Object Orientation

Since I have mutable state and records, an object oriented (OO) model is possible. The 'problem' is in late binding of methods, mostly. Below, a solution, though there are several approaches. It is split into two parts, general definitions which introduce a few combinators for constructing and updating objects and a few examples.

/*
 * There are several manners in which you can try to emulate object
 * orientation (OO) in Egel. This is one manner.
 *
 * An object has a mutable state and a collection of named methods. 
 */
import "prelude.eg"

// a namespace OO which introduces the 'object' datatype and 
// accessors.
namespace OO (
    using System

    data object

    def object_get_state =
        [ object STATE MM -> get STATE ]

    def object_set_state =
        [ object STATE MM, S -> object (set STATE S) MM ]

    def object_get_method0 =
        [ cons NAME0 (cons METHOD MM), NAME1 ->
            if NAME0 == NAME1 then METHOD else object_get_method0 MM NAME1
        | {}, NAME1 -> NAME1 ]

    def object_get_method =
        [ object STATE MM, NAME -> object_get_method0 MM NAME ]

    def @@ = 
        [ SELF, NAME -> (object_get_method SELF NAME) SELF ]

)

using OO
using System

// examples of a box and a circle class.
def box =
    [ W, H -> 
        object (var (W, H)) 
        { "resize", 
          [ SELF, WW, HH -> object_set_state SELF (WW, HH) ],
          "area",
          [ SELF -> [(W, H) -> W*H] (object_get_state SELF) ] }
    ]

def circle =
    [ R -> 
        object (var R) 
        { "resize", 
          [ SELF, R -> object_set_state SELF R ],
          "area",
          [ SELF -> [R -> (tofloat (R*R))*3 ] (object_get_state SELF) ] }
    ]

def main =
    B = box 1 5;
    AREA0 = B @@ "area";
    N = (B @@ "resize") 2 3;
    AREA1 = B @@ "area";
        (AREA0, AREA1)

This just shows it is possible to have OO in the graph rewrite system. There are other approaches. The named fields could be introduced as constants, the attributes and methods could be coalesced into one list, or map, of fields; i.e., essentially a named record. And some system could be supported through syntactic sugar.

It's good enough for now, I rather fix concurrency.