Scheme Interpreter

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Introduction

(http://www.gnu.org/copyleft/fdl.html) in the spirit of which this script is GPL'd. Copyright (C) 2010 Xaviar Czervik

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

This is a scheme interpreter for LSL. Scheme is one of the two major dialects of Lisp, the other being Common Lisp.

This is somewhat related to a program I wrote a while back: a Prefix Calculator, which would evaluate expressions. + 1 2 would evaluate to 3, + * 2 3 4 would evaluate to 10.

Workings

This scheme interpreter is somewhat hackish in how I get things to work. There are two lists. One is a list of memory, the other of defines.

The memory list is called 'array' (quite creative, I know). Entries come in pairs. The first is the data contained by that pair, and the second is a pointer to where the next data is.

When an expression is entered, it is immediately pushed onto the array. If the expression is a 'define', then it adds to the define list the name that is being bound and a pointer to the defined location.


Examples

To enter a command into the interpreter, simply type it in chat.


(+ 2 3)
This will do the obvious, add two and three and return five.
(+ (* 2 3) 4)
The interpreter can handle lists inside of lists. This will evaluate to 10.
(define x 2)
It is also possible to define variables to numbers. Here we set x to be 2.
(+ x 3)
Add x to 3 and get 5.
(define square 
	(lambda (x) 
		(* x x)))
We can also create procedures. Square this case.
(square 5)
Call this procedure like any other; returns 25.
(define factorial 
	(lambda (x) 
		(if (= x 0)
			1
			(* x (factorial (- x 1))))))
This interpreter can evaluate recursive calls like any other.
(factorial 5)
And call this procedure to get 120.
(define operate
	(lambda (op x)
		(op x)))
This creates a procedure which takes two arguments. The first is a lambda function and the second is a variable to call it on.
(operate square 4)
Pass the square procedure like any other variable, along with the number to square, 4. This returns 16.
(define addormultiply
	(lambda (which)
		(if (= which 0)
			(lambda (a b)
				(* a b))
			(lambda (c d)
				(+ c d)))))
Returns a procedure to multiply if called with arg 0, or a procedure to add otherwise.
((addormultiply 1) 39 3)
The call to addormultiply returns an adding procedure, and then adds 39 and 3 to get 42.
(define mylist (quote ((a b) ((c d) e) f (g))))
We can work with lists as well.
(car mylist)
Get the first element of mylist, (a b).
(cdr mylist)
Get the rest of the list, (((c d) e) f (g))
(append (cdr mylist) (car mylist))
Append onto (cdr mylist) the value of (car mylist), (((c d) e) f (g) a b).
(list (car mylist) (cdr mylist))
Make a two element list containing the car and cdr; ((a b) (((c d) e) f (g)))
(define map 
	(lambda (op lst) 
		(if (equal (cdr lst) (quote ())) 
			(list 
				(op (car lst)) (quote ())) 
			(append 
				(list (op (car lst)) (quote ())) 
				(map op (cdr lst))))))
It is possible to rewrite some of scheme's functionality. Here is map, which takes a lambda function and a list to apply it to.
(map square (quote (3 4 5)))
Apply the square function to every element of the list. Returns (9 16 25).

To-Do

There is currently no garbage collection what-so-ever. This means that the interpreter can only compute (+ 2 3) so many times before running out of space.

There are still scheme features that are not implimented. Among these are the following: - There is no way to define macros. - There are no floating point numbers. - Procedures must be created using lambda. It is not possible to write (define (square x) (* x x)) - There is no such thing as a dotted pair! *gasp* - Cond, Let, Display have not been defined.


I am absoultely certian that there are bugs in this interpreter as it exists now. I have attempted to test it for bugs, and all of the above code works as expected, but that is no guarantee that everyhing works. If you find any bugs please let me know, either on the talk page of by IMing me directly.

