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@takezoe
Created November 29, 2011 08:13
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MyLisp implemented using Scala parser combinator library
package mylisp
import scala.util.parsing.combinator.RegexParsers
import scala.collection.mutable.{Map => MutableMap}
object MyLispParser extends App {
val source = """
(defun sayHello (name) (println "Hello " name "!"))
(defun sum (a b)(if (eq a b) a (sum (+ a 1) b)))
(println (sum 1 1000))
"""
val parser = new MyLispParser
println(parser.parse(source))
val ast = parser.parse(source).get
val env = new Environment()
// define global functions
env.set("println", { params: List[Any] => println(params.mkString) })
env.set("+", operator2(_ + _))
env.set("-", operator2(_ - _))
env.set("*", operator2(_ * _))
env.set("/", operator2(_ / _))
env.set("eq", operator2(_ == _))
new ExprVisitor().visit(ast, env)
private def operator2(f: (Int, Int) => Any): List[Any] => Any = {
(params: List[Any]) => {
params match {
case List(a: Int, b: Int) => f(a, b)
case _ => throw new Exception("Invalid arguments.")
}
}
}
}
class Environment(parent:Option[Environment] = None){
val variables = MutableMap[String, Any]()
def get(key:String):Any = {
if(variables.contains(key)){
variables(key)
} else {
parent match {
case Some(p) => p.get(key)
case None => throw new Exception("symbol'%s' not found".format(key))
}
}
}
def set(key:String, value:Any){
variables(key) = value
}
}
class ExprVisitor() {
def visit(ast:AST, env: Environment):Any = {
ast match {
case Expr(ident, params) => {
env.get(ident.name) match {
case f: Func => {
val local = new Environment(Some(env))
f.params.zip(params.map(visit(_, env))).foreach { case(variable, value) =>
local.set(variable.name, value)
}
visit(f.proc, local)
}
case f: ((List[Any]) => Any) => f(params.map(visit(_, env)))
case _ => throw new Exception("function '%s' not found.".format(ident.name))
}
}
case If(cond, expr1, expr2) => {
if(visit(cond, env).asInstanceOf[Boolean] == true){
visit(expr1, env)
} else {
visit(expr2, env)
}
}
case IntVal(value) => value
case StrVal(value) => value
case BooleanVal(value) => value
case Ident(name) => env.get(name)
case Defun(name, func) => env.set(name.name, func)
case Program(exprs) => exprs.foreach(visit(_, env))
}
}
}
trait AST
case class IntVal(value: Int) extends AST
case class StrVal(value: String) extends AST
case class BooleanVal(value: Boolean) extends AST
case class Ident(name: String) extends AST
case class Expr(name: Ident, params:List[AST]) extends AST
case class Defun(name: Ident, func: Func) extends AST
case class Func(params:List[Ident], proc:AST) extends AST
case class If(cond:AST, expr1:AST, expr2:AST) extends AST
case class Program(exprs: List[AST]) extends AST
class MyLispParser extends RegexParsers {
def ident :Parser[Ident] = """[A-Za-z_+\-*/][a-zA-Z0-9]*""".r^?{
case n if n != "defun" && n != "if" => n
}^^Ident
def intLiteral : Parser[AST] = """[1-9][0-9]*|0""".r^^{ value => IntVal(value.toInt) }
def booleanLiteral : Parser[AST] = ("true"|"false")^^{ value => BooleanVal(value.toBoolean) }
def stringLiteral : Parser[AST] = "\""~>"""[a-zA-Z0-9:*/+\- !]*""".r<~"\""^^StrVal
def value: Parser[AST] = expr|intLiteral|stringLiteral|booleanLiteral|ident
def defun: Parser[AST] = ("(defun" ~> ident ~"("~ opt(rep(ident)) ~ ")" ~ expr <~")")^^{
case(ident~_~params~_~proc) => {
Defun(ident.asInstanceOf[Ident], Func(params.get.asInstanceOf[List[Ident]], proc))
}
}
def `if`: Parser[AST] = ("(if" ~> value ~ value ~ value <~ ")")^^{
case(cond~expr1~expr2) => If(cond, expr1, expr2)
}
def expr: Parser[AST] = defun|`if`|("(" ~> ident ~ opt(rep(value)) <~ ")" )^^{
case(ident~params) => {
Expr(ident.asInstanceOf[Ident], params.get)
}
}
def program: Parser[AST] = rep(expr)^^{ exprs => Program(exprs) }
def parse(str:String) = parseAll(program, str)
}
@takezoe
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takezoe commented Nov 29, 2011

I'm trying to implement a simple programing language like Lisp using Scala parser combinator library.
Inspired from https://gist.github.com/1345875/

@takezoe
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takezoe commented Dec 1, 2011

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