The Objective Caml system release 3.06 - The Caml language - Inria

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1.6 Exceptions
Caml provides exceptions for signalling and handling exceptional conditions. Exceptions can also be
used as a general-purpose non-local control structure. Exceptions are declared with the exception
construct, and signalled with the raise operator. For instance, the function below for taking the
head of a list uses an exception to signal the case where an empty list is given.
# exception Empty_list;;
exception Empty_list
# let head l =
# match l with
# [] -> raise Empty_list
# | hd :: tl -> hd;;
val head : ’a list -> ’a =
# head [1;2];;
- : int = 1
# head [];;
Exception: Empty_list.
Exceptions are used throughout the standard library to signal cases where the library functions
cannot complete normally. For instance, the List.assoc function, which returns the data associated
with a given key in a list of (key, data) pairs, raises the predefined exception Not_found when
the key does not appear in the list:
# List.assoc 1 [(0, "zero"); (1, "one")];;
- : string = "one"
# List.assoc 2 [(0, "zero"); (1, "one")];;
Exception: Not_found.
Exceptions can be trapped with the try. . . with construct:
# let name_of_binary_digit digit =
# try
# List.assoc digit [0, "zero"; 1, "one"]
# with Not_found ->
# "not a binary digit";;
val name_of_binary_digit : int -> string =
# name_of_binary_digit 0;;
- : string = "zero"
# name_of_binary_digit (-1);;
- : string = "not a binary digit"
The with part is actually a regular pattern-matching on the exception value. Thus, several
exceptions can be caught by one try. . . with construct. Also, finalization can be performed by
trapping all exceptions, performing the finalization, then raising again the exception: