Eiffel: Analysis, Design and Programming

Bertrand Meyer

Chair ofSoftware Engineering

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- 6 -

Genericity

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What’s wrong with this code?class LIST_OF_CARS feature

extend (v: CAR) is …remove (v: CAR) is …item: CAR is …

end

class LIST_OF_CITIES featureextend (v: CITY) is …remove (v: CITY) is …item: CITY is …

end

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What’s wrong with this code?class LIST_OF_CARS feature

append (other: LIST_OF_CARS) do

from other.start until other.after loop Current.extend (other.item)end

endend

class LIST_OF_CITIES featureappend (other : LIST_OF_CITIES)

dofrom other.start until other.after loop Current.extend (other.item)end

endend

DRY Principle: Don’t Repeat Yourself

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Possible approaches for containers1. Duplicate code, manually or with help of macro

processor.

2. Wait until run time; if types don’t match, trigger a run-time failure. (Smalltalk)

3. Convert (“cast”) all values to a universal type, such as “pointer to void” in C.

4. Parameterize the class, giving an explicit name G to the type of container elements. This is the Eiffel approach, now also found in Java (1.5), .NET (2.0) and others.

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Genericity solution to LIST_OF_...class LIST [G] feature

append (other: LIST [G] ) dodo

from other.start until other.after loop

Current.extend (other.item)end

endend

city_list: LIST [CITY]car_list: LIST [CAR]

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A generic class

class LIST [G ] featureextend (x : G ) ...last : G ...

end

To use the class: obtain a generic derivation, e.g.cities : LIST [CITY ]

Formal generic parameter

Actual generic parameter

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Genericity: Ensuring type safety

How can we define consistent “container” data structures, e.g. list of accounts, list of points? Dubious use of a container data structure:

c : CITY ; p : PERSONcities : LIST ... people : LIST ... ---------------------------------------------------------people.extend ( )cities.extend ( )

c := cities.last

c. some_city_operation

What if wrong?

pc

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Using generic derivationscities : LIST [CITY ]people : LIST [PERSON]c : CITYp : PERSON...

cities.extend (c)people.extend (p)

c := cities.lastc. some_city_operation

STATIC TYPINGThe compiler will reject:

people.extend (c) cities.extend (p)

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Static typing

Type-safe call (during execution):A feature call x.f such that the object attached to x has a feature corresponding to f.

[Generalizes to calls with arguments, x.f (a, b) ]

Static type checker:A program-processing tool (such as a compiler) that guarantees, for any program it accepts, that any call in any execution will be type-safe.

Statically typed language:A programming language for which it is possible to write a static type checker.

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Using genericityLIST [CITY ]LIST [LIST [CITY ]]…

A type is no longer exactly the same thing as a class!

(But every type remains based on a class.)

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Definition: Type

We use types to declare entities, as in

x : SOME_TYPE

With the mechanisms defined so far, a type is one of:

A non-generic class e.g. METRO_STATION

A generic derivation, i.e. the name of a class followed by a list of types, the actual generic parameters, in brackets (also recursive)

e.g. LIST [ARRAY [METRO_STATION ]]

LIST [LIST [CITY ]]TABLE [STRING, INTEGER]

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So, how many types can I possibly get?Two answers, depending on what we are talking about:

• Static types Static types are the types that we use while writing Eiffel code to declare types for entities (arguments, locals, return values) and when creating new objects without explicitly specifying the type

• Dynamic types Dynamic types on the other hand are created at run-time. Whenever a new object is created, it gets assigned to be of some type.

