Add haskell-interactive-copy-to-prompt.

haskell-interactive-mode.el

@@ -178,6 +178,21 @@ be nil.")
       (switch-to-buffer-other-window haskell-interactive-previous-buffer)
     (message "No previous buffer.")))
 
+(defun haskell-interactive-copy-to-prompt ()
+  "Copy the current line to the prompt, overwriting the current
+prompt."
+  (interactive)
+  (let ((l (buffer-substring-no-properties (line-beginning-position)
+                                           (line-end-position))))
+    ;; If it looks like the prompt is at the start of the line, chop
+    ;; it off.
+    (when (and (>= (length l) (length haskell-interactive-prompt))
+               (string= (substring l 0 (length haskell-interactive-prompt))
+                        haskell-interactive-prompt))
+      (setq l (substring l (length haskell-interactive-prompt))))
+
+    (haskell-interactive-mode-set-prompt l)))
+
 (defun haskell-interactive-mode-space (n)
   "Handle the space key."
   (interactive "p")

haskell.el

@@ -83,10 +83,17 @@
   "Handle the return key."
   (interactive)
   (cond
+   ;; At a compile message, jump to the location of the error in the
+   ;; source.
    ((haskell-interactive-at-compile-message)
     (next-error-internal))
+   ;; At the input prompt, handle the expression in the usual way.
+   ((haskell-interactive-at-prompt)
+    (haskell-interactive-handle-expr))
+   ;; At any other location in the buffer, copy the line to the
+   ;; current prompt.
    (t
-    (haskell-interactive-handle-expr))))
+    (haskell-interactive-copy-to-prompt))))
 
 ;;;###autoload
 (defun haskell-session-kill (&optional leave-interactive-buffer)