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//############################################################################
// Lisp interpreter
//############################################################################
// $Id$
import List._;
object Lisp {
type Data = Any;
case class Lambda(f: List[Data] => Data);
var curexp: Data = null;
var trace: boolean = false;
var indent: int = 0;
def lispError[a](msg: String): a =
error("error: " + msg + "\n" + curexp);
trait Environment {
def lookup(n: String): Data;
def extendRec(name: String, expr: Environment => Data) =
new Environment {
def lookup(n: String): Data =
if (n == name) expr(this) else Environment.this.lookup(n);
}
def extend(name: String, v: Data) = extendRec(name, (env1 => v));
}
val EmptyEnvironment = new Environment {
def lookup(n: String): Data = lispError("undefined: " + n);
}
def asList(x: Data): List[Data] =
if (x is List[Data]) x as List[Data]
else lispError("malformed list: " + x);
def asBoolean(x: Data): boolean =
if (x == 0) false else true;
def asInt(x: Data): int =
if (x is int) x as int else lispError("not an integer: " + x);
def asString(x: Data): String =
if (x is java.lang.String) x as java.lang.String else lispError("not a string: " + x);
def normalize(x: Data): Data = x match {
case 'and :: x :: y :: Nil =>
normalize('if :: x :: y :: 0 :: Nil)
case 'or :: x :: y :: Nil =>
normalize('if :: x :: 1 :: y :: Nil)
case 'def :: (name :: args) :: body :: expr :: Nil =>
normalize('def :: name :: ('lambda :: args :: body :: Nil) :: expr :: Nil)
case 'cond :: ('else :: expr :: Nil) :: rest =>
normalize(expr);
case 'cond :: (test :: expr :: Nil) :: rest =>
normalize('if :: test :: expr :: ('cond :: rest) :: Nil)
case 'cond :: 'else :: expr :: Nil =>
normalize(expr)
case h :: t =>
normalize(h) :: asList(normalize(t))
case _ =>
x
}
def eval(x: Data, env: Environment): Data = {
val prevexp = curexp;
curexp = x;
if (trace) {
for (val x <- range(1, indent)) do System.out.print(" ");
System.out.println("===> " + x);
indent = indent + 1;
}
val result = eval1(x, env);
if (trace) {
indent = indent - 1;
for (val x <- range(1, indent)) do System.out.print(" ");
System.out.println("<=== " + result);
}
curexp = prevexp;
result
}
def eval1(x: Data, env: Environment): Data = x match {
case Symbol(name) =>
env lookup name
case 'def :: Symbol(name) :: y :: z :: Nil =>
eval(z, env.extendRec(name, (env1 => eval(y, env1))))
case 'val :: Symbol(name) :: y :: z :: Nil =>
eval(z, env.extend(name, eval(y, env)))
case 'lambda :: params :: y :: Nil =>
mkLambda(params, y, env)
case 'if :: c :: y :: z :: Nil =>
if (asBoolean(eval(c, env))) eval(y, env) else eval(z, env)
case 'quote :: y :: Nil =>
y
case y :: z =>
apply(eval(y, env), z map (x => eval(x, env)))
case Lambda(_) => x
case y: String => x
case y: int => x
case y => lispError("illegal term")
}
def lisp2string(x: Data): String = x match {
case Symbol(name) => name
case Nil => "()"
case y :: ys =>
def list2string(xs: List[Data]): String = xs match {
case List() => ""
case y :: ys => " " + lisp2string(y) + list2string(ys)
}
"(" + lisp2string(y) + list2string(ys) + ")"
case _ => x.toString()
}
def apply(fn: Data, args: List[Data]): Data = fn match {
case Lambda(f) => f(args);
case _ => lispError("application of non-function: " + fn + " to " + args);
}
def mkLambda(params: Data, expr: Data, env: Environment): Data = {
def extendEnv(env: Environment,
ps: List[String], args: List[Data]): Environment =
Pair(ps, args) match {
case Pair(List(), List()) =>
env
case Pair(p :: ps1, arg :: args1) =>
extendEnv(env.extend(p, arg), ps1, args1)
case _ =>
lispError("wrong number of arguments")
}
val ps: List[String] = asList(params) map {
case Symbol(name) => name
