diff --git a/src/cljs/snapshot/lumo/repl.cljs b/src/cljs/snapshot/lumo/repl.cljs
index 6b7e30e8..08211a71 100644
--- a/src/cljs/snapshot/lumo/repl.cljs
+++ b/src/cljs/snapshot/lumo/repl.cljs
@@ -646,6 +646,57 @@
   "Wrap wfn around all (fn) values in fns hashmap."
   (into {} (for [[k v] fns] [k (wfn v)])))
 
+;; --------------------
+;; REPL upgrade
+
+(deftype ^:private SuspensionRequest [f])
+
+(defn suspension-request? [x] (instance? SuspensionRequest x))
+
+(defn suspension-request [f] (SuspensionRequest. f))
+
+(defprotocol AsyncReader
+  "Asynchronous stream of strings."
+  (read-chars [r f] "Calls f with a string or nil (EOF)")
+  (pushback [r s] "Unread s"))
+
+(defn yield-control [suspension-request async-reader resume-cb]
+  ((.-f suspension-request) async-reader resume-cb))
+
+(defn- create-async-pipe []
+  (let [front #js []
+        back #js []
+        cb (volatile! nil)
+        spill! #(when-some [s (.pop back)] (.push front s) (recur))]
+    #js [(fn line-cb
+           ([] ; 0-arg is called on resumption by repl.js to retrieve the content of the buffer
+             (spill!)
+             (when-some [last (.pop front)]
+               ; remove last newline since repl/readline assumes no trailing newline
+               (let [n (dec (.-length last))]
+                 (.push front (if (= \newline (.charAt last n)) (subs last 0 n) last))))
+             (.join front ""))
+           ([s] ; 1-arg is called by repl.js when new input is available 
+             (when (and s (not= "" s)) ; TODO handle EOF
+               (let [s (str s "\n")]
+                 (if-some [f @cb]
+                  (do (vreset! cb nil) (f s))
+                  (if (pos? (.-length front))
+                    (.push back s)
+                    (.push front s)))))))
+         (reify AsyncReader
+           (read-chars [r f]
+             (if-some [s (.pop front)]
+               (f s)
+               (do
+                 (spill!)
+                 (if-some [s (.pop front)]
+                   (f s)
+                   (vreset! cb f)))))
+           (pushback [r s]
+             (when (and s (not= "" s))
+               (.push front s))))]))
+
 (declare execute-path)
 
 (def ^:private repl-special-fns
@@ -917,7 +968,7 @@
   (let [{:keys [ex-kind]} (ex-data e)]
     (keyword-identical? ex-kind :eof)))
 
-(defn- read-chars
+(defn- read-all-chars
   [reader]
   (let [sb (StringBuffer.)]
     (loop [c (rt/read-char reader)]
@@ -982,7 +1033,7 @@
               r/*data-readers* (merge tags/*cljs-data-readers* (load-data-readers! env/*compiler*))
               r/resolve-symbol ana/resolve-symbol
               r/*alias-map* (current-alias-map)]
-      [(r/read {:read-cond :allow :features #{:cljs}} reader) (read-chars reader)])))
+      [(r/read {:read-cond :allow :features #{:cljs}} reader) (read-all-chars reader)])))
 
 (defn- ns-for-source [source]
   (let [[ns-form] (repl-read-string source)
@@ -1009,7 +1060,9 @@
    :*2 *2
    :*3 *3
    :*e *e
-   :ns @current-ns})
+   :ns @current-ns
+   :*print-fn* *print-fn*
+   :*print-err-fn* *print-err-fn*})
 
 (defn- set-session-state!
   "Sets the session state given a sesssion state map."
@@ -1024,7 +1077,9 @@
   (set! *2 (:*2 session-state))
   (set! *3 (:*3 session-state))
   (set! *e (:*e session-state))
-  (vreset! current-ns (:ns session-state)))
+  (vreset! current-ns (:ns session-state))
+  (set! *print-fn* (:*print-fn* session-state))
+  (set! *print-err-fn* (:*print-err-fn* session-state)))
 
 (def ^{:private true
        :doc "The default state used to initialize a new REPL session."}
@@ -1044,7 +1099,10 @@
 (defn- set-session-state-for-session-id!
   "Sets the session state for a given session."
   [session-id]
-  (set-session-state! (get @session-states session-id @default-session-state)))
+  (set-session-state! (or (get @session-states session-id)
+                        (assoc @default-session-state
+                          :*print-fn* *print-fn*
+                          :*print-err-fn* *print-err-fn*))))
 