Code

So finally ... the code! I have commented out all of the debugging code, but left it there in case someone wants to see the inner-workings of the interpreter.

string prog = "";
 
list operators = ["+", "-", "*", "/", "=", "!=", ">", ">=", "<", "<="];
 
list defines;
 
list array;
 
adddefine(integer pointer, string what) {
    //llOwnerSay("DEFINING " + what + " TO BE " + (string)pointer);
    if (llListFindList(defines, [what]) != -1) {
        defines = llListReplaceList(defines, [pointer], 
                llListFindList(defines, [what])+1, llListFindList(defines, [what])+1);
    } else {
        defines += [what, pointer];
    }
}
 
add(string data, integer next) {
    array += [data, next];
}
 
addpointer(integer pointer, integer next) {
    array += [pointer, next];
}
 
editdata(string data, integer addr) {
    array = llListReplaceList(array, [data], addr*2, addr*2);
}
 
editpointer(integer point, integer addr) {
    array = llListReplaceList(array, [point], addr*2, addr*2);
}
 
string get(integer pointer) {
    return llList2String(array, 2*pointer);
}
 
string getpointer(integer pointer) {
    return llList2String(array, 2*pointer+1);
}
 
setpointer(integer pointer, integer addr) {
    array = llListReplaceList(array, [pointer], addr*2+1, addr*2+1);
}
 
integer size() {
    return llGetListLength(array)/2;
}
 
integer isOperator(string data) {
    return llSubStringIndex(data, "(");
}
 
integer prog2list(string prog) {
    //llOwnerSay(prog);
 
    if (isOperator(prog)) {
        add(prog, 0);
        return size()-1;
    }
 
    list l = llParseString2List(prog, [" ", "\n"], ["(", ")"]);
 
    integer parend = 1;
    list argStart;
    list argEnd;
    list arg;
 
    integer i = 1;
    while (i < llGetListLength(l)) {
        if (llList2String(l, i) == "(") {
            if (parend == 1)
                argStart += i;
            parend++;
        } else if (llList2String(l, i) == ")") {
            parend--;
            if (parend == 1)
                argEnd += i;
        } else {
            if (parend == 1) {
                argStart += i;
                argEnd += i;
            }
        }
        i++;
    }
    if (parend != 0) {
        //llOwnerSay("Error: Mismatched parend.");
    }
 
    integer argLocation = size();
 
    i = 0;
    while (i < llGetListLength(argStart)) {
        addpointer(0, size()+1);
        i++;
    }
    addpointer(0, 0);
 
    i = 0;
    while (i < llGetListLength(argStart)) {
        editpointer(prog2list(llDumpList2String(
                llList2List(l, llList2Integer(argStart, i), llList2Integer(argEnd, i)), " ")), argLocation + i);
        //llOwnerSay("Pointer p" + llList2String(array, 2*(argLocation+i)) + " which is " +
            //llDumpList2String(llList2List(l, llList2Integer(argStart, i), llList2Integer(argEnd, i)), " "));
        i++;
    }
    //setpointer(0, argLocation + i - 1);
    //llOwnerSay("RETURNING with " + (string)argLocation);
    return argLocation;
}
 
string car(string s) {
    list l = llParseString2List(s, [" "], ["(", ")"]);
    if (llList2String(l, 1) == "quote")
        l = llList2List(l, 3, -3);
    else
        l = llList2List(l, 1, -2);
    //llOwnerSay(llList2CSV(l));
 
    if (llList2String(l, 0) == "(") {
        integer i = 0;
        while (i < llGetListLength(l)) {
            if (llList2String(l, i) == ")") {
                return llDumpList2String(llList2List(l, 0, i), " ");
            }
            i++;
        }
        return "()";
    } else {
        return llList2String(l, 0);
    }
}
 
string cdr(string s) {
    list l = llParseString2List(s, [" "], ["(", ")"]);
    if (llList2String(l, 1) == "quote")
        l = llList2List(l, 3, -3);
    else
        l = llList2List(l, 1, -2);
 
