Overview of the Haskell 98 Programming Language

1
Overview of the Haskell 98 Programming Language

• (Condensed from Haskell Document by Godisch and
  Sturm)

2
How to Run Haskell Programs

• Use the Hugs interpreter downloaded from
  www.haskell.org/hugs/
• At the command line, enter hugs
• Haskell script files usually end with a ".hs"
  extension
• -- indicates a single-line comment
• - - enclose multi-line comments
• ltcommandgt executes a Haskell command
• All variables and functions start with a
  lowercase letter
• All types and constructors start with an
  uppercase letter

3
Summary of Commands Used in Hugs
4
Built-in Haskell Data Types

• Int
• Integer
• Float
• Double
• Bool
• Char
• String

5
Using the Data Type Int
myFile.hs
functionF Int -gt Int -gt Int functionF x y x
y
Inside Hugs Interpreter
Preludegt load myFile.hs Maingt functionF 1 2 3
6
Operators Available for Type Int

• 5 3
• 5 - 3
• 5 3
• 5 div 3 Integer division (backquotes)
• div 5 3
• 5 mod 2 Modulo division
• mod 5 2
• 5 3 3rd power of 5
• abs (-5) Absolute value, parentheses are needed
• Negate (-5) Negation of -5
• -5 Negative 5

7
Using the Data Type Bool

• Values of type Bool are True or FalsefunctionF
  Int -gt BoolfunctionF (gt 7)
• Boolean Operators
• True False logical AND
• True False - logical OR
• not True - logical negation
• True False - equality
• True / False - inequality

8
Using the Data Type Char

• A character is enclosed in a single quotec
  Charc 'a'
• A character can be converted to its ASCII integer
  value and vice versa using predefined
  functionsgt ord 'a'97gt chr 97'a'

9
Using the Data Type String

• A string is a (possibly empty) sequence of
  characters and is enclosed in double quotes
• It is type synonym for a list of type
  CharfunctionS StringfunctionS "A string
  example"

10
Guards and Pattern Matching
functionG Int -gt Int functionG n n 0
0 otherwise n - 1
functionH Int -gt Int functionH 0
0 functionH (n 1) n
11
Precedence and Binding

• Function application has the highest precedence
• Example functionF n 1
• Correct binding (functionF n) 1
• Wrong binding functionF (n 1)

12
Indentation and the Offside Rule

• Haskell reduces the need for parentheses to a
  minimum by using the offside rule
• To detect the start of another function
  definition, Haskell looks for the first piece of
  text that lies at the same indentation or to the
  left of the start of the current function
• Also, a semicolon can be used to separate several
  definitions on the same line

13
where and let keywords

• Each expression may be followed by a local
  definitions using the keyword where
• A similar approach can be done using the keywords
  let and in

functionF Int -gt Int functionF x g (x 1)
where g (2) functionM Int -gt
Int functionM x let functionP (2) in
functionP (x 1)
14
Tuples

• A tuple is a grouping of two or more possibly
  different types representing a record or
  structure
• person (String, Int)person ("Bill", 24)
• A type synonym using the keyword type can be used
  to give an alternative name to a typetype
  Person (String, Int)

15
Polymorphic Functions

• A polymorphic function has one definition. It
  accepts variables of different types as input at
  runtime
• Polymorphic types begin with a lowercase
  letterfunctionF a -gt b -gt c -gt afunctionF x
  _ _ x
• This is different from overloaded functions
• They have many different definitions
• Example is the equality function, which is
  defined differently for type Int compared to type
  Bool

16
Type Classes

• Members of a type classe are called instances
• Predefined instances of class Eq are Int, Float,
  Double, Bool, Char, and String
• Haskell has other built-in classes
• Eq - equality and inequality
• Ord - ordering over elements of a type
• Enum - enumeration of a type
• Show - conversion of the elements of a type into
  text
• Read - conversion of values to a type from a
  string

17
Function Composition

• The function composition operator is the dot (.)
• It type of the dot is (b -gt c) -gt (a -gt b) -gt
  (a -gt c)
• ExamplefunctionF Char -gt CharfunctionF c
  chr (succ (ord c))functionG Char -gt
  CharfunctionG chr . succ . ord

18
Using Lists

• A list is an ordered set of values of the same
  type
• It is enclosed in brackets with each element
  separated by a comma

listOfInt Int listOfInt 1,2,3,4 listOfCh
ar Char listOfChar 'h', 'e', 'l', 'l',
'o' listOfString Char listOfString
'o', 'n', 'e', "two", "three"
19
Construction of Lists

• A list can be written as xxs, where x is the
  head of the list, xs is the tail, and is the
  cons operator
• Every list is built up recursively using the cons
  operator whose type is a -gt a -gt a
  1 121 2, 132,1
  321 3, 2, 1
• The list concatenation operator is and its
  type is a -gt a -gt a1,2
  3,4,5 1, 2, 3, 4, 5
• Lists of elements can be denoted by giving a
  range2 .. 8 2, 3, 4, 5, 6, 7, 81, 3
  .. 12 1, 3, 5, 7, 9, 11

20
Pattern Matching Over Lists

• An unstructured variable xs matches any list
• (__) matches any non-empty list without binding
  of list elements
• (xxs) matches any non-empty list however, x is
  bound to the head and xs to the tail
• (_) is identical to _ and matches any list
  with one and only one element
• (x1x2xs) extracts the first two elements of a
  list containing two or more elements, with x1 and
  x2 bound to the first two elements and xs bound
  to the tail

21
List Comprehension

• A subset of a list can be generated using list
  comprehension in a common mathematical set
  notationx xlt-g, even x generator
  guard(Translated as x such that x is a
  member of the list g and x is even)
• Generate a list of all possible pairs out of two
  setspairs a -gt b -gt (a,b)pairs xs ys
  (x,y) x lt- xs, y lt- ys

22
Recursion over Lists

• Example Sum up the elements of a listsum
  Int -gt Intsum 0sum (xxs) x sum xs
• Example Filter a list according to a selection
  criteriafilter (a -gt Bool) -gt a -gt
  afilter p filter p (xxs) p x
  x filter p xs otherwise
  filter p xs

Note sum and filter are both predefined
functions in Haskell
23
Some Predefined List Functions

• () a -gt a -gt a
• () a -gt a -gt a
• (!!) a -gt Int -gt a
• map (a-gtb) -gt a -gt b
• filter (a -gt Bool) -gt a -gt a
• head a -gt a
• tail a -gt a
• last a -gt a
• init a -gt a
• length a -gt Int
• null a -gt Bool
• take Int -gt a -gt a
• drop Int -gt a -gt a

24
Algebraic Data Types Enumeration

• This is the simplest kind of algebraic typedata
  Color Red Green Bluedata RoomNumber 102
  214 228 233 245assignColor
  RoomNumber -gt Color

25
Algebraic Data Types Product

• This is used to combine two or more typestype
  Name Stringtype Age Intdata People
  Person Name Age
• Person is referred to as the constructor of type
  People. Its type is Name -gt Age -gt People
• Person is a function that generates an element of
  type People out of the types Name and Age

26
Algebraic Data Types Alternatives

• This is used to combine two or more types but
  allows some choicestype Name Stringdata
  Year Freshman Sophomore
  Junior Seniordata Dept Bus Eng LA
  Math data People Student Name Year
  Staffer Name Dept

27
Algebraic Data Types Recursive

• A recursive type can be created from an
  alternative typedata Tree Nil Node Int Tree
  TreemyTree TreemyTree Node 1 (Node 2
  (Node 4 Nil Nil) (Node 5 Nil Nil)) (Node
  3 Nil Nil)depth Tree -gt Intdepth Nil
  0depth (Node _ t1 t2) 1 max (depth
  t1) (depth t2)

28
Instances of Classes

• When creating a new algebraic type, you may need
  to inherit operations from other type classes.
  This is done using the deriving keyworddata
  Color Red Green Blue deriving (Eq, Ord,
  Enum, Show, Read)

29
Example of an Abstract Data Type
Module Stack (Stack, isEmpty, push, pop)
where data Stack a Empty MyStack a (Stack
a)isEmpty Stack a -gt Bool isEmpty Empty
True isEmpty (MyStack _ _) False push
a -gt Stack a -gt Stack a push element aStack
MyStack element aStack pop Stack a -gt (a,
Stack a) pop Empty error "Empty
stack" pop (MyStack element aStack)
(element, aStack)