Error handling

  1. The functional way: option and result types
    1. The bind combinator
  2. The imperative way: exceptions
    1. Raising and catching exceptions
    2. Built-in exceptions
    3. The types of exception-raising functions
    4. Enabling exception tracing

Every program that deals with user input and the operating system has to handle error conditions. OCaml is a multi-paradigm language and there is more than one way to handle errors.

None of them is inherently superior to the other, and it’s possible to mix them.

The functional way: option and result types

It’s common to indicate error conditions with option of result types. This is how they are defined in the standard library:

type 'a option = None | Some of 'a

type ('a, 'b) result = Ok of 'a | Error of 'b

The 'a option type allows you to signal that the function could not produce a result with None or return a value with Some value.

The ('a, 'b) result type allows you to also indicate the error condition. Note that both data constructors of the result type are polymorphic, so your error message doesn’t have to be a string.

The option type is commonly used when there can be only one, obvious error condition. For example, in functions that search for an element in an collection, it indicates that nothing was found.

Let’s demonstrate it with List.find_opt : ('a -> bool) -> 'a list -> 'a option function from the standard library that takes a function of type 'a -> bool and checks if it’s true for any element of a list until it finds one or the end of the list is reached:

let xs = [1; 3; 5; 7]

let () =
  let res = List.find_opt (fun x -> x mod 2 = 0 ) xs in
  match res with
  | Some value -> Printf.printf "Found even number %d\n" value
  | None -> Printf.printf "List has no even numbers"

If there can be different error conditions, it’s better to use the result type:

(* (//) : int -> int -> (int, string) result *)
let (//) x y =
  if y = 0 then Error "Division by zero"
  else Ok (x / y)

let () =
  let res = 5 // 0 in
  match res with
  | Ok value -> Printf.printf "Result is %d\n" value
  | Error msg -> Printf.printf "Error: %s\n" msg

The advantage of this approach is that function types indicate possible errors conditions, and those errors are impossible to ignore: you have to use pattern matching to unwrap the value from Some or Ok before you can use it.

The disadvantage is that you have to unwrap the value before you can use it, so errors must be handled immediately.

The bind combinator

Handling all option or result values by hand can be a tedious task. The standard library provides so called bind functions for them to allow working with option or result types more easily. With a bind, you can write a pipeline of functions where Some or Ok values are passed down the pipeline, but error values stop it and propagate back up.

For a type 'a t, a bind function has this type: 'a -> ('a -> 'a t) -> 'a t.

Thus for the option we have a function Option.bind : 'a option -> ('a -> 'a option) -> 'a option.

From its type we can see that it takes a value of type 'a option and a function of type 'a -> 'a option. Internally, it passes the value to that function. What’s important is that the the input of the function it takes is 'a, not 'a option—the bind function deals with the errors on its own so that your function only needs to know how to produce one, but not how to handle it.

This is how it’s implemented:

let bind o f =
  match o with
  | None -> None
  | Some v -> f v

As you can see, it simply returns None if the value is None or applies the function f to the value attached to Some.

For ease of use, the bind function is usually aliased to an infix operator, traditionally >>=. Let’s rewrite our example using the Option.bind:

let (>>=) = Option.bind

let handle_search_result o =
  match o with
  | None -> Printf.printf "Nothing found\n"
  | Some v -> Printf.printf "Found %d\n" v

let is_even x = x mod 2 = 0

let () =
  List.find_opt is_even [1; 3; 5] >>= handle_search_result

If we had multiple functions of type 'a -> 'a option, we could write a much longer pipeline, and still have to deal with unwrapping the option type explicitly only at the last step:

let y = x >>= f >>= g >>= h in
match y with
| None -> ...

This approach amplifies the disadvantage of the option type: if something in the pipeline returns None, we don’t know what went wrong or even where in the pipeline it happened.

The Result.bind works in a similar way, but since you can attach information about the error, you can make your pipeline much easier to debug and give the user a sensible error message.

A type t with functions bind : 'a t -> ('a -> 'a t) -> 'a t and return : 'a -> 'a t defined for it is known as a monad1. In our case we use Some and Ok data constructors as “virtual” return functions. Error handling is the simplest use case for the monad pattern, it can encapsulate different kinds of complexity: for example, the Lwt library uses it for asynchronous computations and multitasking.

The imperative way: exceptions

Like many other languages, OCaml has exceptions and exception handling. Even if you prefer purely functional style, you still need to learn about them because many standard library functions raise them.

Some functions exist in both pure and exception-raising variants. For example, apart from List.find_opt, there’s also List.find : ('a -> bool) -> 'a list -> 'a function that will raise Not_found exception if it fails to find anything:

let () = 
  let x = List.find (fun x -> x mod 2 = 0) [1; 3; 5] in
  Printf.printf "%d\n" x

In OCaml, exceptions are not objects, and there are no exception hierarchies. It may look unusual now, but in fact exceptions predate the rise of object oriented languages and it’s more in line with original implementations. The advantage is that they are very lightweight.

The syntax for defining exceptions is reminiscent of sum types with a single data constructor. Exception constructors can be either nullary or can have a value of any type attached to them.

All exceptions are essentially members of a single extensible (open) sum type exn, with some language magic to allow raising and catching them.

