From 53a3cc7b17f4cf97075b7e71720777fd84109696 Mon Sep 17 00:00:00 2001
From: Gilles Dubochet <gilles.dubochet@epfl.ch>
Date: Fri, 16 Dec 2005 18:44:33 +0000
Subject: Created proper 'docs' folder for new layout.

---
 docs/examples/monads/simpleInterpreter.scala | 75 ++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 docs/examples/monads/simpleInterpreter.scala

(limited to 'docs/examples/monads/simpleInterpreter.scala')

diff --git a/docs/examples/monads/simpleInterpreter.scala b/docs/examples/monads/simpleInterpreter.scala
new file mode 100644
index 0000000000..d728c620cf
--- /dev/null
+++ b/docs/examples/monads/simpleInterpreter.scala
@@ -0,0 +1,75 @@
+object simpleInterpreter {
+
+  case class M[A](value: A) {
+    def bind[B](k: A => M[B]): M[B]     =  k(value);
+    def map[B](f: A => B): M[B]         =  bind(x => unitM(f(x)));
+    def flatMap[B](f: A => M[B]): M[B]  =  bind(f);
+  }
+
+  def unitM[A](a: A): M[A]          =  M(a);
+
+  def showM(m: M[Value]): String = m.value.toString();
+
+  type Name = String;
+
+  trait Term;
+  case class Var(x: Name) extends Term;
+  case class Con(n: int) extends Term;
+  case class Add(l: Term, r: Term) extends Term;
+  case class Lam(x: Name, body: Term) extends Term;
+  case class App(fun: Term, arg: Term) extends Term;
+
+  trait Value;
+  case object Wrong extends Value {
+   override def toString() = "wrong"
+  }
+  case class Num(n: int) extends Value {
+    override def toString() = n.toString();
+  }
+  case class Fun(f: Value => M[Value]) extends Value {
+    override def toString() = "<function>"
+  }
+
+  type Environment = List[Pair[Name, Value]];
+
+  def lookup(x: Name, e: Environment): M[Value] = e match {
+    case List() => unitM(Wrong)
+    case Pair(y, b) :: e1 => if (x == y) unitM(b) else lookup(x, e1)
+  }
+
+  def add(a: Value, b: Value): M[Value] = Pair(a, b) match {
+    case Pair(Num(m), Num(n)) => unitM(Num(m + n))
+    case _ => unitM(Wrong)
+  }
+
+  def apply(a: Value, b: Value): M[Value] = a match {
+    case Fun(k) => k(b)
+    case _ => unitM(Wrong)
+  }
+
+  def interp(t: Term, e: Environment): M[Value] = t match {
+    case Var(x) => lookup(x, e)
+    case Con(n) => unitM(Num(n))
+    case Add(l, r) => for (val a <- interp(l, e);
+			   val b <- interp(r, e);
+			   val c <- add(a, b))
+                      yield c
+    case Lam(x, t) => unitM(Fun(a => interp(t, Pair(x, a) :: e)))
+    case App(f, t) => for (val a <- interp(f, e);
+			   val b <- interp(t, e);
+			   val c <- apply(a, b))
+		      yield c
+  }
+
+  def test(t: Term): String =
+    showM(interp(t, List()));
+
+  val term0 = App(Lam("x", Add(Var("x"), Var("x"))), Add(Con(10), Con(11)));
+  val term1 = App(Con(1), Con(2));
+
+  def main(args: Array[String]) = {
+    System.out.println(test(term0));
+    System.out.println(test(term1));
+  }
+}
+
-- 
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