Yan Cui
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Throwing Exceptions
In F#, you can define a custom exception type by creating a type that inherit from System.Exception or using the lightweight exception syntax to define them with the same syntax as discriminated unions.
You can throw exceptions in F# using the raise keyword or using failwith or failwithf:
exception MyExceptionA
exception MyExceptionB of string
let exThrower (isA : bool option) =
match isA with
| Some(true) -> raise MyExceptionA
| Some(false) -> raise <| MyExceptionB("Blah Blah")
| _ -> raise <| new System.Exception()
| _ -> failwith "Not a valid input"
Erlang also has a number of different exception types, the most common ways to raise an exception in Erlang is via:
- error(Reason) – terminates the current process and includes a stack trace.
- exit(Reason) – similar to errors, but does not return the stack trace.
- throw(Reason) – throws exceptions that the caller can be expected to handle, also used as a way to return from deep recursion.
1> throw("Blah Blah").
** exception throw: "Blah Blah"
2> error(boo).
** exception error: boo
3> exit(foo).
** exception exit: foo
Catching Exceptions
In F#, you can implement a try-catch block using the try-with keywords, and it’s really easy to use pattern matching to make handling multiple types of exceptions really easy.
Borrowing from the wonderfully humorous example from Learn you some Erlang for great good! here’s how it’ll look in F#:
| exception Slice | |
| exception CutArm | |
| exception CutLeg | |
| exception Punch | |
| exception CrossBridge | |
| let blackKnight (attack : unit -> unit) = | |
| try | |
| attack() | |
| printfn "None shall pass." | |
| with | |
| | :? Slice -> printfn "It is but a scratch." | |
| | :? CutArm -> printfn "I've had worse." | |
| | :? CutLeg -> printfn "Come on you pansy!" | |
| | _ -> printfn "Just a flesh wound." | |
| let sword n = | |
| match n with | |
| | 1 -> raise Slice | |
| | 2 -> raise CutArm | |
| | 3 -> raise CutLeg | |
| | 4 -> raise Punch | |
| | 5 -> raise CrossBridge | |
| | _ -> () | |
| let talk () = "blah blah" |> ignore | |
| blackKnight talk | |
| blackKnight (fun () -> sword(1)) | |
| blackKnight (fun () -> sword(2)) | |
| blackKnight (fun () -> sword(3)) | |
| blackKnight (fun () -> sword(4)) | |
| blackKnight (fun () -> sword(5)) | |
| (* the above prints: | |
| None shall pass. | |
| It is but a scratch. | |
| I've had worse. | |
| Come on you pansy! | |
| Just a flesh wound. | |
| Just a flesh wound. | |
| *) |
by
In Erlang, the syntax is surprisingly similar:
| -module(exceptions). | |
| -export([sword/1, talk/0, black_knight/1]). | |
| sword(1) -> throw(slice); | |
| sword(2) -> erlang:error(cut_arm); | |
| sword(3) -> exit(cut_leg); | |
| sword(4) -> throw(punch); | |
| sword(5) -> exit(cross_bridge). | |
| talk() -> "blah blah". | |
| black_knight(Attack) when is_function(Attack, 0) -> | |
| try Attack() of | |
| _ -> "None shall pass." | |
| catch | |
| throw:slice -> "It is but a scratch."; | |
| error:cut_arm -> "I've had worse."; | |
| exit: cut_leg -> "Come on you pansy!"; | |
| _:_ -> "Just a flesh wound." | |
| end. | |
| %% run these into the shell | |
| 1> c(exceptions). | |
| {ok,exceptions} | |
| 2> exceptions:black_knight(fun exceptions:talk/0). | |
| "None shall pass." | |
| 3> exceptions:black_knight(fun () -> exceptions:sword(1) end). | |
| "It is but a scratch." | |
| 4> exceptions:black_knight(fun () -> exceptions:sword(2) end). | |
| "I've had worse." | |
| 5> exceptions:black_knight(fun () -> exceptions:sword(3) end). | |
| "Come on you pansy!" | |
| 6> exceptions:black_knight(fun () -> exceptions:sword(4) end). | |
| "Just a flesh wound." | |
| 7> exceptions:black_knight(fun () -> exceptions:sword(5) end). | |
| "Just a flesh wound." |
Another thing to keep in mind is that whilst F# has try-with and try-finally blocks there is no equivalent of C#’s try-catch-finally block. In Erlang the try-catch-finally block looks like this:
black_knight(Attack) when is_function(Attack, 0) –>
try Attack() of
_ -> "None shall pass."
catch
throw:slice -> "It is but a scratch.";
error:cut_arm -> "I’ve had worse.";
exit: cut_leg -> "Come on you pansy!";
_:_ -> "Just a flesh wound."
after
io:format("Attack finished…~n", [])
end.
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