: Based on the Haskell 98 standard
Copyright (c) 1994-1999 World Wide Web: /hugs Report bugs to: hugs-bugs@
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Meaning load script name reload current script edit script name edit current script show type of expr show all commands quit
• Calculating the length of a list:
> length [1,2,3,4] 4
May 2004 SEU Slide 7
> reverse [1,2,3,4] [4,3,2,1] > [1,2,3] ++ [4,5,6] [1,2,3,4,5,6] > head [1,2,3,4] 1 > tail [1,2,3,4] [2,3,4] > last [1,2,3,4] 4
May 2004 SEU Slide
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Function Types
A function is a mapping from values of one type to values of another type: not :: Bool Bool
isDigit :: Char Bool
Version: February 2000
_____Leabharlann ______________________
Haskell 98 mode: Restart with command line option -98 to enable extensions
Reading file "/usr/local/share/hugs/lib/Prelude.hs":
May 2004 SEU Slide 10
Larger
larger :: Int -> Int -> Int larger a b = if a > b then a else b
May 2004 SEU Slide
11
If we modify file “test.hs” must reload
increment :: Int -> Int increment n = (n+1)
May 2004 SEU Slide
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The Map Function
The higher-order library function called map applies a function to every element of a list. map :: (a b) [a] [b]
> search 5 [3,4,5,6] True > search 8 [3,4,5,6] False
May 2004 SEU Slide 13
count_els :: [Int] -> Int count_els [] = 0 count_els (x:xs) = 1 + count_els xs
• Hugs is available on the web from:
/hugs Download has type „.msi‟ may have to download a mircosoft application to unzip this also
May 2004 SEU Slide 3
> count_els [3,4,5,6] 4 count_occur :: Int -> [Int] -> Int count_occur t [] = 0 count_occur t (x:xs) = if x==t then 1 + count_occur t xs else 0 + count_occur t xs
For example:
> map increment [1,2,3,4] [2,3,4,5]
May 2004 SEU Slide 17
The map function can be defined as map1 :: (a -> b) -> [a] -> [b] map1 f [] = [] map1 f (x:xs) = f x : map1 f xs
> :r
May 2004 SEU Slide
12
factorial n = if n==0 then 1 else n * factorial (n - 1) search :: Int -> [Int] -> Bool search t [] = False search t (x:xs) = if x == t then True else search t xs
Start an editor, type in the following two function definitions, and save the script as test.hs
Save in: c:/Program Files/Hugs98/lib double x = x + x
quadruple x = double (double x) From Hugs prompt type > :load “test.hs”
Need a pure Functional Language to implement these fully
we will briefly look at Haskell
May 2004 SEU Slide
2
What is Hugs?
• Hugs is an interactive Haskell system, and is the most widely used implementation of Haskell;
Hugs session for: /usr/local/share/hugs/lib/Prelude.hs Type :? for help
May 2004 5 Slide
Prelude>
SEU
The Hugs > prompt means that the Hugs system is ready to evaluate an expression. For example: > 5+2*3 11
May 2004 SEU Slide
> add_fl[3,4,5,6] 9
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Higher Order Functions A Function which takes another function as parameter Function can also be returned as a result I.e Functions treated as Objects
> map1 increment [1,2,3,4]
[2,3,4,5]
May 2004 SEU Slide 18
Write a function which : - takes a function & list of Ints as paramemets - applies function to first & last element - return result as sum of these two wlaues >sum_fl increment [3,4,5,6] 11 i.e sum_fl :: (Int -> Int) -> [Int] -> Int sum_fl f (x:xs) = f x + f (last xs) 4+7
Starting Hugs
•From File Manager
•Change to folder c:/Program Files/Hugs98/lib •double click on “Prelude.hs
May 2004 SEU Slide
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Starting Hugs
% hugs __ __ || || ||___|| ||---|| || || || || __ __ ____ ___ || || || || ||__ ||__|| ||__|| __|| ___||