 (defn- capture-session-state-for-session-id
   "Captures the session state for a given session."
@@ -1113,53 +1171,82 @@
         (handle-error (ex-info (str "Could not load file " file) {}) true)))))
 
 (defn- execute-text
-  [source {:keys [expression? print-nil-result? filename session-id] :as opts}]
-  (try
-    (set-session-state-for-session-id! session-id)
-    (binding [ana/*cljs-warning-handlers* (if expression?
-                                              [warning-handler]
-                                              [ana/default-warning-handler])
-              cljs/*eval-fn*   caching-node-eval
-              cljs/*load-fn*   load
-              ana/*cljs-ns*    @current-ns
-              *ns*             (create-ns @current-ns)
-              env/*compiler*   st
-              r/resolve-symbol ana/resolve-symbol
-              tags/*cljs-data-readers* (merge tags/*cljs-data-readers* (load-data-readers! env/*compiler*))
-              r/*alias-map*    (current-alias-map)]
-      (let [form (and expression? (first (repl-read-string source)))
-            eval-opts (merge (make-eval-opts)
-                        (when expression?
-                          {:context :expr
-                           :def-emits-var true}))]
-        (if (repl-special? form)
-          ((get repl-special-fns (first form)) form (merge opts eval-opts))
-          (cljs/eval-str
-            st
-            source
-            (cond
-              expression? source
-              filename (or (ns-for-source source) filename)
-              :else "source")
-            eval-opts
-            (fn [{:keys [ns value error] :as ret}]
-              (if-not error
-                (when expression?
-                  (when (or (true? print-nil-result?)
-                            (not (nil? value)))
-                    (js/$$LUMO_GLOBALS.doPrint print-value value))
-                  (process-1-2-3 form value)
-                  (when (def-form? form)
-                    (let [{:keys [ns name]} (meta value)]
-                      (swap! st assoc-in [::ana/namespaces ns :defs name ::repl-entered-source] source)))
-                  (vreset! current-ns ns))
-                (handle-error error true)))))))
-    (catch :default e
-      ;; `;;` and `#_`
-      (when-not (identical? (.-message e) "Unexpected EOF.")
-        (handle-error e true)))
-    (finally (capture-session-state-for-session-id session-id)))
-  nil)
+  [source {:keys [expression? print-nil-result? filename session-id host-yield-control] :as opts}]
+  (let [suspended (volatile! false)]
+    (try
+      (set-session-state-for-session-id! session-id)
+      (binding [ana/*cljs-warning-handlers* (if expression?
+                                                [warning-handler]
+                                                [ana/default-warning-handler])
+                cljs/*eval-fn*   caching-node-eval
+                cljs/*load-fn*   load
+                ana/*cljs-ns*    @current-ns
+                *ns*             (create-ns @current-ns)
+                env/*compiler*   st
+                r/resolve-symbol ana/resolve-symbol
+                tags/*cljs-data-readers* (merge tags/*cljs-data-readers* (load-data-readers! env/*compiler*))
+                r/*alias-map*    (current-alias-map)]
+        (let [form (and expression? (first (repl-read-string source)))
+              eval-opts (merge (make-eval-opts)
+                          (when expression?
+                            {:context :expr
+                             :def-emits-var true}))]
+          (if (repl-special? form)
+            ((get repl-special-fns (first form)) form (merge opts eval-opts))
+            (cljs/eval-str
+              st
+              source
+              (cond
+                expression? source
+                filename (or (ns-for-source source) filename)
+                :else "source")
+              eval-opts
+              (fn eval-cb [{:keys [ns value error] :as ret}]
+                (when @suspended
+                  (set-session-state-for-session-id! session-id))
+                (if (and expression? (suspension-request? value))
+                  (if host-yield-control
+                   (if-let [re-yield @suspended]
+                    (re-yield value)
+                    (do
+                      (capture-session-state-for-session-id session-id)
+                      ; host-yield-control is the function for readline yielding control
+                      ; this could be avoided by using .once and .pause but readline seems to have
+                      ; issues with pauses, see https://github.com/nodejs/node-v0.x-archive/issues/8340
+                      (host-yield-control
+                        (fn [async-reader done-cb]
+                          (let [resume #(try
+                                          (eval-cb %)
+                                          (finally
+                                            ; eval-cb may have resuspended (see re-yield above)
+                                            (when-not @suspended (done-cb))))]
+                            (vreset! suspended #(yield-control % async-reader resume))
+                            (yield-control value async-reader resume))))))
+                   (throw (js/Error. "This REPL can't be upgraded.")))
+                  (try
+                    (vreset! suspended false)
+                    (if-not error
+                      (when expression?
+                        (when (or (true? print-nil-result?)
+                                (not (nil? value)))
+                          (js/$$LUMO_GLOBALS.doPrint print-value value))
+                        (process-1-2-3 form value)
+                        (when (def-form? form)
+                          (let [{:keys [ns name]} (meta value)]
+                            (swap! st assoc-in [::ana/namespaces ns :defs name ::repl-entered-source] source)))
+                        (vreset! current-ns ns))
+                      (handle-error error true))
+                    (finally
+                      (when @suspended
+                        (capture-session-state-for-session-id session-id))))))))))
+      (catch :default e
+        ;; `;;` and `#_`
+        (when-not (identical? (.-message e) "Unexpected EOF.")
+          (handle-error e true)))
+      (finally
+        (when-not @suspended
+          (capture-session-state-for-session-id session-id))))
+  nil))
 
