Misc Functionality

Pretty Printing

Pretty printing support was added in nREPL 0.6 and the API is still considered experimental.

The namespace nrepl.middleware.print contains some dynamic vars you can set! at the REPL to alter how evaluation results will be printed. Note that if your nREPL client supports passing these options in requests, then it may override some or all of these options.

  • *print-fn*: the function to use for printing. Defaults to the equivalent of clojure.core/pr. The function must take two arguments:

    • value: the value to print.

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

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

    • This lets you see results being printed incrementally, and optionally interrupt the evaluation while it is printing.

    • Your nREPL client may not fully support this mode of operation.

  • *buffer-size*: size of the buffer to use when streaming results. Defaults to 1024.

  • *quota*: a hard limit on the number of bytes printed for each value. Defaults to nil (no limit).

    • Your nREPL client may not indicate if truncation has occurred.

For example, to prevent printing infinite lazy sequences from causing the REPL to hang:

user=> (set! nrepl.middleware.print/*quota* 32)
user=> (range)
(0 1 2 3 4 5 6 7 8 9 10 11 12 13

Or to use clojure.pprint to print evaluation results:

user=> (set! nrepl.middleware.print/*print-fn* clojure.pprint/pprint)
#object[clojure.pprint$pprint 0x2bc11aa0 "clojure.pprint$pprint@2bc11aa0"]

user=> (meta #'int)
{:added "1.0",
 :ns #object[clojure.lang.Namespace 0xea12515 "clojure.core"],
 :name int,
 :file "clojure/core.clj",
 :column 1,
 :line 882,
 :arglists ([x]),
 :doc "Coerce to int",
 #object[clojure.core$int__inliner__5509 0x3c79d89 "clojure.core$int__inliner__5509@3c79d89"]}


However, note well that clojure.pprint/pprint rebinds *out* internally, and so if anything else prints to *out* while evaluating, that output will end up intermingled in the result. See the ../design/middleware.adoc#_pretty_printing for a more detailed explanation.

Evaluation Errors

The dynamic var nrepl.middleware.caught/*caught-fn* can be set! at the REPL to alter how evaluation errors will be handled. Like the :caught option to clojure.main/repl, this is a function that takes a java.lang.Throwable (default clojure.main/repl-caught) and is called when either read, eval, or print throws an exception or error.

For example, to automatically print the stacktrace of each error:

user> (set! nrepl.middleware.caught/*caught-fn* clojure.repl/pst)
user> (first 1)
IllegalArgumentException Don't know how to create ISeq from: java.lang.Long
	clojure.lang.RT.seqFrom (RT.java:542)
	clojure.lang.RT.seq (RT.java:523)
	clojure.lang.RT.first (RT.java:668)
	clojure.core/first--4339 (core.clj:55)
	clojure.core/first--4339 (core.clj:55)
	user/eval11339 (form-init6612168545889071220.clj:12)
	user/eval11339 (form-init6612168545889071220.clj:12)
	clojure.lang.Compiler.eval (Compiler.java:6927)
	clojure.lang.Compiler.eval (Compiler.java:6890)
	clojure.core/eval (core.clj:3105)
	clojure.core/eval (core.clj:3101)
	clojure.main/repl/read-eval-print--7408/fn--7411 (main.clj:240)


Or to use the pretty stacktrace printing library to print stacktraces:

user=> (set! nrepl.middleware.caught/*caught-fn* io.aviso.repl/pretty-pst)
user=> (first 1)
 clojure.core/eval   core.clj: 3214
     user/eval5945  REPL Input
clojure.core/first   core.clj:   55
java.lang.IllegalArgumentException: Don't know how to create ISeq from: java.lang.Long


Hot-loading dependencies

From time to time you’d want to experiment with some library without adding it as a dependency of your project. You can easily achieve this with tools.deps or pomegranate. Let’s start with a tools.deps example:

$ clj -Sdeps '{:deps {nrepl {:mvn/version "0.6.0"}
                      org.clojure/tools.deps.alpha {:git/url "https://github.com/clojure/tools.deps.alpha.git"
                                                    :sha "d492e97259c013ba401c5238842cd3445839d020"}}}' -m nrepl.cmdline --interactive
Clojure 1.9.0
user=> (require '[clojure.tools.deps.alpha.repl :refer [add-lib]])
user=> (add-lib 'org.clojure/core.memoize {:mvn/version "0.7.1"})
user=> (require 'clojure.core.memoize)

Alternatively with pomegranate you can do the following:

$ clj -Sdeps '{:deps {nrepl {:mvn/version "0.6.0"} com.cemerick/pomegranate {:mvn/version "1.0.0"}}}' -m nrepl.cmdline --interactive
Clojure 1.9.0
user=> (require '[cemerick.pomegranate :refer [add-dependencies]])
user=> (add-dependencies :coordinates '[[org.clojure/core.memoize "0.7.1"]]
                         :repositories (merge cemerick.pomegranate.aether/maven-central
                                             {"clojars" "https://clojars.org/repo"}))
{[org.clojure/core.memoize "0.7.1"] #{[org.clojure/core.cache "0.7.1"] [org.clojure/clojure "1.6.0"]}, [org.clojure/core.cache "0.7.1"] #{[org.clojure/data.priority-map "0.0.7"]}, [org.clojure/data.priority-map "0.0.7"] nil, [org.clojure/clojure "1.6.0"] nil}
user=> (require 'clojure.core.memoize)