Better REPL behavior on syntax errors in meta commands.

Fixes <https://bugs.gnu.org/30600>.

* module/system/repl/command.scm (define-meta-command): Flush all
remaining input after handling a read error.
* module/system/repl/common.scm (flush-all-input): New public procedure.
* module/system/repl/repl.scm: Remove local flush-all-input definition.

module/system/repl/command.scm

@@ -229,7 +229,10 @@
                                  (lp (cons x out)))))))
                      (lambda (k . args)
                        (handle-read-error #f k args)))))
-           (lambda (k) #f)))))           ; the abort handler
+           ;; The abort handler:
+           (lambda (k)
+             (flush-all-input)
+             #f)))))
 
     ((_ ((name category) repl . datums) docstring b0 b1 ...)
      (define-meta-command ((name category) repl () . datums)

module/system/repl/common.scm

@@ -33,7 +33,7 @@
             repl-expand repl-optimize repl-optimize-cps
             repl-parse repl-print repl-option-ref repl-option-set!
             repl-default-option-set! repl-default-prompt-set!
-            puts ->string user-error
+            puts ->string user-error flush-all-input
             *warranty* *copying* *version*))
 
 (define *version*
@@ -297,3 +297,10 @@ See <http://www.gnu.org/licenses/lgpl.html>, for more details.")
 
 (define (user-error msg . args)
   (throw 'user-error #f msg args #f))
+
+(define (flush-all-input)
+  (if (and (char-ready?)
+           (not (eof-object? (peek-char))))
+      (begin
+        (read-char)
+        (flush-all-input))))

module/system/repl/repl.scm

@@ -92,13 +92,6 @@
                  *unspecified*)
                 (else ((language-reader lang) port env))))))))
         
-(define (flush-all-input)
-  (if (and (char-ready?)
-           (not (eof-object? (peek-char))))
-      (begin
-        (read-char)
-        (flush-all-input))))
-
 ;; repl-reader is a function defined in boot-9.scm, and is replaced by
 ;; something else if readline has been activated. much of this hoopla is
 ;; to be able to re-use the existing readline machinery.