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.dr...@gmail.com> 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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