I am interested in this view that exceptions are an anti pattern. I have
heard it voiced before.
I am not sure that I understand why.
As I see it you have a choices:
1. Handle in the result - and test this result repeatedly all the way back
to the caller
2. Handle "out of band" - Throw an exception, allow the stack to unwind and
catch where it matters
[And maybe - but I am not very knowledgeable on this and it won't work
today on the JVM anyway
3. Use continuation passing style with TCO to shortcut the return to
follow an exception path]
So, ignoring 3.
Why is 2 preferable over 1? There are certainly pros and cons.
Dave
On Wednesday, 20 March 2013 09:42:11 UTC+11, James Reeves wrote:
>
> I'd argue that using exceptions for control flow is something of an
> anti-pattern, even in Java.
>
> In this case a better mechanism might be to use polymorphism. For instance:
>
> (defprotocol Validatable
> (validation-errors [x] "Return the validation errors for x."))
>
> (defn valid? [x]
> (empty? (validation-errors x)))
>
> Then you can define a general function to validate and store that item in
> a database:
>
> (defn store-valid [db x]
> (if (valid? x)
> (store db x)
> (validation-error-response x)))
>
> - James
>
>
> On 19 March 2013 16:43, Julien Dreux <julien...@gmail.com <javascript:>>wrote:
>
>> Hi all,
>>
>> Coming from a Java background, I am having a hard time understanding how
>> validation error propagation should work in clojure web APIs.
>>
>> To be clear, this is similar to how my Java web service would be setup:
>>
>> /** Method that validates the model, accesses the DB. If something went
>> wrong, throw an exception */
>> public void validateAndCreateUser(User u) throws ValidationException,
>> EmailAlreadyInUseException, ... {
>> ...
>> if(...) {
>> throw new ValidationException(fieldName);
>> } else if (...) {
>> throw new EmailAlreadyInUseException(u.getEmail());
>> }
>> }
>>
>> /** Endpoint method, catches & formats the exceptions thrown by the db
>> method. **/
>> @POST("/api/user/create")
>> public Response createUser (User u) {
>> ..
>> try{
>> validateAndCreateUser(u);
>> return Response.ok();
>> } catch (Exception e) {
>> return generateExceptionResponse(e); //Method that maps exceptions to
>> responses.
>> }
>> }
>>
>> For all of Java's clunkiness, this had the benefit of not having to write
>> tons of if/else statements for validation handling. Exception were just
>> thrown from anywhere, bubbling back up to inital call, and if not handled
>> in the endpoint method, a specialized class mapped them into a proper
>> response. The exceptions contained all the information needed to generate
>> 'rich' error messages back to the client.
>>
>> Being a Clojure newbie, I wonder what a good pattern is for a similar
>> situation. So far, I have a method that validates models based on a schema,
>> that returns
>>
>> {:success true}
>>
>> or
>>
>> {:success false :errors ["error 1" "error 2" ...]}
>>
>> But I don't know how to avoid having to write if/else conditions of the
>> sort in each function between my endpoint and db functions.
>>
>> (if (validation :success)
>> (follow-normal-path)
>> (handle-validation-errors validation))
>>
>>
>> Any guidance appreciated.
>>
>> Cheers,
>>
>> Julien
>>
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