add exceptions example and explanation

2 files changed, 72 insertions, 5 deletions

diff --git a/Exceptions/README.md b/Exceptions/README.md
index df654d3..9350a28 100644
--- a/Exceptions/README.md
+++ b/Exceptions/README.md
@@ -6,14 +6,39 @@ The example in this section is a derivative of the function reference summary in
 
 ## Source
 
-### 
+### exceptions.b:17,26
 
+This section calculates numbers within the fibonacci sequence to a given interval. In this case, the first 5 numbers are calculated within the sequence.
 
+Note the call to the `raise` keyword after the fibonacci calculations have completed, but before the program exits.
 
-## Demo
+### exceptions.b:31,47
+
+The `fibonacci()` function attempts to perform a `fib()` call and, if an exception `e` is thrown, attempts to handle the exception in a manner depending on the exception type. 
+
+Note that the attempted operation is wrapped in a `{ ... }` block.
+
+### exceptions.b:49,63
 
+The `raises` clause allows for warnings to be generated for un-caught exception potential within a program. Note that these exception types can be user-defined. 
+
+The `FIB` exception type is defined at: exceptions.b:12
+
+## Demo
 
+	; limbo exceptions.b
+	; exceptions
+	F(0) = 1
+	F(1) = 1
+	F(2) = 2
+	F(3) = 3
+	F(4) = 5
+	sh: 798 "Exceptions":going down!
+	; 
 
 ## Exercises
 
-- 
+- Try removing the raise for "going down!".
+- Remove the `i < 5` portion of the for loop, how many values are printed and why?
+- Try removing the exception-catching block from around calls to `fib()`, do you get warnings?
+- Try changing the raise type within `fib()`, does `fibonacci()` handle the exception `e` differently?
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);
+	}
+}