    //llOwnerSay("Now it is " + llList2CSV(l));
    if (llList2String(l, 0) == "(") {
        integer i = 0;
        integer lv;
        while (i < llGetListLength(l)) {
            if (llList2String(l, i) == "(")
                lv++;
            if (llList2String(l, i) == ")") {
                lv--;
                if (lv == 0) {
                    //llOwnerSay("Here: " + (string)(llGetListLength(l)-i) + " "+  (string)(i+1) + " " +(string)llList2List(l, i+1, -1));
                    if (llGetListLength(l)-i == 1)
                        return "()";
                    //llOwnerSay("Returning " + llDumpList2String(llList2List(l, i+1, -1), " "));
                    return "( " + llDumpList2String(llList2List(l, i+1, -1), " ") + " )";
                }
            }
            i++;
        }
        return "()";
    } else {
        if (llGetListLength(l) == 1)
            return "()";
        return "( " + llDumpList2String(llList2List(l, 1, -1), " ") + " )";
    }
}
 
string tostring(list l) {
    if (1 == 1)
        return "";
    string ret;
    integer i = 0;
    while (i < llGetListLength(l)) {
        if (llGetListEntryType(l, i) == TYPE_STRING)
            ret += (string)(i/2) + " (" + llList2String(l, i) + ", " + llList2String(l, i+1) + "); ";
        else
            ret += (string)(i/2) + " (p" + llList2String(l, i) + ", " + llList2String(l, i+1) + "); ";
        i += 2;
    }
    return ret;
}
 
printOut() {
    //llOwnerSay((string)llGetListLength(array));
    integer k = 0;
    while (k <= llGetListLength(array)) {
        string ret;
        integer i = k;
        while (i < 50+k && i < llGetListLength(array)) {
            if (llGetListEntryType(array, i) == TYPE_STRING)
                ret += (string)(i/2) + " (" + llList2String(array, i) + ", " + llList2String(array, i+1) + "); ";
            else
                ret += (string)(i/2) + " (p" + llList2String(array, i) + ", " + llList2String(array, i+1) + "); ";
            i += 2;
        }
        //llOwnerSay(ret);
        k += 50;
    }
}
 
string format(integer i) {
    //llOwnerSay("Format " + (string)i);
    string ret = "";
    if (llGetListEntryType(array, 2*i) != TYPE_INTEGER) {
        return get(i);
    } else {
        ret += "( ";
        while (i) {
            integer p = (integer)getpointer(i);
            ret += format(llList2Integer(array, 2*i)) + " ";
            i = p;
        }
        ret += ")";
        //llOwnerSay(ret);
        //llOwnerSay("Finding " + (string)llSubStringIndex(ret, " ( )"));
        while (llSubStringIndex(ret, " ( )") != -1)
            ret = llDeleteSubString(ret, llSubStringIndex(ret, " ( )"), llSubStringIndex(ret, " ( )")+3);
        return ret;
    }
}
 
 
list evaluate(integer i) {
    //llOwnerSay("Evaluate " + (string)i);
    if (llGetListEntryType(array, 2*i) == TYPE_INTEGER && get(i) == "0" && getpointer(i) == "0")
        return [0];
    if (0 == (integer)getpointer(i) && llGetListEntryType(array, 2*i) == TYPE_STRING) {
        string gt = get(i);
        if (llListFindList(defines, [gt]) != -1) {
            //llOwnerSay(llList2CSV(defines));
            integer loc = (integer)llList2String(defines, llListFindList(defines, [gt])+1);
            //llOwnerSay(gt + " is " + (string)loc);
            if (get((integer)get(loc)) == "quote")
                return [loc];
            else
                return evaluate(loc);
        }
        return [gt];
    }
 
    if (get((integer)get(i)) == "define") {
        adddefine((integer)get(i+2), get((integer)get(i+1)));
        //llOwnerSay(llList2CSV(defines));
        return [get((integer)get(i+1))];
    }
    if (get((integer)get(i)) == "lambda") {
        return [i];
    }
    if (get((integer)get(i)) == "if") {
        if ((integer)llList2String(evaluate((integer)get(i+1)), 0)) {
            return evaluate((integer)get(i+2));
        } else {
            return evaluate((integer)get(i+3));
        }
    }
    if (get((integer)get(i)) == "quote") {
        return [i];
    }
 
    integer cont = 1;
    list things;
    while (cont) {
        integer pointerTo = (integer)get(i);
        integer pointer = (integer)getpointer(i);
        if (pointerTo != 0) {
            things += evaluate(pointerTo);
            i = pointer;
        } else {
            cont = 0;
        }
    }
    //llOwnerSay(llList2CSV(things));
 
