Download Multi-Methods in Cecil

Multi-Methods in Cecil
Objects and Aspects (Fall 2004)
September 20, 2004
Kevin Bierhoff
Agenda
Generic functions
 Cecil

 Object
model
 Method dispatch
 Encapsulation

Open questions
Discussion any time
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Generic functions allow dynamic
dispatch on multiple arguments

Define the generic function add(a, b) with
 add(Integer
a, Integer b)
 add(Integer a, Double b)
 add(Double a, Integer b)
 add(Double a, Double b)
Which method receives add(x, y) ?
 Depends on runtime type of x and y

 Java’s
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x.add(y) would only look at x!
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Generic functions can be simulated
in object-oriented languages …

class Object
 boolean

return other.equalsOther(this)
 boolean


equals(Object other)
equalsOriginal(Object original)
return this == original
class Point extends Object
 protected
“Double
Dispatch”
int x, y;
 boolean equalsOriginal(Point original)

return x == original.x && y == original.y;
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… but it’s a real pain

class Point [cont.]

boolean equals(Object other)


return other.equalsOther(this)
class ShadowPoint extends Point

boolean equals(Object other)



return other.equalsOther(this)
boolean equalsOriginal(Point original)

return x == (original.x – 5) && y == (original.y – 5)
return x == original.x && y == original.y
class Point [cont.]

Consistent
behavior
boolean equalsOriginal(ShadowPoint original)


Static
binding
Code both sides
boolean equalsOriginal(ShadowPoint original)

return x == (original.x + 5) && y == (original.y + 5)
Behavior for new classes cannot be added later
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Generic functions as in CLOS
foster functional programming

Generic functions are external to objects
 Leave

only data fields within the objects
Implicit generic functions for accessor methods
 Prohibit

data encapsulation
Objects are
really structs
Incredibly complex inheritance rules
 To
resolve dispatch ambiguities (cf.)
 Really not transparent to programmer
Cecil addresses both problems mentioned here
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Cecil builds its object model on Self

Object model based on prototypes
 Supports
multiple inheritance
 Root object called “any”

Adds or modifies several ideas
 Inheritance
vs. subtyping of objects
 Abstract, template, and unique objects
 Local vs. shared fields and their accessors
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Cecil distinguishes inheritance and
subtyping of objects

Adopt behavior through inheritance
 int

= object inherits number
Override methods to adapt behavior
 Get


new concrete object at runtime
var ::= object inherits int
Adopt interface through subtyping
 type
collection
 type list subtypes collection
 set = object inherits array subtypes collection
Inheritance crucial for method dispatch
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Cecil can enforce special roles of
objects at runtime

abstract objects are not completely defined
 Can
contain abstract methods
 Similar to abstract classes in Java

template objects for concrete incarnations
 Cannot be manipulated at runtime
 int = template object inherits number
 var ::= object inherits int
 int.set_value(5) -- is invalid

unique objects exist exactly once
 zero
= unique object inherits number
 var ::= object inherits zero -- is invalid
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Objects can have state

Define data fields for objects
 value(n@number)
{ field }
 By default fields are local to an object


Can also be shared between objects
Each field implicitly defines pair of accessors
 Get with value(num), set it with set_value(num, 5)
 Fields can be declared read-only / init-only
 var ::= object inherits int [value := 5] -- for initialization

Note: Self shares fields on inheritance
 Thus
usually need a “traits” object with behavior and
an inheriting “template” that holds fields and is cloned
 This separation is not necessary in Cecil
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Cecil associates multi-methods
with all objects involved

Constrained arguments for multi-methods
 Dynamic
dispatch on all constrained arguments
 OOP as special case




No constrained argument  static function
One constrained argument  ~ OOP (Single-Dispatch)
Two or more constrained arguments  Multi-Dispatch
Multi-methods belong to all constrained args
 Code
is now best viewed as a graph
int + int
int
int + double
double + int
double
double + double
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Constrain arguments with @

Template objects
 int
= template object inherits number
 double = template object inherits number

Methods
 a@int
+ b@int { … }
 a@int + b@double { … }
 a@double + b@int { … }
 a@double + b@double { … }
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Multi-methods provoke ambiguities
similar to multiple inheritance …

“Diamond” relationships
collection
contains(c@collection, elem)
unsorted_list
sorted_list
contains(c@unsorted_list, elem)
contains(c@sorted_list, elem)
set
Now what happens if you call “contains” with a set?
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… as well as new ambiguities
related to multiple dispatch
collection
merge(c1@collection, c2@collection)
sorted_list
merge(s1@sorted_list, c2@collection)
merge(c1@collection, s2@sorted_list)
Now what happens if you call “merge” with two sorted_list objects?
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Cecil leaves these ambiguities to
the programmer

Cecil’s dispatch mechanism
 Find
syntactically fitting methods
 Ignore those with a too specialized argument
 Find the most specialized under the remaining
 Ambiguity  runtime exception
Programmer must resolve manually
 Note: CLOS avoids ambiguities

 By
totally ordering all methods
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Cecil allows information hiding …

Fields and methods can be private


Really means protected
Access based on caller privileges



Analyze signature of calling method
Match calling to requested objects
Grant access to all super- and sub-objects
visible
draw(v@visible)
draw(s@shape)
shape
private draw_shape(s@shape)
x
rectange
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y
size_x
size_y
private draw_shape(s@shape)
draw(s@shape) {
if(has_changed(s),
{ draw_shape(s) },
{}
What is
)
the difference to
}
OO?
draw(v@visible) {
draw_shape(v)
}
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… but tolerates serious
encapsulation breaches

Want to break into an object?
 Just
define a new method that takes this
object as a constrained argument
 breach(a@rectangle)
{ access private fields now }
 breach(a@any)
{ access whatever you want }
Is that good or evil?
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Cecil solves issues in CLOS …
CLOS
 Generic functions
Cecil
 Multi-methods
 External
to object
hierarchy
 Foster functional
programming
approach

Fails to address
object encapsulation
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 Belong
to all dynamic
arguments
 Graph-based objectoriented programming

Offers (some)
encapsulation
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… but raises new questions

Do you like
 the
object model better than Self’s?
 how Cecil (does not re)solve ambiguities?
 that methods belong to multiple objects?

Hey, where is the code hierarchy?
Is this really better than double dispatch?
 Does this really feel

 like
(multi-) object-oriented programming?
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