Guards

صفحه 1:
Guards Guards 

صفحه 2:
; or conditions are used to express various cases in the defi iction. Example maxi :: Int -> Int -> Int maxi nm | << ‏مد‎ =n | otherwise =m Let’s test this function. Main> maxi 3 2 3 Here is a trace of the function: maxi3 2 29 3 < 2 < ۳ =3 maxi 2 5 29 2 << 5 = False ?? Otherwise = True Guards 2 

صفحه 3:
yeneral form of function definitions using gu function name pl p2 pn | gl =el | g2 = e2 | otherwise = e Guards 

صفحه 4:
maximum then the first guard will succeed d be the Example: maxiOf3 :: Int -> Int -> Int-> Int maxiOf3 mn p |m>=n &&m>=p =m | << ‏و‎ ‎If m is th and the result Withherwise If the first guard fails then either n or p shou maximum and this is what the second guard is checking. If the second guard succeeds then the result will be n. Otherwise, if both (first and second) guards fail then the last guard will cause p to be returned as maximum of 4 the three arguments. Guards 

صفحه 5:
ets test maxiOf3. Main> maxiOf3 95 4 9 True && True = True False && False = False maxiOf3 mn p |m>=n &&n>=p =m |n>=p =n | otherwise 2201013 4 29 9 << 5 66 5 << 4 =9 maxiOf3 1 2 5 221 << 2 66 2 << 5 202 << 5 = False ?? otherwise = True =5 

صفحه 6:
\nother Example: دهع مه سک mnp allEqual 2 3 4 =(3 = = 2) && (3 = = 4) =False && False =False m n Pp allEqual (maxil5) 5 (maxi 4 2) = (5 = = (maxi 1 5)) && (5 = = (maxi 4 2)) 901 <> 5 = False ?? otherwise = True =(5 == 5) && (5 = = (maxi 4 2)) ??4>=2 =True =(5==5) &&(5==4) = True && False ‏ب‎ False Guards 

صفحه 7:
Error Messages Guards 

صفحه 8:
The script on the next slide contains errors Y We will first use Notepad to create a file containing the script Y We will then use Hugs to interpret this script and find the errors Y We will use Notepad again to edit the file and correct the errors Y We will also test our fungtigns by evaluating some, expressions 

صفحه 9:
add3 :: Int -> Int -> Int -> Int add3nmp=n+m+p sig :: Int -> Int sig a_number |amumber <0 < 1 |amumber==0 = 0 | otherwise = 1 abs :: Int -> Int abs x Ix<0 | otherwise Int answe: answer = 42 Guards 

صفحه 10:
Il ILI UM WL.» Hugs 98: Based on the Haskell 98 standard ‏ال( ااا الا‎ Il Copyright (c) 1994-2001 II---ll ‏ات‎ World Wide Web: http://haskell.org/hugs ۱۱ || Report bugs to: hugs-bugs@haskell.org || || Version: December 2001 Hugs mode: Restart with command line option +98 for Haskell 98 mode Reading file "C:\Program Files\Hugs98\\lib\Prelude.hs": Hugs session for: C:\Program Files\Hugs98\\lib\Prelude.hs 10 Time -? for heln 

صفحه 11:
Prelude>:1__ d:\haskell\small.hs Reading file "d:\haskell\small.hs": ERROR "d:\haskell\small.hs":12 - Definition of variable "abs" clashes with import Prelude> The problem here is that abs is a built-in function. One way of solving this problem is to use a different name for our function. We will now edit the file and fix the error. Prelude> :e This command ( :e ) tells Hugs to edit the last module. Guards 11 

صفحه 12:
Reading file "d:\haskell\small.hs": Dependency analysis ERROR "d:\haskell\small.hs":7 - Undefined variable “a_mumber" There is an error here to be fixed and we do that using the :e command. Prelude> :e Reading file "d:\haskell\small.hs": Hugs session for: C:\Program Files\Hugs98\\lib\Prelude.hs d:\haskell\small.hs Main> Guards 12 