    string op = llList2String(things, 0);
    //llOwnerSay("Op is " + op);
    if (llListFindList(operators ,[op]) != -1) {
        //llOwnerSay("Doing " + op);
        if (llGetListEntryType(things, 1) == TYPE_INTEGER) {
            //llOwnerSay("Thing1 of type Integer");
            things = llListReplaceList(things, [get(llList2Integer(things, 1))], 1, 1);
        }
        if (llGetListEntryType(things, 2) == TYPE_INTEGER) {
            //llOwnerSay("Thing2 of type Integer");
            things = llListReplaceList(things, [get(llList2Integer(things, 2))], 2, 2);
        }
        return [(string)integerOp(op, 
                                    (integer)valueOf(llList2String(things, 1)),
                                    (integer)valueOf(llList2String(things, 2)))];
    } else if (op == "atom") {
        if (llGetListEntryType(things, 1) != TYPE_INTEGER)
            return ["1"];
        //llOwnerSay("GEGH is " + get(llList2Integer(things, 1)));
        if (llGetListEntryType(array, (integer)get(llList2Integer(things, 1))) == TYPE_INTEGER) {
            //llOwnerSay("Part is " + get((integer)get(llList2Integer(things, 1))));
            if (get((integer)get(llList2Integer(things, 1))) == "quote") {
                //llOwnerSay("It is " + get((integer)getpointer(llList2Integer(things, 1))));
                if (llGetListEntryType(array, 2*(integer)get((integer)getpointer(llList2Integer(things, 1)))) == TYPE_INTEGER)
                    return ["0"];
                else
                    return ["1"];
 
            }
        }
        return ["0"];
    } else if (op == "car") {
        string s = format((integer)llList2String(things, 1));
        //llOwnerSay("S is " + s);
        string c = car(s);
        //llOwnerSay("CAR IS " + c);
        if ((string)((integer)c) == c)
            return [c];
        return [prog2list("(quote " + c +")")];
    } else if (op == "cdr") {
        string s = format((integer)llList2String(things, 1));
        //llOwnerSay("S is " + s);
        //llOwnerSay("CDR IS " + cdr(s));
        return [prog2list("(quote " + cdr(s) + ")")];
    } else if (op == "list") {
        string lst;
        if (get((integer)get(llList2Integer(things, 1))) == "quote") {
            lst = format((integer)get((integer)getpointer(llList2Integer(things, 1)))) + " ";
        } else {
            lst += llList2String(things, 1) + " ";
        }
        if (get((integer)get(llList2Integer(things, 2))) == "quote") {
            lst += format((integer)get((integer)getpointer(llList2Integer(things, 2))));
        } else {
            lst += llList2String(things, 2);
        }
        //llOwnerSay("HERE: '" + lst + "'");
        //llOwnerSay((string)size());
        integer r = prog2list("(quote (" + lst + "))");
        //llOwnerSay("Final answer is " + (string)r);
        return [r];
    } else if (op == "append") {
        string lst;
        if (get((integer)get(llList2Integer(things, 1))) == "quote") {
            lst = format((integer)get((integer)getpointer(llList2Integer(things, 1))));
            lst = llGetSubString(lst, 1, -2) + " ";
        }
        if (get((integer)get(llList2Integer(things, 2))) == "quote") {
            string part = format((integer)get((integer)getpointer(llList2Integer(things, 2))));
            lst = lst + llGetSubString(part, 1, -2);
        } else {
            lst += llList2String(things, 2);
        }
        return [prog2list("(quote (" + lst + "))")];
    } else if (op == "equal") {
        if (get((integer)get(llList2Integer(things, 1))) == "quote") {
            things = llListReplaceList(things, [get((integer)get((integer)getpointer(llList2Integer(things, 1))))], 1, 1);
        } else if (llGetListEntryType(things, 1) == TYPE_INTEGER) {
            things = llListReplaceList(things, [get(llList2Integer(things, 1))], 1, 1);
        }
        if (get((integer)get(llList2Integer(things, 2))) == "quote") {
            things = llListReplaceList(things, [get((integer)get((integer)getpointer(llList2Integer(things, 2))))], 2, 2);
        } else if (llGetListEntryType(things, 2) == TYPE_INTEGER) {
            things = llListReplaceList(things, [get(llList2Integer(things, 2))], 2, 2);
        }
        //llOwnerSay(llList2CSV(things));
        if (llList2String(things, 1) == llList2String(things, 2)) {
            return ["1"];
        } else {
            return ["0"];
        }
    } else {
        //llOwnerSay("Here!");
        //llOwnerSay(tostring(array));
        string opold = op;
        if (llListFindList(defines, [op]) != -1) {
            //llOwnerSay("The integer is " + llList2String(defines, llListFindList(defines, [op])+1));
            //llOwnerSay("Getting that is " + get((integer)llList2String(defines, llListFindList(defines, [op])+1)));
            if (isdef(get((integer)llList2String(defines, llListFindList(defines, [op])+1))))
                op = llList2String(defines, llListFindList(defines, 
                        [get((integer)llList2String(defines, llListFindList(defines, [op])+1))])+1);
            if (isdef(op))
                op = llList2String(defines, llListFindList(defines, [op])+1);
        }
        integer start = (integer)op;
        //llOwnerSay(opold + " has been defined to be " + (string)start);
        if (get((integer)get(start)) == "lambda") {
            list tmp = defines;
            if (llGetListLength(things) > 1)
                defineVars((integer)get(start+1), llList2List(things, 1, -1));
            //llOwnerSay(tostring(array));
            //llOwnerSay("Defined things: " + llList2CSV(defines));
            //llOwnerSay("So now evaling " + get(start+2));
            list ret = evaluate((integer)get(start+2));
            defines = tmp;
            return ret;
        }
    }
 