 (defn- execute-source
   [source-or-path {:keys [type] :as opts}]
@@ -1168,14 +1255,17 @@
     (execute-text source-or-path opts)))
 
 (defn- ^:export execute
-  [type source-or-path expression? print-nil-result? setNS session-id]
-  (clear-fns!)
-  (when setNS
-    (vreset! current-ns (symbol setNS)))
-  (execute-source source-or-path {:type type
-                                  :expression? expression?
-                                  :print-nil-result? print-nil-result?
-                                  :session-id session-id}))
+  ([type source-or-path expression? print-nil-result? setNS session-id]
+    (execute type source-or-path expression? print-nil-result? setNS session-id nil))
+  ([type source-or-path expression? print-nil-result? setNS session-id host-yield-control]
+    (clear-fns!)
+    (when setNS
+      (vreset! current-ns (symbol setNS)))
+    (execute-source source-or-path {:type type
+                                    :expression? expression?
+                                    :print-nil-result? print-nil-result?
+                                    :session-id session-id
+                                    :host-yield-control host-yield-control})))
 
 (defn- ^:export is-readable?
   [form]
diff --git a/src/js/cljs.js b/src/js/cljs.js
index 624224fa..5d68b18d 100644
--- a/src/js/cljs.js
+++ b/src/js/cljs.js
@@ -207,35 +207,31 @@ function setRuntimeOpts(opts: CLIOptsType): void {
   );
 }
 
-let cljsSender: stream$Writable;
-
-function printFn(...args: string[]): void {
-  if (utilBinding.watchdogHasPendingSigint()) {
-    throw interruptSentinel;
-  }
-  cljsSender.write(args.join(' '));
+function mkPrintFn(cljsSender: stream$Writable): () => void {
+    return (...args: string[]): void => {
+        if (utilBinding.watchdogHasPendingSigint()) {
+            throw interruptSentinel;
+        }
+        cljsSender.write(args.join(' '));
+    };
 }
 
-function printErrFn(...args: string[]): void {
-  if (utilBinding.watchdogHasPendingSigint()) {
-    throw interruptSentinel;
-  }
-
-  process.stderr.write(args.join(' '));
-}
+const printErrFn = mkPrintFn(process.stderr);
+const printOutFn = mkPrintFn(process.stdout);
 
 export function setPrintFns(stream?: stream$Writable): void {
   if (stream == null || stream === process.stdout) {
-    cljsSender = process.stdout;
+    // $FlowIssue: context can have globals
+    ClojureScriptContext.cljs.core.set_print_fn_BANG_(printOutFn);
     // $FlowIssue: context can have globals
     ClojureScriptContext.cljs.core.set_print_err_fn_BANG_(printErrFn);
   } else {
-    cljsSender = stream;
+    const printFn = mkPrintFn(stream);
+    // $FlowIssue: context can have globals
+    ClojureScriptContext.cljs.core.set_print_fn_BANG_(printFn);
     // $FlowIssue: context can have globals
     ClojureScriptContext.cljs.core.set_print_err_fn_BANG_(printFn);
   }
-  // $FlowIssue: context can have globals
-  ClojureScriptContext.cljs.core.set_print_fn_BANG_(printFn);
 }
 
 function initClojureScriptEngine(opts: CLIOptsType): void {
@@ -263,6 +259,8 @@ function initClojureScriptEngine(opts: CLIOptsType): void {
   setRuntimeOpts(opts);
 }
 
