linenoise: visual feedback for incomplete entry.

When a line of input is incomplete and the cursor is at the
end of that line, hitting Enter causes an uncomfortable
ambiguity. Although the cursor moves to the next line, it is
not clear whether that is because the input is being accepted,
or whether the expression which was entered is executing.

For instance, these appear to behave the same way:

  > (while t[Enter][Enter]...

  > (while t)[Enter][Enter]...

One is just waiting for more input; the other is sitting in
an infinite loop just echoing the newline characters.

To partially address this issue, we introduce a visual
feedback mechanism. When Enter is issued at the end of an
incomplete line, then immediately after the insertion of
Enter, the character ! is flashed twice, alerting the user
that the line is incomplete.

In other situations, there isn't any feedback. An infinite
loop or lengthy calculation like (while t) looks the same as
code which is reading input like (get-line).

* linenoise/linenoise.c (LINNOISE_FLASH_DELAY): New macro.
(flash): New static function.
(edit): Call flash to flash the ! character when Enter is
issued at the end of an incomplete line, and we are not in
paste mode.

1 files changed, 25 insertions, 0 deletions

diff --git a/linenoise/linenoise.c b/linenoise/linenoise.c
index a80ca3ab..09bc4c10 100644
--- a/linenoise/linenoise.c
+++ b/linenoise/linenoise.c
@@ -76,6 +76,7 @@
 #define LINENOISE_MAX_LINE 1024
 #define LINENOISE_MAX_DISP (LINENOISE_MAX_LINE * 8)
 #define LINENOISE_PAREN_DELAY 400000
+#define LINENOISE_FLASH_DELAY 200000
 #define LINENOISE_MAX_UNDO 200
 
 /* The lino_state structure represents the state during line editing.
@@ -1355,6 +1356,26 @@ static void paren_jump(lino_t *l)
     }
 }
 
+static void flash(lino_t *l, int ch)
+{
+    int i;
+
+    if (l->dlen >= (int) sizeof l->data - 1)
+        return;
+
+    for (i = 0; i < 2; i++) {
+        if (i > 0)
+            usec_delay(l, LINENOISE_FLASH_DELAY);
+        l->data[l->dlen++] = ch;
+        l->data[l->dlen] = 0;
+        refresh_line(l);
+        usec_delay(l, LINENOISE_FLASH_DELAY);
+        l->data[--l->dlen] = 0;
+        refresh_line(l);
+    }
+}
+
+
 static void update_sel(lino_t *l)
 {
     if (l->selmode) {
@@ -2023,6 +2044,8 @@ static int edit(lino_t *l, const char *prompt)
                         l->error = lino_ioerr;
                         goto out;
                     }
+                    if (l->dpos == l->dlen)
+                        flash(l, '!');
                     break;
                 }
                 if (l->mlmode)
@@ -2086,6 +2109,8 @@ static int edit(lino_t *l, const char *prompt)
                     l->error = lino_ioerr;
                     goto out;
                 }
+                if (!paste && l->dpos == l->dlen)
+                    flash(l, '!');
                 break;
             }
             if (l->mlmode)