Docs / Language Manual / Exception

Exception

Exceptions are just a special kind of variant, "thrown" in exceptional cases (don't abuse them!).

Usage

RE
let getItem = (theList) => if (callSomeFunctionThatThrows()) { /* return the found item here */ } else { raise(Not_found) }; let result = try (getItem([1, 2, 3])) { | Not_found => 0 /* Default value if getItem throws */ };

Note that the above is just for demonstration purposes; in reality, you'd return an option(int) directly from getItem and avoid the try altogether.

You can directly match on exceptions while getting another return value from a function:

RE
switch (List.find((i) => i === theItem, myItems)) { | item => print_endline(item) | exception Not_found => print_endline("No such item found!") };

You can also make your own exceptions like you'd make a variant (exceptions need to be capitalized too).

RE
exception InputClosed(string); ... raise(InputClosed("the stream has closed!"));

Tips & Tricks

When you have ordinary variants, you often don't need exceptions. For example, instead of throwing when item can't be found in a collection, try to return an option(item) (None in this case) instead.

Design Decisions

The above tip seems to contradict what's happening in the OCaml standard library; prominent functions in modules such as List and String seems to throw exceptions overly often. This is partially a historical sediment, and partially out of extreme care for performance. Native OCaml/Reason is incredibly performant; exception throwing was designed to be very cheap, cheaper than allocating and returning e.g. an option. This is unfortunately not the case for JavaScript.

Newer standard library alternatives usually come with option-returning functions rather than exception-throwing ones. For example, List.find has the option-returning counterpart List.find_opt, which doesn't throw.

Exceptions are actually just variants too. In fact, they all belong to a single variant type, called exn. It's an extensible variant, meaning you can add new constructors to it, such as InputClosed above. exception Foo is just a sugar for adding a constructor to exn.