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Static typesclass EMPLOYEEfeature

name: STRINGbirthday: DATE

end

class DEPARTMENTfeature

staff: LIST [EMPLOYEE]end

bound by the program text:EMPLOYEESTRINGDATEDEPARTMENTLIST[G]

becomes LIST[EMPLOYEE]

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Object creation, static and dynamic typesclass TEST_DYNAMIC _CREATIONfeature

ref_a: A ref_b: B--Let’s suppose B, with creation feature make_b,

--inherits from A, with creation feature make_a

do_somethingdo

create ref_a.make_a-- All that matters is the static type A

create {B} ref_a.make_b-- This is ok, because of the dynamic

type endend

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Dynamic types: another exampleclass SET[G] feature powerset: SET[SET[G]] is do create Result

-- More computation… end

i_th_power (i: INTEGER): SET[ANY] require i >= 0 local n: INTEGER do Result := Current from n := 1 until n > i loop Result := Result.powerset n := n + 1

end endend __

Dynamic types from i_th_power : SET[ANY]SET[SET[ANY]]SET[SET[SET[ANY]]]…

From http://www.eiffelroom.com/article/fun_with_generics

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Genericity: summary 1Type extension mechanism

Reconciles flexibility with type safety

Enables us to have parameterized classes

Useful for container data structures: lists, arrays, trees, …

“Type” now a bit more general than “class”

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Extending the basic notion of class

LIST_OF_CARS

BAG_OF_CARS

LINKED_LIST_OF_CARS

LIST_OF_CITIES

LIST_OF_PERSONS

Abstraction

Specialization

Type parameterization

Type parameterization

Genericity

Inheritance

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The static and the dynamic

For a feature call x.f :

Static typing:There is at least one feature f applicable to x

Dynamic binding:If more than one possible feature,execution will select the right feature

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The static and the dynamicdeferred class ANIMAL feature

talk is-- Talk, or as least make sound.

deferredend

end

class CAT inherit ANIMAL featuretalk is do io.put_string(“Miao”) end

end

class HUMAN inherit ANIMAL featuretalk is do io.put_string(“Hello”) end

end

CAT HUMAN

ANIMAL

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The static and the dynamic

a: ANIMALc: CATh: HUMAN

a := ca.talk

a := ha.talk

CAT HUMAN

ANIMAL

When compiling, type checker checks if there is at least one feature named talk in type ANIMAL, which is the declared type of a.

At run-time, execution will select the right feature to invoke, which will be talk from CAT here.

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More Genericity

UnconstrainedLIST [G]

e.g. LIST [INTEGER], LIST [PERSON]

Constrained HASH_TABLE [G, H ―> HASHABLE ]VECTOR [G ―> NUMERIC ]

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Genericity + inheritance 1: Constrained genericity

class VECTOR [G ] feature plus alias "+" (other : VECTOR [G]): VECTOR [G]

-- Sum of current vector and otherrequire

lower = other.lowerupper = other.upper

locala, b, c: G

do... See next ...

end... Other features ...

end

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Adding two vectors

i a b c=+

+ =u v w

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Constrained genericity

Body of plus alias "+":

create Result.make (lower, upper)from

i := lower until

i > upper loop

a := item (i)b := other.item (i)c := a + b -- Requires “+” operation on G!

Result.put (c, i)i := i + 1

end

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The solutionDeclare class VECTOR as

class VECTOR [G –> NUMERIC ] feature... The rest as before ...

end

Class NUMERIC (from the Kernel Library) provides features plus alias "+", minus alias "-"and so on.

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Improving the solution

Make VECTOR itself a descendant of NUMERIC,effecting the corresponding features:

class VECTOR [G –> NUMERIC ] inheritNUMERIC

feature... Rest as before, including infix "+"...

endThen it is possible to define

v : VECTOR [INTEGER ]vv : VECTOR [VECTOR [INTEGER ]]vvv : VECTOR [VECTOR [VECTOR [INTEGER ]]]

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Enforcing a type: the problemfl : LINKED_LIST [FIGURE]fl.store ("FILE_NAME")...

-- Two years later:fl := retrieved ("FILE_NAME") – See nextx := fl.last -- [1]print (x.diagonal ) -- [2]

What’s wrong with this?

