11 Exceptions

Exceptions are extensible variants.

type exn

You can define your own exception with the keyword exception. This is just a constructor for exn.

exception ABadThing
exception OhNo of string

Type exn is a built-in extensible variant. It allows us to provide constructors later.

exception OhNo of string;;

exception OhNo of string

Here we created an exception called OhNo that carries type string We can create an exception now with our OhNo constructor:

OhNo "oops";;

- : exn = OhNo "oops"

Note that we now have a value of type exn! We can use the raise function to "throw" an exception:

raise (OhNo "oops")

Exception: OhNo "oops".
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27

Look at that! There's been an exception, the same way that if we were to divide by zero!

1 / 0;;

Exception: Division_by_zero.
Raised by primitive operation at unknown location
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27

Exceptions don't need to carry any data:

exception ABadThing;;

exception ABadThing

raise ABadThing;;

Exception: ABadThing.
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27

OCaml programmers are more likely to create their own exceptions than Java programmers, just because it is so easy to do so. The built in function raise has type exn -> 'a.

raise;;

- : exn -> 'a = <fun>

What's interesting is that raise has a return type of 'a. We can treat it as any type we want because it never will actually return.

let x : int = raise ABadThing;;

Exception: ABadThing.
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27

Note that the error we got here was ABadThing, not a type error. This is because the type system was okay with our work, because raise will never actually return.

OCaml provides some predefined exceptions that are used pretty often

• exception Failure of string
  • Exception raised by library functions to signal that they are undefined on the given argument
  • Built in function failwith : string -> 'a
• exception InvalidArgument of string
  • Exception raised by library functions to signal that the given arguments do not make sense
  • Built in function invalid_arg : string -> 'a

raise (Failure "my error message")

Exception: Failure "my error message".
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27

is the same as

failwith "my error message"

Exception: Failure "my error message".
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Toploop.load_lambda in file "toplevel/toploop.ml", line 212, characters 17-27