Eiffel Programming Language Chad Frommeyer CSC 407 507

Eiffel Programming Language

Chad Frommeyer CSC 407/507 Fall 2005 Dr. Richard Fox

History • • • 1985 Language Development Began Developed by Bertrand Meyer of Interactive Software Engineering 1986 First Compiler Delivered 1987 Commercial Applications Developed 1991 NICE Founded (Non-Profit International Consortium for Eiffel) NICE controls evolution, standardization and basic class definitions for Eiffel • 1996 Commercial Eiffel based development system becomes available (iss-base). • Decendent of Algol and ADA

Overview • • • • Pure object oriented language Strong statically typed Automatic memory management/Garbage collection Supports ADTs Supports Generic Classes Supports By Value and By reference semantics Promotes simplicity (Readable, not Writeable) Non-Object Oriented Capabilities/Low Level Functionality (C-Lang) Some Compilers only generate C source code Design By Contract (Reliability) Exception Handling Supports separate library compilation Supports In-Class Documentation Root class starts execution

Overview (Continued) • Statement delimiter (; ) allowed, not required, typically used in mutistatement line • Is not case sensitive • Doesn’t support global variables • Doesn’t support union types • Doesn’t support goto instructions • Doesn’t contain side-effect expression operators • Doesn’t support pointers or pointer arithmetic

Hello World indexing description: “Basic Hello World author : “Jim Developer” class HELLO_WORLD creation make feature make is do io. put_string ("Hello, world!%N") end

Basic Instructions • • • Assignment Object Creation Routine Call Conditional Iteration Choice/Switch/Case

Assignment • i : = i + 1 • Assignments cannot happen directly to class attributes. Methods/Routines must be used. • Shortcut Assignments don’t exist

Object Creation • • Declaration acc: ACCOUNT acc is “void” when declared At runtime acc obtains an object by calling create acc Which creates a new instance of ACCOUNT and attaches acc can now be used: acc. open(“John”) acc. deposit(5000)

Data Types • The following data types exist, the initialization of variables of these types is automatically initialized • INTEGER initializes to zero • REAL and DOUBLE initialize to 0. 0 • BOOLEAN initializes to false • CHARACTER initializes to null • Reference types initialize to a null reference • INTEGER, REAL, DOUBLE, BOOLEAN and CHARACTER are all considered Expanded types

Reference Types • A reference is the default object type, when an instance of a class is created it is called a reference, this is created by either calling “create” or using “!!”

Expanded Types • An expanded type is one that is already initialized • INTEGER, REAL, DOUBLE, BOOLEAN and CHARACTER are all considered Expanded types • Expanded types don’t require calling “create” or “!!” • If the class is not defined with the keyword “expanded” then it will be a reference type. expanded class INTEGER feature. . . end -- INTEGER

Routine/Function Call acc. open ("Jill") acc. deposit (5000) if acc. may_withdraw(3000) then acc. withdraw(3000); print(acc. balance) end

Conditional if x > 10 then. . . statements. . . elseif x > 5 then. . . elsif statements. . . elseif x > 0 then. . . more elsif statements. . . else statements. . . end

Looping/Iteration • Only one looping statement exists, but it could be used to emulate other types of loops. For Loop While Loop Repeat Loop from i : = 1 until i = 10 loop io. put_int (i); io. new_line; i : = i + 1; end from node : = first; until node = Void; loop io. put_string (node. text); node : = node. next; end; from done : = False until done and pages = 10; loop done : = True; printer. output_next; pages : = pages + 1; end

Choice/Switch/Case • The inspected value must be enumerable State : INTEGER; State_1, State_2, State_3 : INTEGER is unique; . . . statements. . . inspect State when State_1 then. . . statements. . . when State_2 then. . . statements. . . when State_3 then. . . statements. . . else. . . statements. . . end;

Generic Classes class STACK [T] feature push( element : T ) is do storage( pos ) : = element; pos : = pos + 1; end -- push end -- STACK class LUNCH_COUNTER feature tray_stack : STACK[ TRAY ]; napkin_dispenser : STACK[ NAPKIN ]. . . init is do. . . napkin_dispenser. pop( end -- init end -- LUNCH_COUNTER

Creation Routines/Constructors • A creation routine is called when the reference type is created with “create” or “!!” class CONVERTER creation make; feature temperature : DOUBLE make is do temperature : = 98. 6; end -- make. . . end -- CONVERTER

