ref: 5cd0b7d8139f0cd5f4af1f9f3b82fe2cd3f37288
dir: /Exceptions/README.md/
# Exceptions
Limbo supports the throwing, catching, and custom definition of exceptions.
The example in this section is a derivative of the exceptions summary in the [Limbo Addendum](http://www.vitanuova.com/inferno/papers/addendum.pdf).
## 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.
### 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?