readline/readline: add readline-newline' and readline-redisplay'

mzrl.rkt:
  Add and provide readline-newline and readline-readisplay.

readline.scrbl:
  Documentation and example code.

collects/readline/mzrl.rkt

@@ -4,7 +4,8 @@
 (provide readline readline-bytes
          add-history add-history-bytes
          history-length history-get history-delete
-         set-completion-function!)
+         set-completion-function!
+         readline-newline readline-redisplay)
 
 ;; libtermcap needed on some platforms
 (define libtermcap  (with-handlers ([exn:fail? void]) (ffi-lib "libtermcap")))
@@ -117,3 +118,14 @@
 ;; make it possible to run Scheme threads while waiting for input
 (set-ffi-obj! "rl_event_hook" libreadline (_fun -> _int)
               (lambda () (sync/enable-break real-input-port) 0))
+
+
+;; force cursor on a new line
+(define readline-newline
+  (get-ffi-obj "rl_crlf" libreadline (_fun -> _void)
+               (lambda ()
+                 (get-ffi-obj "rl_newline" libreadline (_fun -> _void)))))
+
+;; force redisplay of prompt and current user input
+(define readline-redisplay
+  (get-ffi-obj "rl_forced_update_display" libreadline (_fun -> _void)))

collects/readline/readline.scrbl

@@ -5,6 +5,7 @@
                      readline/pread
                      readline/readline
                      racket/contract
+                     ffi/unsafe/atomic
                      (except-in ffi/unsafe ->)))
 
 @(define readline "Readline")
@@ -240,6 +241,32 @@ Sets @|readline|'s @tt{rl_completion_entry_function} to
 from @racketmodname[ffi/unsafe], determines the type of value supplied
 to the @racket[proc].}
 
+@defproc[(readline-newline) void?]{
+
+Sets the cursor to the start of a new line.}
+
+
+@defproc[(readline-redisplay) void?]{
+
+Forces a redisplay of the @|readline| prompt and current user input.
+
+The @racket[readline-redisplay] function can be used together with
+@racket[readline-newline] to prevent a background thread from
+cluttering up the user input by interleaving its output.  For example,
+an unsafe wrapper function for the thread's output might look like the
+following:
+
+@racketblock[
+(define (with-thread-safe-output output-thunk)
+  (dynamic-wind
+    (lambda ()
+      (start-atomic)
+      (readline-newline))
+    output-thunk
+    (lambda ()
+      (readline-redisplay)
+      (end-atomic))))]}
+
 
 @section[#:tag "readline-license"]{License Issues}