    return [];
}
 
integer isdef(string op) {
    return llListFindList(defines + operators, [op]) != -1;
}
 
 
defineVars(integer ptr, list things) {
    if ((integer)get(ptr) != 0) {
        //llOwnerSay("Ptr: " + (string)ptr);
        string var = get((integer)get(ptr));
        //llOwnerSay("Var is " + var);
        adddefine(size(), var);
        //llOwnerSay("'"+llList2String(things, 0)+"'");
        add(llList2String(things, 0), 0);
        if ((integer)getpointer(ptr) != 0) {
            defineVars((integer)getpointer(ptr), llList2List(things, 1, -1));
        }
    }
}
 
string valueOf(string x) {
    //llOwnerSay("What is val of " + x + "?");
    if ((string)((integer)x) == x) {
        //llOwnerSay("= " + x);
        return x;
    } else {
        //llOwnerSay(" = " + llList2String(evaluate(llList2Integer(defines, llListFindList(defines, [x])+1)), 0));
        return llList2String(evaluate(llList2Integer(defines, llListFindList(defines, [x])+1)), 0);
    }
}
 
integer integerOp(string op, integer a, integer b) {
    if (op == "+") return a + b;
    if (op == "-") return a - b;
    if (op == "*") return a * b;
    if (op == "/") return a / b;
    if (op == "=") return a == b;
    if (op == "!=") return a != b;
    if (op == ">") return a > b;
    if (op == "<") return a < b;
    if (op == ">=") return a >= b;
    if (op == "<=") return a <= b;
    return 0;
}
 
default {
    state_entry() {
        llListen(0, "", llGetOwner(), "");
    }
    listen(integer i, string n, key id, string m) {
        //llOwnerSay("A");
        integer loc = size();
        prog2list(m);
        //llOwnerSay(tostring(array));
        //llOwnerSay("Eval: " + (string)loc);
        list l = evaluate(loc);
        if (llGetListEntryType(l, 0) == TYPE_INTEGER)
            llOwnerSay("Answer is " + format((integer)get((integer)getpointer(llList2Integer(l, 0)))));
        else
            llOwnerSay("Answer is " + llList2String(l, 0));
 
        //printOut();
 
        //llOwnerSay("Defines: " + llList2CSV(defines));
    }
}