Guards Guards 1 s or conditions are used to express various cases in the defin nction. Example: maxi :: Int -> Int -> Int maxi n m | n >= m =n | otherwise =m Let’s test this function. Main> maxi 3 2 3 Here is a trace of the function: n m maxi 3 2 ?? 3 > 2 = True =3 maxi 2 5 ?? 2 >= 5 = False ?? Otherwise = True =5 Guards 2 general form of function definitions using gua function_name p1 … pn | g1 = e1 | g2 = e2 p2 … | otherwise = e Guards 3 Example: maxiOf3 :: Int -> Int -> Int -> Int maxiOf3 m n p | m >= n && m >=p =m | n >= p =n If m is the maximum then the first guard will succeed | otherwise and the result will be m. = p fails then either n or p should be the If the first guard maximum and this is what the second guard is checking. If the second guard succeeds then the result will be n. Otherwise, if both (first and second) guards fail then the last guard will cause p to be returned as maximum of Guards 4 the three arguments. maxiOf3 m n p Let’s test maxiOf3. | m >= n && n >=p =m | n >= p =n Here| otherwise is a trace of the function: =p m n Main> maxiOf3 9 5 4 9 p maxiOf3 9 5 4 ?? 9 >= 5 && 5 >= 4 = True && True = True =9 maxiOf3 1 2 5 ?? 1 >= 2 && 2 >= 5 ?? 2 >= 5 = False && False = False = False ?? otherwise = True Guards 5 Another Example: allEqual :: Int -> Int -> Int -> Bool allEqual m n p = (n = = m) && (n = = p) m n p allEqual 2 3 4 =(3 = = 2) && (3 = = 4) =False && False =False m n p allEqual (maxi 1 5) 5 (maxi 4 2) = (5 = = (maxi 1 5)) && (5 = = (maxi 4 2)) ?? 1 >= 5 = False ?? otherwise = True = (5 = = 5) && (5 = = (maxi 4 2)) ?? 4 >= 2 = True = (5 = = 5) && (5 = = 4) = True && False Guards = False 6 Error Messages Guards 7 The script on the next slide contains errors We will first use Notepad to create a file containing the script We will then use Hugs to interpret this script and find the errors We will use Notepad again to edit the file and correct the errors We will also test our functions by evaluating some Guards 8 expressions add3 :: Int -> Int -> Int -> Int add3 n m p = n + m + p sig :: Int -> Int sig a_number | a_number < 0 = -1 | a_mumber == 0 = 0 | otherwise = 1 abs :: Int -> Int abs x |x<0 = x * -1 | otherwise = x answer :: Int answer = 42 Guards 9 __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| ||---|| ___|| || || Copyright (c) 1994-2001 World Wide Web: http://haskell.org/hugs Report bugs to: hugs-bugs@haskell.org || || Version: December 2001 _________________________________________ Hugs mode: Restart with command line option +98 for Haskell 98 mode Reading file "C:\Program Files\Hugs98\\lib\Prelude.hs": Hugs session for: C:\Program Files\Hugs98\\lib\Prelude.hs Guards 10 Prelude> :l d:\haskell\small.hs Reading file "d:\haskell\small.hs": ERROR "d:\haskell\small.hs":12 - Definition of variable "abs" clashes with import Prelude> The problem here is that abs is a built-in function. One way of solving this problem is to use a different name for our function. We will now edit the file and fix the error. Prelude> :e This command ( :e ) tells Hugs to edit the last module. Guards 11 Reading file "d:\haskell\small.hs": Dependency analysis ERROR "d:\haskell\small.hs":7 - Undefined variable "a_mumber" There is an error here to be fixed and we do that using the :e command. Prelude> :e Reading file "d:\haskell\small.hs": Hugs session for: C:\Program Files\Hugs98\\lib\Prelude.hs d:\haskell\small.hs Main> Guards 12