add exceptions example and explanation

1 files changed, 44 insertions, 2 deletions

diff --git a/Exceptions/exceptions.b b/Exceptions/exceptions.b
index 336c448..b9addaa 100644
--- a/Exceptions/exceptions.b
+++ b/Exceptions/exceptions.b
@@ -4,18 +4,60 @@ include "sys.m";
 include "draw.m";
 
 sys: Sys;
-print: import sys;
 
 Exceptions: module {
 	init: fn(nil: ref Draw->Context, nil: list of string);
 };
 
+FIB: exception(int, int);
+
 init(nil: ref Draw->Context, nil: list of string) {
 	sys = load Sys Sys->PATH;
 
-	
+	for(i := 0; i < 5 ; i++) {
+		f := fibonacci(i);
+
+		if(f < 0)
+			break;
+
+		sys->print("F(%d) = %d\n", i, f);
+	}
 
 	raise "going down!";
 
 	exit;
 }
+
+fibonacci(n: int): int {
+	{
+		fib(1, n, 1, 1);
+	} exception e {
+		FIB =>
+			(x, nil) := e;
+			return x;
+
+		"*" =>
+			sys->print("unexpected string exception %s raised\n", e);
+
+		* =>
+			sys->print("unexpected exception raised\n");
+	}
+
+	return 0;
+}
+
+fib(n, m, x, y: int) raises (FIB) {
+	if(n >= m)
+		raise FIB(x, y);
+	{
+		fib(n+1, m, x, y);
+	} exception e {
+		FIB =>
+			(x, y) = e;
+
+			x = x+y;
+			y = x-y;
+
+			raise FIB(x, y);
+	}
+}