+export type AsyncReader = {};
+
 export function execute(
   code: string,
   type?: string = 'text',
@@ -270,6 +268,7 @@ export function execute(
   printNilResult?: boolean = true,
   sessionID?: number = 0,
   setNS?: string,
+  yieldControl?: (f: (async_reader: AsyncReader, resume_cb: ()=>void) => void) => void,
 ): void {
   // $FlowIssue: context can have globals
   return ClojureScriptContext.lumo.repl.execute(
@@ -279,6 +278,7 @@ export function execute(
     printNilResult,
     setNS,
     sessionID,
+    yieldControl,
   );
 }
 
@@ -315,6 +315,11 @@ export function clearREPLSessionState(sessionID: number): void {
   return ClojureScriptContext.lumo.repl.clear_state_for_session(sessionID);
 }
 
+export function createAsyncPipe(): [(s?: string)=>string, AsyncReader] {
+  // $FlowIssue: context can have globals
+  return ClojureScriptContext.lumo.repl.create_async_pipe();
+}
+
 function executeScript(
   code: string,
   type: string,
diff --git a/src/js/repl.js b/src/js/repl.js
index 6827f6cf..af43c53a 100644
--- a/src/js/repl.js
+++ b/src/js/repl.js
@@ -11,6 +11,7 @@ import { currentTimeMicros, isWhitespace, indentationSpaces } from './util';
 import { close as socketServerClose } from './socketRepl';
 
 import type { CLIOptsType } from './cli';
+import type { AsyncReader } from './cljs';
 
 type KeyType = {
   name: string,
@@ -23,6 +24,7 @@ type KeyType = {
 export type REPLSession = {
   id: number,
   rl: readline$Interface,
+  linecb: ?(s?: string) => string,
   isMain: boolean,
   isReverseSearch: boolean,
   reverseSearchBuffer: string,
@@ -86,7 +88,24 @@ export function processLine(replSession: REPLSession, line: string): void {
   const session = replSession;
   const { input, rl, isMain } = session;
 
-  let extraForms;
+  if (session.linecb) {
+      session.linecb(line);
+      return undefined;
+  }
+
+  let suspended;
+  function yieldControl(f: (async_reader: AsyncReader, resume_cb: ()=>void)=>void): void {
+      suspended = true;
+      const [linecb, reader] = cljs.createAsyncPipe();
+      session.linecb = linecb;
+      const bakprompt = rl._prompt;
+      rl.setPrompt('');
+      f(reader, () => {
+          session.linecb = null;
+          rl.setPrompt(bakprompt);
+          processLine(session, linecb());
+      });
+  }
 
   if (exitCommands.has(line.trim())) {
     // $FlowIssue - use of rl.output
@@ -99,6 +118,7 @@ export function processLine(replSession: REPLSession, line: string): void {
     session.input = `${input}\n${line}`;
   }
 
+  let extraForms;
   for (;;) {
     const currentInput = session.input;
     extraForms = cljs.isReadable(currentInput);
@@ -117,7 +137,15 @@ export function processLine(replSession: REPLSession, line: string): void {
       cljs.setPrintFns(rl.output);
       currentREPLInterface = rl;
 
-      cljs.execute(session.input, 'text', true, true, session.id);
+      suspended = false;
+      cljs.execute(session.input, 'text', true, true, session.id, undefined, yieldControl);
+      if (suspended) {
+          // yieldControl has been called, user code is in control
+          session.input = '';
+          // $FlowIssue: linecb is guaranteed to be defined when suspended
+          session.linecb(extraForms);
+          break;
+      }
 
       currentREPLInterface = null;
       cljs.setPrintFns();
@@ -265,6 +293,8 @@ function handleKeyPress(
   const { rl, isReverseSearch } = session;
   const isReverseSearchKey = ctrl && name === 'r';
 
+  if (session.linecb) return;
+
   // TODO: factor this out into own function
   if (isReverseSearch || isReverseSearchKey) {
     let failedSearch = false;
@@ -375,6 +405,7 @@ export function createSession(
     id: sessionCount,
     rl,
     input: '',
+    linecb: null,
     isMain,
     reverseSearchBuffer: '',
     isReverseSearch: false,