If x is declared of type RECTANGLE, [1] is invalid.If x is declared of type FIGURE, [2] is invalid.

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Object-Test Local

Enforcing a type: the Object Test

if {r : RECTANGLE } fl.last then

print (r.diagonal)… Do anything else with r, guaranteed… to be non void and of dynamic type

RECTANGLEelse

print ("Too bad.")end

Expression to be tested

SCOPE of the Object-Test Local

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Assignment attempt: an older mechanismx ?= y

withx : A

Semantics: If y is attached to an object whose type conforms

to A, perform normal reference assignment.

Otherwise, make x void.

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Assignment attempt examplef : FIGUREr : RECTANGLE...fl.retrieve ("FILE_NAME")f := fl.last

r ?= f

if r /= Void thenprint (r.diagonal)

elseprint ("Too bad.")

end

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More examples on constrained genericsdeferred class FIGURE feature

draw is-- Draw Current figure

deferredend

end

class CIRCLE inherit FIGUREfeature

draw do -- Draw circle.end

end

LINE, RECTANGLE, …

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What about a composite figure?class COMPOSITE_FIGUREinherit

feature

end

FIGURELINKED_LIST [G]

[G -> FIGURE]

draw do

-- To be finished.end

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Composite figureclass COMPOSITE_FIGURE [G -> FIGURE] inherit

FIGURELINKED_LIST [G]

featuredraw is

dofrom

startuntil

afterloop

item.drawforth

endend

end

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Using FIGURE classesf : FIGUREc1, c2 : CIRCLEl : LINEcc: COMPOSITE_FIGURE [ CIRCLE ]cf: COMPOSITE_FIGURE [ FIGURE ]

cc.extend (c1)cc.extend (c2)cc.draw

cf.extend (c2)cf.extend (l)cf.draw

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Are we really type safe?animal_list: LINKED_LIST [ANIMAL]sheep_list: LINKED_LIST [SHEEP]sheep: SHEEP

sheep_list.extend (sheep)animal_list := sheep_list

SHEEP WOLF

ANIMAL

wolf: WOLF

animal_list.extend (wolf)

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CAT callsCAT stands for Changing Availability or Type

A routine is a CAT if some redefinition changes its export status or the type of any of its arguments

A call is a catcall if some redefinition of the routine would make it invalid because of a change of export status or argument type.

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Catcall cases

• Covariant redefinition• Non-generic case• Generic case

• Descendant hiding

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Covariant redefinition – Nongeneric caseclass ANIMAL feature

eat (a_food: FOOD) is deferred … endend

class WOLFinherit ANIMAL redefine eat endfeature

eat (a_meat: MEAT) is do … endend

MEAT GRASS

FOOD

WOLF

ANIMAL

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Covariant redefinition – Nongeneric caseanimal: ANIMALwolf: WOLFfood: FOODgrass: GRASS

create wolfcreate grassanimal := wolffood := grassanimal.eat (grass)

MEAT GRASS

FOOD

WOLF

ANIMAL

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Descendant hidingclass RECTANGLE

inherit POLYGON export{NONE} add_vertex endend

feature…

invariantvertex_count = 4

end

RECTANGLE

POLYGON

What will happen?

r: RECTANGLE

p: POLYGON

create r

p := r

p.add_vertex (…)

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Covariant redefinition – Generic caseanimal_list: LINKED_LIST [ANIMAL]sheep_list: LINKED_LIST [SHEEP]sheep: SHEEP

sheep_list.extend (sheep)animal_list := sheep_list

SHEEP WOLF

ANIMAL

wolf: WOLF

animal_list.extend (wolf)

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Covariant redefinition – Generic caseclass LINKED_LIST [ANY]feature

extend (v: ANY) do … endend

class LINKED_LIST [SHEEP]feature

extend (v: SHEEP) do … endend

class LINKED_LIST [WOLF]feature

extend (v: WOLF) do … endend