Ocaml Exceptions

OCaml has an exception mechanism similar to many other programming languages. A new type of OCaml exception is defined with this syntax:

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exception E of t

To create an exception value, use the same syntax you would for creating a variant value. Here, for example, is an exception value whose constructor is Failure, which carries a string:

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Failure "something went wrong"

To catch an exception, use this syntax:

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

The expression e is what might raise an exception. If it does not, the entire try expression evaluates to whatever e does. If e does raise an exception value v, that value v is matched against the provided patterns, exactly like match expression.

Pattern Matching

There is a pattern form for exceptions. Here’s an example of its usage:

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match List.hd [] with
  | [] -> "empty"
  | _ :: _ -> "non-empty"
  | exception (Failure s) -> s

Exception patterns are a kind of syntactic sugar. Consider this code for example:

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match e with
  | p1 -> e1
  | exception p2 -> e2
  | p3 -> e3
  | exception p4 -> e4

We can rewrite the code to eliminate the wxception pattern:

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try 
  match e with
    | p1 -> e1
    | p3 -> e3
with
  | p2 -> e2
  | p4 -> e4

In general if there are both exception and non-exception patterns, evaluation proceeds as follows:

Try evaluating e. If it produces an exception packet, use the exception patterns from the original match expression to handle that packet.
If it doesn’t produce an exception packet but instead produces a non-exception value, use the non-exception patterns from the original match expression to match that value.