Middleware are higher-order functions that accept a handler and return a new handler that may compose additional functionality onto or around the original. For example, some middleware that handles a hypothetical "time?" :op by replying with the local time on the server:

 '[nrepl.misc :refer (response-for)]
 '[nrepl.transport :as t])

(defn current-time
  (fn [{:keys [op transport] :as msg}]
    (if (= "time?" op)
      (t/send transport (response-for msg :status :done :time (System/currentTimeMillis)))
      (h msg))))

A little silly, but this pattern should be familiar to you if you have implemented Ring middleware before. Nearly all of the same patterns and expectations associated with Ring middleware should be applicable to nREPL middleware.

All of nREPL’s provided default functionality is implemented in terms of middleware, even foundational bits like session and eval support. This default middleware "stack" aims to match and exceed the functionality offered by the standard Clojure REPL, and is available at nrepl.server/default-middlewares. Concretely, it consists of a number of middleware functions' vars that are implicitly merged with any user-specified middleware provided to nrepl.server/default-handler. To understand how that implicit merge works, we’ll first need to talk about middleware "descriptors".

(See this documentation listing for details as to the operations implemented by nREPL’s default middleware stack, what each operation expects in request messages, and what they emit for responses.)

Middleware descriptors and nREPL server configuration

It is generally the case that most users of nREPL will expect some minimal REPL functionality to always be available: evaluation (and the ability to interrupt evaluations), sessions, file loading, and so on. However, as with all middleware, the order in which nREPL middleware is applied to a base handler is significant; e.g., the session middleware’s handler must look up a user’s session and add it to the message map before delegating to the handler it wraps (so that e.g. evaluation middleware can use that session data to stand up the user’s dynamic evaluation context). If middleware were "just" functions, then any customization of an nREPL middleware stack would need to explicitly repeat all of the defaults, except for the edge cases where middleware is to be appended or prepended to the default stack.

To eliminate this tedium, the vars holding nREPL middleware functions may have a descriptor applied to them to specify certain constraints in how that middleware is applied. For example, the descriptor for the nrepl.middleware.session/add-stdin middleware is set thusly:

(set-descriptor! #'add-stdin
  {:requires #{#'session}
   :expects #{"eval"}
   :handles {"stdin"
             {:doc "Add content from the value of \"stdin\" to *in* in the current session."
              :requires {"stdin" "Content to add to *in*."}
              :optional {}
              :returns {"status" "A status of \"need-input\" will be sent if a session's *in* requires content in order to satisfy an attempted read operation."}}}})

Middleware descriptors are implemented as a map in var metadata under a :nrepl.middleware/descriptor key. Each descriptor can contain any of three entries:

  • :requires, a set containing strings or vars identifying other middleware that must be applied at a higher level than the middleware being described. Var references indicate an implementation detail dependency; string values indicate a dependency on any middleware that handles the specified :op.

  • :expects, the same as :requires, except the referenced middleware must exist in the final stack at a lower level than the middleware being described.

Another way to think of :expects and :requires would be before and after. Middleware you’re expecting should have already been applied by the time the middleware that expects it gets applied, and middleware that’s required should be applied afterwards. We’ll expand on this in the paragraphs to come.
  • :handles, a map that documents the operations implemented by the middleware. Each entry in this map must have as its key the string value of the handled :op and a value that contains any of four entries:

    • :doc, a human-readable docstring for the middleware

    • :requires, a map of slots that the handled operation must find in request messages with the indicated :op

    • :optional, a map of slots that the handled operation may utilize from the request messages with the indicated :op

    • :returns, a map of slots that may be found in messages sent in response to handling the indicated :op

The values in the :handles map are used to support the "describe" operation, which provides "a machine- and human-readable directory and documentation for the operations supported by an nREPL endpoint" (see nrepl.impl.docs/generate-ops-info and the results of lein with-profile +maint run nrepl.impl.docs here).

There’s also lein with-profile +maint run nrepl.impl.docs --output md if you’d like to generate an ops listing in Markdown format.

The :requires and :expects entries control the order in which middleware is applied to a base handler. In the add-stdin example above, that middleware will be applied after any middleware that handles the "eval" operation, but before the nrepl.middleware.session/session middleware. In the case of add-stdin, this ensures that incoming messages hit the session middleware (thus ensuring that the user’s dynamic scope — including in — has been added to the message) before the add-stdin’s handler sees them, so that it may append the provided `stdin content to the buffer underlying in. Additionally, add-stdin must be "above" any eval middleware, as it takes responsibility for calling clojure.main/skip-if-eol on in prior to each evaluation (in order to ensure functional parity with Clojure’s default stream-based REPL implementation).

The specific contents of a middleware’s descriptor depends entirely on its objectives: which operations it is to implement/define, how it is to modify incoming request messages, and which higher- and lower-level middlewares are to aid in accomplishing its aims.

nREPL uses the dependency information in descriptors in order to produce a linearization of a set of middleware; this linearization is exposed by nrepl.middleware/linearize-middleware-stack, which is implicitly used by nrepl.server/default-handler to combine the default stack of middleware with any additional provided middleware vars. The primary contribution of default-handler is to use nrepl.server/unknown-op as the base handler; this ensures that unhandled messages will always produce a response message with an :unknown-op :status. Any handlers otherwise created (e.g. via direct usage of linearize-middleware-stack to obtain a ordered sequence of middleware vars) should do the same, or use a similar alternative base handler.


There are two types of sessions: ephemeral sessions and long-lived sessions (or registered sessions).

