Custom REPLs

This page documents recent changes to requirements for custom REPLs that use the functionality provided in cljs.repl. These changes have been made towards the goal of dramatically diminishing the start time of all ClojureScript REPLs and simplifying the synchronization of REPL state with compiled source. This is accomplished by reusing the globally available compilation caching infrastructure. In fact it is currently possible to launch a REPL with :output-dir set to an existing compilation cache and incur no analysis or compilation.

Under the new infrastructure all the builtin REPLs are capable of booting on modern hardware in a second or less.


In order to boot REPLs as quickly as possible REPLs must implement the new 2-arg arity of -setup which take the typical compiler build options. In the past -setup was permitted to be asynchronous - this is no longer supported, REPLs must now compile and load cljs.core and all of its dependencies during -setup. In -setup REPLs should use the build options to cache compiled JavaScript and analysis information to the expected location. Note, while it is OK to stream compiled forms the user has entered this should be avoided at all costs for loading namespaces - REPLs should rely on the target environment to interpret goog.require. This has many benefits including precise source mapping information.

The new Node.js REPL is a good example of the new pattern. The Node.js REPL is short because it relies on the Node.js runtime itself to interpret goog.require.

Examining cljs.repl/load-file and cljs.repl/load-namespace will clarify the new approach:

  • Given a namespace ensure that it’s compiled.

  • Compute the goog.addDependency string for the file and evaluate it.

  • Emit goog.require statement for the namespace and evaluate it.

REPLs should override the global CLOSURE_IMPORT_SCRIPT function to get custom goog.require behavior.

Eliminating Loaded Libs tracking

Under the new changes REPLs no longer need to bother with explicitly tracking loaded libs directly within their Clojure implementation. Instead, REPLs should arrange to ensure that the JavaScript evaluation environment honors cljs.core/loaded-libs, embedding the required logic in CLOSURE_IMPORT_SCRIPT if need be.

History: This was only previously done because goog.provide throws if the namespace has already been loaded. This is a completely bogus error intended to teach "beginners". By monkey-patching goog.isProvided_ to be a function that always returns false - the error can be suppressed. Again the Node.js REPL is a good example of such patching as well as honoring loaded-libs in the CLOSURE_IMPORT_SCRIPT implementation.

Special Functions

All REPLs support several "special functions". Special functions must take the REPL environment, an analysis environment, the form, and (optionally) compiler build options. Out of the box in-ns, require, load-file, and load-namespace are provided.


Custom REPLs should not call println, print, or flush directly, but should instead honor values associated with :print, :print-no-newline, and :flush in the opts (second argument) passed to -setup. Also note that the functions associated with :print and :print-no-newline take exactly one argument.

Source Mapping

All REPLs can now implement a new protocol in order to get source mapping support for "free". In the case of an :exception result from evaluation the REPL infrastructure will invoke -parse-stacktrace if the REPL evaluation environment satisfies cljs.repl/IParseStacktrace. The REPL evaluation environment will receive the original JavaScript stacktrace string, the entire original error value, as well as all build options passed into the REPL. The REPL evaluation environment may then return a canonical stacktrace which must take the form of:

[{:function <string>
  :file <string>
  :line <integer>
  :column <integer>}*]

:file must be a URL style path (forward slashes) without a URI protocol relative to :output-dir.

With this commit, the contract has been relaxed slightly to accommodate REPL-defined functions: The :file value may begin with < to indicate that no source is present, and "NO_SOURCE_FILE" will be emitted in the trace.

Custom REPLs may still want to further customize or control printing of stacktraces. A hook is provided, the REPL evaluation environment may implement cljs.repl/IPrintStacktrace. -print-stacktrace takes the mapped canonical stacktrace, the entire original error value, and all build options passed to the REPL.

Original author: David Nolen