case _ => error("illegal parameter list");
}
Lambda(args => eval(expr, extendEnv(env, ps, args)))
}
val globalEnv = EmptyEnvironment
.extend("=", Lambda{
case List(arg1, arg2) => if(arg1 == arg2) 1 else 0})
.extend("+", Lambda{
case List(arg1: int, arg2: int) => arg1 + arg2
case List(arg1: String, arg2: String) => arg1 + arg2})
.extend("-", Lambda{
case List(arg1: int, arg2: int) => arg1 - arg2})
.extend("*", Lambda{
case List(arg1: int, arg2: int) => arg1 * arg2})
.extend("/", Lambda{
case List(arg1: int, arg2: int) => arg1 / arg2})
.extend("nil", Nil)
.extend("cons", Lambda{
case List(arg1, arg2) => arg1 :: asList(arg2)})
.extend("car", Lambda{
case List(x :: xs) => x})
.extend("cdr", Lambda{
case List(x :: xs) => xs})
.extend("null?", Lambda{
case List(Nil) => 1
case _ => 0});
def evaluate(x: Data): Data = eval(normalize(x), globalEnv);
def evaluate(s: String): Data = evaluate(string2lisp(s));
def string2lisp(s: String): Data = {
val it = new LispTokenizer(s);
def parseExpr(token: String): Data = {
if (token == "(") parseList
else if (token == ")") error("unbalanced parentheses")
else if (Character.isDigit(token.charAt(0))) Integer.parseInt(token)
else if (token.charAt(0) == '\"' && token.charAt(token.length()-1)=='\"')
token.substring(1,token.length() - 1)
else Symbol(token)
}
def parseList: List[Data] = {
val token = it.next;
if (token == ")") Nil else parseExpr(token) :: parseList
}
parseExpr(it.next)
}
}
class LispTokenizer(s: String) extends Iterator[String] {
private var i = 0;
private def isDelimiter(ch: Char) = ch <= ' ' || ch == '(' || ch == ')';
def hasNext: Boolean = {
while (i < s.length() && s.charAt(i) <= ' ') { i = i + 1 }
i < s.length()
}
def next: String =
if (hasNext) {
val start = i;
var ch = s.charAt(i); i = i + 1;
if (ch == '(') "("
else if (ch == ')') ")"
else {
while (i < s.length() && !isDelimiter(s.charAt(i))){ i = i + 1 }
s.substring(start, i)
}
} else error("premature end of string")
}
//############################################################################
object M0 {
def test = {
import Lisp._;
System.out.println();
System.out.println("( '(1 2 3)) = " + evaluate(" (quote(1 2 3))"));
System.out.println("(car '(1 2 3)) = " + evaluate("(car (quote(1 2 3)))"));
System.out.println("(cdr '(1 2 3)) = " + evaluate("(cdr (quote(1 2 3)))"));
System.out.println("(null? '(2 3)) = " + evaluate("(null? (quote(2 3)))"));
System.out.println("(null? '()) = " + evaluate("(null? (quote()))"));
System.out.println();
System.out.println("faculty(10) = " + evaluate(
"(def (faculty n) " +
"(if (= n 0) " +
"1 " +
"(* n (faculty (- n 1)))) " +
"(faculty 10))"));
System.out.println("faculty(10) = " + evaluate(
"(def (faculty n) " +
"(cond " +
"((= n 0) 1) " +
"(else (* n (faculty (- n 1))))) " +
"(faculty 10))"));
System.out.println("foobar = " + evaluate(
"(def (foo n) " +
"(cond " +
"((= n 0) \"a\")" +
"((= n 1) \"b\")" +
"((= (/ n 2) 1) " +
"(cond " +
"((= n 2) \"c\")" +
"(else \"d\")))" +
"(else " +
"(def (bar m) " +
"(cond " +
"((= m 0) \"e\")" +
"((= m 1) \"f\")" +
"(else \"z\"))" +
"(bar (- n 4)))))" +
"(val nil (quote ())" +
"(val v1 (foo 0) " +
"(val v2 (+ (foo 1) (foo 2)) " +
"(val v3 (+ (+ (foo 3) (foo 4)) (foo 5)) " +
"(val v4 (foo 6) " +
"(cons v1 (cons v2 (cons v3 (cons v4 nil))))))))))"));
System.out.println();
}
}
//############################################################################
object Test {
def main(args: Array[String]): Unit = {
System.out.println(Lisp.string2lisp("(lambda (x) (+ (* x x) 1))") as Object);
System.out.println(Lisp.lisp2string(Lisp.string2lisp("(lambda (x) (+ (* x x) 1))")));
M0.test;
()
}
}
//############################################################################
|