When a message arrives without a session id, an ephemeral session is created and assigned to it. Ephemeral sessions are bound to the processing of a single message.

The only way to create a long-lived session is to clone an existing session (even an ephemeral one). Cloning a session creates a new session that initially shares the dynamic bindings of the original session.

Sessions (long-lived ones since they are the useful ones) serialize evaluations and make dynamic bindings available for inspection to other ops.

All other ops have no serialization guarantee (they are serialized in the current implementation since they run on the server IO thread). However evals run on a dedicated thread so a running eval can’t block another op.

Sessions become even more useful when different nREPL extensions start taking advantage of them. debug-repl uses sessions to store information about the current breakpoint, allowing debugging of two things separately. piggieback uses sessions to allow host a ClojureScript REPL alongside an existing Clojure one.

Pretty Printing

Pretty printing support was added in nREPL 0.5 and the API is still considered experimental.

nREPL includes a print middleware to print the results of evaluated forms as strings for returning to the client. This enables using libraries like puget to pretty print the evaluation results automatically. The middleware options may be provided in either requests or responses (the former taking precedence over the latter if any options are specified in both). The following options are supported:

  • :nrepl.middleware.print/print: a fully-qualified symbol naming a var whose function to use for printing. Defaults to the equivalent of clojure.core/pr.

    • The var must point to a function of three arguments:

      • value: the value to print.

      • writer: the java.io.Writer to print on.

      • options: a (possibly nil) map of options.

    • Note well that the printing function is expected to not interact with *out* or *err* at all, even rebinding them (e.g. via with-out-str). Output may be printed to either of those streams during its operation – consider the following example:

    (->> [1 2 3]
         (map (fn [n]
                (println n)
    • The result of the expression is (1 2 3), and evaluating it will result in each of the three numbers being printed to *out*. However, because map is lazy, the calls to println will be interleaved with the operation of the printer function. Hence if the printer function is coupled to *out*, its output might be interleaved with that of the calls to println.

      • Technically, map is not fully lazy – it returns a chunked sequence – but the principle still applies.

    • Further, note that clojure.pprint/pprint rebinds *out* internally (even when using its explicit writer arity). It is not possible to prevent the interleaving of output when using clojure.pprint.

  • :nrepl.middleware.print/options: a map of options to pass to the printing function. Defaults to nil.

  • :nrepl.middleware.print/stream?: if logical true, the result of printing each value will be streamed to the client over one or more messages. Defaults to false.

  • :nrepl.middleware.print/buffer-size: size of the buffer to use when streaming results. Defaults to 1024.

    • Note that this only represents an upper bound on the number of bytes per message – the printing function may also call flush on writer, which will result in a response being sent immediately.

  • :nrepl.middleware.print/quota: a hard limit on the number of bytes printed for each value.

    • A status of :nrepl.middleware.print/truncated will be returned by the middleware if the quota is exceeded. In streamed mode, this will be conveyed as a discrete response after the final printing result. Otherwise, it will be added to the status of the response, and additionally the response will include :nrepl.middleware.print/truncated-keys, indicating which keys in the response were truncated.

  • :nrepl.middleware.print/keys: a seq of the keys in the response whose values should be printed. Defaults to [:value] for eval and load-file responses.

{:op :eval
 :code "(+ 1 1)"
 :nrepl.middleware.print/print 'my.custom/print-value
 :nrepl.middleware.print/options {:print-width 120}
 :nrepl.middleware.print/stream? true
 :nrepl.middleware.print/buffer-size 1024
 :nrepl.middleware.print/quota 8096}

The functionality of the print middleware is reusable by other middlewares. If a middleware descriptor’s :requires set contains #'nrepl.middleware.print/wrap-print, then it can expect:

  • Any responses it returns will have its values printed according to the above options, as provided in the request and/or response.

    • For example, to ensure that :value is printed, responses from the eval middleware look like this:

    {:ns "user"
     :value '(1 2 3)
     :nrepl.middleware.print/keys #{:value}}
  • Any requests it handles will contain the key :nrepl.middleware.print/print-fn, whose value is a function that calls the given printer function with the given options – i.e. its signature is [value writer].

Evaluation Errors

nREPL includes a caught middleware which provides a configurable hook for any java.lang.Throwable that should be conveyed interactively (generally by printing to *err*). Like the print middleware, any options may be provided in either requests or responses (the former taking precedence over the latter if any options are specified in both). The following options are supported:

  • :nrepl.middleware.caught/caught: a fully-qualified symbol naming a var whose function to use to convey interactive errors. Must point to a function that takes a java.lang.Throwable as its sole argument. Defaults to clojure.main/repl-caught.

  • :nrepl.middleware.caught/print?: if logical true, the printed value of any interactive errors will be returned in the response (otherwise they will be elided). Delegates to nrepl.middleware.print to perform the printing. Defaults to false.

{:op :eval
 :code "(/ 1 0)"
 :nrepl.middleware.caught/caught 'my.custom/print-stacktrace
 :nrepl.middleware.caught/print? true}

The functionality of the caught middleware is reusable by other middlewares. If a middleware descriptor’s :requires set contains #'nrepl.middleware.caught/wrap-caught, then it can expect:

  • Any returned responses containing the key :nrepl.middleware.caught/throwable will have that key’s corresponding value passed to the hook.

  • Any handled requests will contain the key :nrepl.middleware.caught/caught-fn, whose value is a function that can be called on a java.lang.Throwable to convey errors interactively.