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Stuart Hinson

Rust & Clojure programmer. Open for freelancing.

stuarthinson<at>gmail

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Pedestal 0.4.0 includes io.pedestal.interceptor.error/error-dispatch, which uses core.match to make error handling very tidy. For example

(def my-error-handler
  (error-int/error-dispatch [context ex]
    ;; match errors from body-params interceptor
    {:interceptor :io.pedestal.http.body-params/body-params}
    (let [response (-> (ring.util.response/response "Invalid data given") :status 400)]
      (assoc context :response response))

     ;; match FileNotFoundException in the :enter stage
    {:exception-type :java.io.FileNotFoundException :stage :enter}
    ;; respond to this error differently

    ;; pass along the error if we don't match
    :else (assoc context :io.pedestal.impl.interceptor/error ex)
    ))

creates an error handler that tries to matches any Throwable from ,

The error-dispatch macro splices its forms after the binding vector ([context ex] above) into a core.match expression matching against the ex-data of our exception, which is defined in io.pedestal.impl.interceptor, and contains :execution-id, :stage, :interceptor, :exception-type, and :exception.

So my-error-handler will respond with a 400 for any Throwable from the interceptor named :io.pedestal.http.body-params/body-param, etc.

Giving a nice separation between the semantically-correct error for an interceptor to throw (e.g. a parser error for invalid JSON) and the response that’s generated (a status 400 for invalid input).