This is how you can define new exceptions:

exception Access_denied

exception Invalid_value of string

(* Parse error: line, column, error message *)
exception Parse_error of (int * int * string)

Raising and catching exceptions

Built-in the division functions / and /. raise a Division_by_zero exception when the divisor is zero.

To learn how to raise an exception ourselves, we can reimplement a division function to raise our own exception. They are raised using the raise : exn -> 'a function:

exception Invalid_value of string

let (//) x y =
  if y <> 0 then x / y
  else raise (Invalid_value "Attempted division by zero")

Now let’s learn how to catch exceptions. Instead of inventing our own exception, we will use a built-in exception named Division_by_zero:

let () =
    let x = 4 / 0 in
    Printf.printf "%d\n" x
  with Division_by_zero -> print_endline "Cannot divide by zero"

Note that try ... with constructs are expressions rather than statements, and can be easily used inside let-bindings, for example to provide a default value in case an exception is raised:

let x = try 4 / 0 with Division_by_zero -> 0 (* x = 0 *)

let () = Printf.printf "%d\n" x

Another implication of the fact that try ... with is an expression is that all expressions in the try and with clauses must have the same type. This would cause a type error:

let x = try 4 / 0 with Division_by_zero -> print_endline "Division by zero"

You can catch multiple exceptions using a syntax similar to that of match expressions:

let () =
    let x = 4 / 0 in
    Printf.printf "%d\n" x
    Failure s -> print_endline s
  | Division_by_zero -> print_endline "Division by zero"

So far our examples assumed that we know all exceptions our functions can possibly raise, or do not want to catch any other exceptions, which is not always the case. Since there are no exception hierarchies, we cannot catch a generic exception, but we can use the wildcard pattern to catch any possible exception. The downside of course is that if even if an exception comes with an attached value, we cannot destructure it and extract the value since the type of that value is not known in advance.

let () =
    let x = 4 / 0 in
    Printf.printf "%d\n" x
    Division_by_zero -> print_endline "Cannot divide by zero"
  | _ -> print_endline "Something went wrong"

Built-in exceptions

These are some exceptions that are commonly raised by standard library functions and you’ll likely encounter them often:

exception Not_found

exception Failure of string

exception Sys_error of string

exception Match_failure of string * int * int

The Not_found and Failure exceptions are normally used to signal error conditions caused by user input. They are expected to be caught.

The Sys_error exception is commonly raised by functions that call the operating system, for example you will get it when trying to open a file that doesn’t exist or that you have no permission to read.

The Match_failure exception is rather more special. It is raised when pattern matching is not exhaustive and the function hits the unhandled case. The compiler will always warn you that your pattern matching is not exhaustive, but as we discussed earlier, the checking algorithm can sometimes incorrectly assume that it’s not exhaustive when it actually is, so by default it is a warning rather than error. But if the compiler was right, it will cause runtime errors, ending with Match_failure. Encountering that exception thus indicates a programming error and supressing it with exception handling is never the right thing to do—if you genuinely do not care about remaining cases, you should indicate it explicitly by adding a wildcard match clause.

The types of exception-raising functions

As you’ve already seen, it’s impossible to know if a function raises any exceptions just from its type. That’s the advantage of exceptions: you don’t need to deal with unwrapping anything, and if you don’t want to handle them, you can just let them pass and crash the program. It may be even better than the result type for dealing with unexpected errors, since it allows you to find out where exactly in the program it happened.

The raise function has type exn -> 'a. This is unusual because the type variable 'a doesn’t appear in the left-hand side of the type. It’s a function from the type exn to any other type.

The exit : int -> 'a function has similar type. It takes an exit code value and terminates the program with it.

The key to understanding this is to stop thinking about return values and think about types that can be substituted for the 'a type variable without compromising type safety.

Since raise and exit abandon the current computation, they can be substituted for anything because nothing can possibly get a value of an unexpected type from them — they produce no value at all. This is why it has a free type variable in the right hand side — it isn’t runtime safe, but type safety is unaffected by exceptions.

Generalizations of exceptions that can be reflected in function types and automatically checked is an area of active research.

Enabling exception tracing

By default, if an exception occurs, OCaml programs only show its type and associated value.

Consider the following program that always fails with an exception.

let _ = raise (Failure "Error!")

If you compile it with default options and run it, you will see the following output:

$ ocamlopt -o failure ./

$ ./failure
Fatal error: exception Failure("Error!")

However, if you enable debug information in the compiler options (-g), it’s possible to get an exception trace with the call stack and line numbers.

Even for programs compiled with debug information, exception tracing is still disabled by default to avoid performance overhead. One way to enable it is to set the OCAMLRUNPARAM environment variable to b (for “backtrace”).

$ ocamlopt -g -o failure ./

$ OCAMLRUNPARAM=b ./failure
Fatal error: exception Failure("Error!")
Raised at Failure in file "./", line 1, characters 8-32

It’s also possible to enable exception tracing from within a program by calling Printexc.record_backtrace true.

let () = Printexc.record_backtrace true

let _ = raise (Failure "Error!")

If the program above is compiled with debug information, it will always produce a trace.

$ ocamlopt -g -o failure ./

$ ./failure
Fatal error: exception Failure("Error!")
Raised at Failure in file "./", line 3, characters 8-32

1For a good introduction, see Philip Wadler’s paper Monads for functional programming.