Definitions

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‘Definitions 

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is of ‏بح‎ ‎type Definitions associate a nam with a value of a certain ty] answer :: Int answer = 12+13 newline :: Char newline = ‘\n’ yes Bool yes = True greater :: Bool greater = (answer > 56) 

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Expressions and evaluation 

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Evaluating expressions To begin with, we have to start the Hugs interpreter; the way to do this is by using the command hugs, which produces a startup han ner lsometh 1۳۵۲0۲۲6۷۲۱30 )1(۰ 100-00 (ها اس اما الحا مايه ساديدا ‎f =I Lj ae‏ ‎Il Report bap to! huebane@kehel.ory,‏ 9 امه تسود || | موی ای و 0 تون سا اج موی لت تنعل 96 ماما ‎(Reactor Pile "/ Ware /ib/Pretrde.bs":‏ يونا مه ‎Was‏ Mawel! Prekrde be ‘Dupe 2? Por key یب یسوط مس سا و سپ پوس ‎bs Soe‏ ایب پم ‎(Sig pte pe‏ 

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(مبم) مس 0 Prekde> suv [0.00] ss Prete? 1+ O+9+F+64+047404+9400 ‏وه‎ Prekrle> (aot ‏له || (ص‎ Gree (ee eter eters یا [0.0] مس بط ‎Preknle>‏ ]9, 6,6, 9, [ Prekrle> the OO Bibs where Phe = O:0:2ip(Dik (+) Bibs (tal Pos) [O, 4, 4, 6, 9, 9, 6, 65, G4, SF] 

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rekide> puGir "kelly, ۲ << ‏ی انم‎ kell, word Prokide> ‏ان‎ "Wels, work" Welle, wort rekde> “Well” ++") "++ "World" “Welly, world" ‏مه حطس‎ ]0.( 6080008: ‏م6‎ error ia expression (uaexpevied *)') @rekide> suc 'o! CGRROR: Pye error ‏ماو و‎ oxpressiva 1 ‏معد‎ ‏و‎ ‏عد‎ + Olek ** does ant watch | [ul] ‏.]مه حطس‎ ‏لول :03000ع)‎ vartuble "a" Prehide> 

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User Defined Functions 

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Defining functions > eS fact 33 Int 5 fact n = product [1..n] fact is of type Int t Important: Y Aname starts with a letter and is followed by a sequence of letters, digits, underscore and single quotes. Y There are some reserved words like case, do and if which can not be used as identifiers. Y Only types start with a capital letter. 

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Examples square :: Int -> Int squaren=n*n allEqual :: Int -> Int -> Int->Bool allEqual m n p = (n==m) &&(n maxi :: Int -> Int -> Int maxi mn | << =n | otherwise =m exOr :: Bool -> Bool -> Bool exOr a b =(a || b) && not(a && b) 

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Working with functions @rekxe> SQuare 5 25 @retee> allEqual 8 8 8 True ‎allEqual 8 4 8‏ مم ‎False‏ ‎@rete> Maxi 7 3 ‎2 ‏0 3 2281 یه ‎9 ‏ات ‎True‏ ‎@rette> EXOr True True False ‎@retse> EXOr True (not False) False 

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ts ‘ipt looks like what you saw on slide 16. It contains definitic nitions of functions and other values) aS well as comments. ¥ Comments are preceeded by -- or enclosed in braces: -- this is a comment Arfotthis alse a comment -} fact :: Int -> Int fact n jn== =1 |n>0 =fact (n-1)*n jotherwise =0 comb :: Integer -> Integer -> Integer comb nr = fact n ‘div’ (fact r * fact (n - r)) ane number of different ways 90 electing r objects from a collection of n objects using the f ormula n!/ 0 -r)!) 

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۲۷0 5 -- myFirst.hs -- Haskell is fun -- function to raise an integer to the power of 2. squ::Int -> Int squ n = n*¥ n Literate Style FirstLit.lhs Haskell is fun function to raise an integer to the power of 2. >squ:: Int -> Int >squ n=n*n 

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‎with Scripts‏ ای ‎An expression entered at the prompt may not be longer than a line. ‎7” You can not define functions at the “prelude>” prompt. ‎” To work with a script you have to use an editor. ‎Y You have to put your scripts into files and load them when you want to use them. ‎Y You may use editors like Notepad or Wordpad to create the file and edit it. ‎Steps to follow ‎i. Open an existing file or create one using an editor ‎ii. Save the file (use only -hs or .Ihs extensions depending the on the style) ‎iii. Launch Hugs ‎iv. Load (reload ) the file you have saved ‎v. Test your functions ‎vi. edit the file to correct any errors ‎xni Save the aditted file and reneat from ctean iv 

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Getting help from Hugs 

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Summary We have learned how to "Define functions =Construct expressions using the functions we define and built-in functions "Evaluate expressions (similar to the way the numeric expressions are evaluated in a calculator.) 

