F# Kung Fu #3: Exceptions recap.

Define

Usually, you do not need to define custom exceptions during programming in F#. If you do scripting in F#, in the most cases you will be happy with standard .NET exception types and F# built-in helper functions. But when you create a custom library or design complex enterprise software, you will need to use power of .NET exceptions. How you can do it from F#:

// C# style exception (possible, but not recommended)
type MyCsharpException(msg, id:int) =
  inherit System.Exception(msg)
  member this.Id = id

// F# style exceptions
exception MyFsharpSimpleException
exception MyFsharpException of string * int

Note that F# has a special keyword exception for defining “handy” exceptions.

Raise(Throw)

F# provides set of functions (failwith, failwithf, invalidArg, invalidOp, nullArg) that help to raise most common exceptions. They are very convenient especially for F# scripts.

let rnd = System.Random()

let rnd = System.Random()

let raiseException() =
    match rnd.Next(8) with
    | 0 -> failwith "throws a generic System.Exception"
    | 1 -> failwithf "throws a generic Exception with formatted message (%d)" 1
    | 2 -> invalidArg "_" "throws an ArgumentException"
    | 3 -> invalidOp "throws an InvalidOperationException"
    | 4 -> nullArg "throws a NullArgumentException"
    | 5 -> raise <| MyFsharpException("throws a MyFsharpException", 5)
    | 6 -> raise <| MyCsharpException("throws a MyCsharpException", 6)
    | 7 -> assert (2>1)
    | _ -> raise <| System.Exception("Impossible case")

The assert expression is syntactic sugar for System.Diagnostics.Debug.Assert. The assertion is triggered only if the DEBUG compilation symbol is defined.

Try-With/Finally (Catch)

The last step is to catch exceptions and handle them in a proper way.

let catchException() =
    try
        raiseException()
    with
    | Failure(msg) // 'Failure' active pattern catches only Exception objects
     -> printfn "%s" msg
    | MyFsharpException(msg, id)
     -> printfn "Catch F# exceptions using pattern matching"
    | : ? System.ArgumentException as ex
     -> printfn "Invalid argument '%s'" ex.ParamName
    | : ? MyCsharpException | : ? System.InvalidOperationException
     -> printfn "You can handle multiple exceptions at a time"
    | _ as ex
     -> printfn "Log: exception '%s'" (ex.GetType().Name)
        reraise() // re-raise exception

let finaly() =
    try
        catchException()
    finally
        printfn "Now, I am ready for exceptions!"

‘: ?‘ is a two-symbol operator without space inside.

Note:

• Failure active pattern catches only System.Exception objects. It is useful to handle exceptions raised by failwith & failwithf functions.
• Exceptions defined using exception keyword could be handled automatically using pattern matching.
• F# provides reraise function that helps to raise a current exception one more time. This function can be used only from pattern matching rules of try-with expressions.

If you want to learn more about exceptions, read an amazing The “Expressions and syntax” series from Scott Wlaschin.