Exception Handling • • • Exceptions can be thrown in few cases: Design by contract failures Low level functions Rescue (Catch) Retry (can only exist within a rescue clause) • Retry starts execution of the routine again without the variable initialization

Exception Handling (Cont. ) read_next_character (f: FILE) is -- Make next character available in last_character ; -- if impossible, set failed to True. require readable: file. readable local impossible: BOOLEAN do if impossible then failed : = True else last_character : = low_level_read_function ( f ) end rescue impossible : = True retry end

Design By Contract • • Facilitates more reliable techniques Defines correct usage Assertions (Boolean Expression): Precondition (require) Postcondition (ensure) Class invariants (invariant) Class invariant must be satisfied upon exit of the creation procedure (constructor)

Assertion Monitoring • Assertion monitoring levels are set at compile time • The levels vary from no monitoring to monitoring all require/ensure/invariant • Exceptions will be thrown if assertions are being monitored • If the exception isn’t handled, a run-time error will result

Sample DBC indexing description: "Simple bank accounts" class ACCOUNT feature -- Access balance: INTEGER -- Current balance deposit_count: INTEGER is -- Number of deposits made since opening do … As before … end feature -- Element change deposit (sum: INTEGER) is -- Add sum to account. require non_negative: sum >= 0 do … As before … ensure one_more_deposit: deposit_count = old deposit_count + 1 updated: balance = old balance + sum end

Sample DBC (Cont. ) feature { NONE } -- Implementation all_deposits: DEPOSIT_LIST invariant consistent_balance: (all_deposits /= Void) implies (balance = all_deposits. total) zero_if_no_deposits: (all_deposits = Void) implies (balance = 0) end -- class ACCOUNT

Object Oriented • • • Purely Object Oriented Everything is an object Information Hiding Encapsulation Polymorphism/Dynamic Binding Inheritance (Including Multiple)

Object Oriented (Cont. ) • Supports information hiding/encapsulation • No attributes of a class are allowed to be modified directly. All modifications must happen within a subroutine • No global variables are allowed • No static functions are allowed • Secret attributes/routines are equivalent to private variables/methods

Polymorphism • Dynamic binding allows polymorphic object to call the appropriate versioned routine acc : ACCOUNT ; sav : SAVINGS_ACCOUNT acc : = sav acc. deposit( 1000 )

Inheritance • • • Inheritance is supported Multiple inheritance is supported Routine can be overridden (redefine) Parent is known as a precursor Keyword precursor can be used to reference the parent • Renaming allows conflicts with multiple inheritance to be resolved

Inheritance (Cont. ) indexing description: "Savings accounts" class SAVINGS_ACCOUNT inherit ACCOUNT redefine deposit end feature -- Element change deposit ( sum : INTEGER ) is -- Add sum to account. do precursor (sum) … end … Other features … end -- class SAVINGS_ACCOUNT

Inheritance (Cont. ) • The implementation of inheritance supplies many keywords to define how the parents members are recognized • Rename allows renaming of a parent attribute/routine • Export allows changing protection of a parent attribute/routine • Redefine allows overriding a routine • Undefine allows removing a routine

Inheritance (Cont. ) class D inherit A rename g as f -- g was effective in A export {X, Y, …} feature 1, feature 2 undefine f end B undefine f end -- f was effective in B C -- C also has an effective feature f , which will serve as -- implementation for the result of the join. feature …

In-Class Documentation • • Keyword Indexing Items (author, description, other) Developer defined indexing items Developer tools exist to generate documentation

class ACCOUNT feature balance: INTEGER -- Attribute owner: PERSON minimum_balance: INTEGER is 1000 open (who: PERSON) is -- Routine -- Assign the account to owner who. do owner : = who end deposit (sum: INTEGER) is -- Deposit sum into the account. do add (sum) end withdraw (sum: INTEGER) is -- Withdraw sum from the account. do add (-sum) end may_withdraw (sum: INTEGER): BOOLEAN is -- Function -- Is there enough money to withdraw sum? do Result : = (balance >= sum + minimum_balance) –- Return value end feature {NONE} add (sum: INTEGER) is –- Private/Secret Routine -- Add sum to the balance. do balance : = balance + sum end -- class ACCOUNT

Bibliography • http: //en. wikipedia. org/wiki/Eiffel_program ming_language • http: //archive. eiffel. com/doc/online/eiffel 50/ intro/language/ • http: //www. eiffel. com/index. html • http: //www. comp. ufla. br/~monserrat/eiffel/ advanced_introduction/eiffel. html