Definitions is of type name :: Type Definitions associate a nam with a value of a certain typ answer :: Int name = answer = 12+13 newline :: Char newline = ‘\n’ yes :: Bool yes = True greater :: Bool greater = (answer > 56) expression Expressions and evaluation Evaluating expressions To begin with, we have to start the Hugs interpreter; the way to do this is by using the command hugs,__________________________________ which produces a startup __ __ __ __ ____ ___ || || || || || || ||__ Hugs 98: Basedlike on the Haskell 98 standard banner something the following (1): ||___|| ||__|| ||__|| __|| Copyright (c) 1994-1999 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs@haskell.org || || Version: September 1999 _________________________________ Haskell 98 mode:Restart with command line option -98 to enable extensions Reading file "/Hugs/lib/Prelude.hs": Hugs session for: /Hugs/lib/Prelude.hs Type :? for help Prelude> Prelude is a special module that contains definitions for built-in functions Prelude> (2+3)*8 40 Prelude> sum [1..10] 55 Prelude> 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 55 Prelude> (not True) || False False Prelude> reverse "Hugs is cool" "looc si sguH" Prelude> filter even [1..10] [2, 4, 6, 8, 10] Prelude> take 10 fibs where fibs = 0:1:zipWith (+) fibs (tail fibs) [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] Prelude> putStr "hello, " >> putStr "world" hello, world Prelude> putStr "Hello, world" Hello, world Prelude> "Hello" ++ ", " ++ "world" "Hello, world" Prelude> sum [1..) ERROR: Syntax error in expression (unexpected `)') Prelude> sum 'a' ERROR: Type error in application *** expression : sum 'a' *** term : 'a' *** type : Char *** does not match : [a] Prelude> sum [1..n] ERROR: Undefined variable "n" Prelude> User Defined Functions Defining functions functio n name fact fact Type of argum ent :: Int -> n = product [1..n] argum type Int to Int ent Important: result Type of result Int fact is of  A name starts with a letter and is followed by a sequence of letters, digits, underscore and single quotes.  There are some reserved words like case, do and if which can not be used as identifiers.  Only types start with a capital letter. Examples square :: Int -> Int square n = n * n allEqual :: Int -> Int -> Int -> Bool allEqual m n p = (n == m) && (n == p) maxi :: Int -> Int -> Int maxi m n | n >= m =n | otherwise =m exOr :: Bool -> Bool -> Bool exOr a b = (a || b) && not (a && b) Working with functions Prelude> square 5 25 Prelude> allEqual 8 8 8 Prelude> allEqual 8 4 8 True False Prelude> maxi 7 Prelude> maxi 3 Prelude> exOr 7 3 Prelude> exOr True True Prelude> exOr True (not False) True False False 3 0 True False ts ript looks like what you saw on slide 16. It contains definitio finitions of functions and other values) as well as comments. Comments are preceeded by - - or enclosed in braces: -- this is a comment {- this Script is also a comment -} Another fact :: Int –> Int fact n |n = = 0 =1 |n>0 =fact (n-1)*n |otherwise =0 comb :: Integer -> Integer -> Integer comb n r = fact n `div` (fact r * fact (n - r)) The number of different ways of selecting r objects from a collection of n objects using the formula n!/(r!(n-r)!) Two styles -- myFirst.hs -- Haskell is fun -- function to raise an integer to the power of 2. squ :: Int squ n -> = Int n * n Literate Style FirstLit.lhs Haskell is fun function to raise an integer to the power of 2. > squ :: > squ Int -> n=n * n Int Working with Scripts  An expression entered at the prompt may not be longer than a line.  You can not define functions at the “prelude>” prompt.  To work with a script you have to use an editor.  You have to put your scripts into files and load them when you want to use them.  You may use editors like Notepad or Wordpad to create the file and edit it. Steps to follow i. Open an existing file or create one using an editor ii. Save the file (use only .hs or .lhs extensions depending the on the style) iii. iv. v. vi. Launch Hugs Load (reload ) the file you have saved Test your functions edit the file to correct any errors Getting help from Hugs The :? command displays the following summary of all Hugs commands: Prelude> :? LIST OF COMMANDS: Any command may be abbreviated to :c where c is the first character in the full name. :load <filenames> load modules from specified files :load clear all files except prelude :also <filenames> read additional modules :reload repeat last load command :project <filename> use project file :edit <filename> edit file :edit edit last module :module <module> set module for evaluating expressions <expr> evaluate expression :type <expr> print type of expression :? display this list of commands :set <options> set command line options :set help on command line options :names [pat] list names currently in scope :info <names> describe named objects :browse <modules> browse names defined in <modules> :find <name> edit module containing definition of name :!command shell escape :cd dir change directory :gc force garbage collection :version print Hugs version :quit exit Hugs interpreter Prelude> :l myFirst.hs :e myFirst.hs :e :q Summary We have learned how to Define functions Construct expressions using the functions we define and built-in functions Evaluate expressions (similar to the way the numeric expressions are evaluated in a calculator.)