Add middleware example, reaadme

babashka.nrepl

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-Subproject commit ad763a78f1bc327a493ff0b650aa5408ecbf4819
+Subproject commit 24c5d08eb4795dff04d67cfaed2e4f4ebcf91404

examples/user_middleware/Readme.md

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+# User Middleware
+
+- Your middleware is defined in bb user sources.
+- Middleware is a function in wrap-middleware style.
+
+# Example
+
+Middleware should be a function in the form of:
+
+```clojure
+(defn my-middleware [handler]
+  (fn [request]
+    ;; ...
+    (handler request)
+    ;; ...
+    ))
+```
+
+# Usage
+
+```shell
+bb nrepl-server --middleware [my.middleware/println-middleware]
+```
+
+--middleware is a vector of fully qualified function symbols.
+They are required to be located on the babashka classpath.
+
+This will start babashka with a nrepl server with the middlware defined in `user_middleware/my_middleware/src/my/middleware.clj`.
+
+You can now connect to the nrepl like usual.
+
+It is possible to redefine the middleware function from within the running nrepl, because we keep a reference
+to the sci-var of the middlware.

examples/user_middleware/my_middleware/deps.edn

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+{:paths ["src"]}

examples/user_middleware/my_middleware/src/my/middleware.clj

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+(ns my.middleware)
+
+(defn println-middleware [handler]
+  (fn [request]
+    (println (:op (:msg request)))
+    (handler request)))

examples/user_middleware/my_project/bb.edn

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+{:deps
+ {my/middleware {:local/root "../my_middleware/"}}}

examples/user_middleware/my_project/hello_middleware.clj

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+(ns hello-middleware)