COCA-ML

coca-ml is a small extension to ocaml 3-02 using camlp4 3.02 to allow "down cast" of objects between classes in relations of inheritence and sub-typing.

Warning there was a bug inside the version 0.1c (see Changes). To correct it the new version 0.1d has several important changes.

The main idea is to allow an "upcast" only at one level inside the inheritence tree to check the sub-typing relation.

Description

This extension introduces three new syntatic constructions :

	"instance of"	"upcast"	"downcast"
Syntax	expr instanceof c	`upcast var from c1 to c2`	`downcast var from c1 to c2`
Typing	bool	c2	c2
Evaluation	true or false	value of var	value of var

Semantics and implementation

o instanceof c1

	semantic	implementation
1.	o "is-a" c1	( Cast.check loc o key_c1)

upcast o from c1 to c2

semantic implementation

1. o is exactly an instance of c1
and c1 inherits directly from c2
Cast.check_sonof var c1 c2

2. type of var is a sub-type of c2 ( var :> c2)

1/ is at running level
2/ is at running level

downcast o from c1 to c2

semantic implementation

1. o has type c1 ( o : c1)

2. c2 is a sub-type of c1 ( c2 :> c1)

3. o "is-a" c1 ( Cast.check loc o key_c1)

4. o "is-a" c2 ( Cast.check loc o key_c2)

5. ((Obj.magic o) : c2)

1/ and 2/ are at typing level
3/ and 4/ are at running level

Examples

testd1.ml : different levels of inheritence
testd2.ml : 2 sons from an abstract class
testd3.ml : multiple inheritence

There are more complete examples in the distribution.

Changes

New version 0.1d (09/28/2001) to correct the previous bug

 using "upcast" o from c2 to c1 (at one level of inheritence) to allow
  to downcast at any level 
 or 
 using  o ":>" c1 to downcast to the class of construction, 
 
 if you use ":>" one time, you can't downcast at any level!

Bug inside the version 0.1d (09/26/2001)

z inherits from m
m inherits from a
z is sub-type of a
m is sub-type of a
  Then we can write 
 cast ((new z) :>a) from a to m
that is wrong if z is not a sub-type of m

For example m has a binary method which is not in a 
but which is inherited by z.

version 0.1c ( 09/19/2001) : signatures.
version 0.1b (du 09/07/2001)
```
  o instanceof c
```
to check if an instance "is-a" c (its class is c or inherits from c).

Distribution

distribution : cocaml-0.1d.tar

ocaml : O'Caml 3.02

camlp4 : camlp4 3.02

Updating : 28/09/2001

	semantic	implementation
1.	o is exactly an instance of c1 and c1 inherits directly from c2	Cast.check_sonof var c1 c2
2.	type of var is a sub-type of c2	( var :> c2)

	semantic	implementation
1.	o has type c1	( o : c1)
2.	c2 is a sub-type of c1	( c2 :> c1)
3.	o "is-a" c1	( Cast.check loc o key_c1)
4.	o "is-a" c2	( Cast.check loc o key_c2)
5.		((Obj.magic o) : c2)