CSc 520Principles of Programming Languages11: Haskell — BasicsChristian CollbergDepartment of Computer ScienceUniversity of [email protected] 2005 Christian CollbergApril 22, 20051 Basic Types – Int• As we’ve seen, Haskell supports the integer (Int) type.Integer Operators:Op Precedence Associativity Description^ 8 right Exponentiation*, / 7 left Mul, Divdiv 7 free Divisionrem 7 free Remainder2 Basic Types – Int. . .Op Precedence Associativity Descriptionmod 7 free Modulus+, - 6 left Add, Subtract==,/= 4 free (In-) Equality<,<=,>,>= 4 free Relational Comparison1+2-3 ⇒ (1+2)-31+2*3 ⇒ 1+(2*3)2^3^4 ⇒ 2^(3^4)4==5==6 ⇒ ERROR12/6/3 ⇒ ERROR12/(6/3) ⇒ 63 Basic Types – Bool• There are two boolean literals, True and False1Op Precedence Associativity Description&& 3 right logical and|| 2 right logical ornot 9 – logical not3 < 5 && 4 > 2 ⇔ (3 < 5) && (4 > 2)True || False && True ⇔ True || (False && True)4 Haskell Functions• Here’s the ubiquitous factorial function:fact :: Int -> Intfact n = if n == 0 then1elsen * fact (n-1)• The first part of a function definition is thetype signature, which gives the domain and range of thefunction:fact :: Int -> Int• The second part of the definition is thefunction declaration, the implementation of the function:fact n = if n == 0 then · · ·5 Haskell Functions. . .• The syntax of a type signature isfun name :: arg typesfact takes one integer input argument and returns one integer result.• The syntax of function declarations:fun name param names = fun body• fact is defined recursively, i.e. the function body contains an application of the function itself.• Function application examples:fact 1 ⇒ 1fact 5 ⇒ 120fact (3+2) ⇒ 12026 Basic Types – Char• Literals: ’a’, ’b’. Special characters: ’\n’ (newline).• ASCII: ’\65’ (decimal), ’\x41’ (hex).• toUpper, isAlpha, etc, are defined in the standard prelude.Built-in Functions:ord :: Char -> Intchar :: Int -> ChartoUpper, toLower :: Char -> CharisAscii,isDigit,· · · :: Char -> BoolisUpper,isLower,· · · :: Char -> Boolord ’a’ ⇒ 97 toUpper ’a’ ⇒ ’A’chr 65 ⇒ ’A’ isDigit ’a’ ⇒ False7 Basic Types – Tuples• A Haskell tuple is similar to a Pascal record – it is a collection of objects of (a limited number of)objects, possibly of different types. Each Pascal record elements has a uniquename, whereas in Haskellyou distinguish between elements by their position in the tuple.• Syntax: (t1, t2, · · · , tn).Examples:type Complex = (Float,Float)mkComplex :: Float -> Float -> ComplexmkComplex re im = (re, im)8 Basic Types – Tuples. . .type Complex = (Float,Float)mkComplex :: Float -> Float -> ComplexmkComplex re im = (re im)mkComplex 5 3 ⇒ (5, 3)addComplex :: Complex -> Complex -> ComplexaddComplex (a,b) (c,d) = (a+c,b+d)addComplex (mkComplex 5 3) (mkComplex 4 2) ⇒ (9,5)9 The Offside Rule• When does one function definition end and the next one begin?square x = x * x+2cube x = · · ·• Textual layout determines when definitions begin and end.310 The Offside Rule. . .• The first character after the "=" opens up a box which holds the right hand side of the equation:square x =x * x+2• Any character to the left of the line closes the box and starts a new definition:square x =x * x+2cube x = ...11 Readings and References• A free implementation ftp://ftp.dcs.gla.ac.uk/pub/haskell/gofer.• Compiler and interpreter available on linux: /home/cs520/2003/bin/linux/gofer.• You can also use the Haskell compiler on lectura: /home/cs520/2003/ghc- 5.04.1/bin/sparc- sun-solaris2/ghci.• http://dmoz.org/Computers/Programming/Languages/Haskell.12 Summary• Haskell has all the basic types one might expect: Ints, Chars, Floats, and Bools.• Haskell functions come in two parts, the signature and the declaration:fun name :: argument typesfun name param names = fun body• Many Haskell functions will use recursion.• Haskell doesn’t have assignment statements, loop statements, or procedures.• Haskell tuples are similar to records in other languages.13 Homework1. Start (Mac || Unix) Haskell.2. Enter the commaint function and try it out.3. Enter the addComplex and mkComplex functions and try them out.4. Turn on tracing (Options:Trace Reductions in MacHaskell) and try the functions again.5. Try the standard functions fst x and snd x on complex values. What do fst and snd do?6. Try out the Eliza application in Demos:Eliza.414 Homework. . .• Write a Haskell function to check if a character is alphanumeric, i.e. a lower case letter, upper caseletter, or digit.? isAlphaNum ’a’True? isAlphaNum ’1’True? isAlphaNum ’A’True? isAlphaNum ’;’False? isAlphaNum ’@’False15 Homework. . .• Define a Haskell exclusive-or function.eOr :: Bool -> Bool -> BooleOr x y = · · ·? eOr True TrueFalse? eOr True FalseTrue? eOr False TrueTrue? eOr False FalseFalse16 Homework. . .• Define a Haskell function charToInt which converts a digit like ’8’ to its integer value 8. The valueof non-digits should be taken to be 0.charToInt :: Char -> IntcharToInt c = · · ·? charToInt ’8’8? charToInt ’0’0? charToInt
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