12 Handling Exceptions

Handling exceptions that have been raised can be done with pattern matching

let safe_div x y =
	try x / y with 
	| Division_by_zero -> 0

val safe_div : int -> int -> int = <fun>

safe_div 4 2;;
safe_div 4 0;;

- : int = 2

- : int = 0

A try exception is essentially pattern matching, but only on exceptions

Syntax

try e with 
    | p1 -> e1
    | ...
    | pn -> en

You can have $[1, \infty)$ branches.

Evaluation

if e ==> v then (try e...) ==> v

if e raises an exception x, then match x against the patterns. If none match, then re-raise x.

Type checking

(try e ...) : t if e : t and e : t and ei : t and pi : exn.