Multi-parameter OOP

Pixel [email protected]
29 Oct 2001 00:25:05 +0100


Richard Uhtenwoldt <[email protected]> writes:

[...]

> My reason for posting is to
> bring attention to how something similar is done in the language merd,
> whose designer posts here under the name pixel, and which
> is actually implemented I think.

not really. still coding... :)

[...]

> http://merd.sourceforge.net/types.png

Was in too small resolution, fixed. 
Try http://merd.sourceforge.net/types.pdf for nicer display.

> In addition, merd has a type called a struct, written a |&| b.
> For reasons I do not understand, instead of defining, eg,
> 
> point = (double,double)
> 
> or 
> 
> point = (Point,double,double)
> 
> pixel prefers to define
> 
> point   = (X,double) |&| (Y,double)

The reason is tuples do not have any subtyping relationship (unless you add
it, but it gets ugly ;p)
(it also enforces that record fields are not ordered)


The |&| is defined elsewhere[1] as /\  aka the intersection type operator
(maybe i should rename it, |&| is ugly, /\ is not bad...)

It is generally used as a record subtyping operator.
It is also used for ad'hoc overloading:

  show !! String -> String |&| Int -> String  [2]

but in fact, it can be useful in many areas, if you allow |&| to work on
values:

  default_val = 0 |&| []

  [ 1, "foo" ].map(x -> x + 1|&|"bar")
     #=> [ 2, "foobar" ]

- type of [ 1, "foo" ] is List(1 | "foo")
(just like type of [ True, False ] is List(Bool) where Bool = True | False)
- (x -> x + 1|&|"bar") is alike (x -> x + 1) |&| (x -> x + "bar")
so the type is Int->Int |&| String->String [3]
- the result is of type List(String | Int)  [4]

for pattern matching:

  "(\s+):(\d+)"  returns String, String|&|Int  allowing the use with no cast
     as an Int and/or as a String
  

[1] Intersection Types and Bounded Polymorphism http://citeseer.nj.nec.com/54630.html
[2] can be factorised as show !! String|Int -> String
[3] well in fact + !! o,o -> o  where o !< &Addable
    so the type is x -> x where 1|&|"bar" !< x !< &Addable 
       which gives x -> x where x !< String|Int  
         when going to the closed world
         where + is defined for types Int and String
[4] hum, in fact merd evaluates what it can at compile time, so the type is
the type of [ 2, "foobar" ] which is List(2 | "foobar")
--
Pixel