You should (setq lexical-binding t) before running it (and note that's
buffer-local, I've been bitten by that several times).  With lexical
binding my original example should work (with the fix, as John pointed out,
that the argument should be &optional).

Thanks Andreas for the quick fix! :-)

On Wed, Mar 9, 2016 at 10:31 PM, Kaushal Modi <kaushal.modi@gmail.com>
wrote:

> I just re-built using this commit (
> http://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-25&id=711ca362e7f8ca7c4f664dc2fe60bc5fa4e4f4fe
> ).
>
> Now the earlier error went away, but I get
>
>   Symbol’s value as variable is void: prev
>
> I guess the read call should be inside the let:
>
> (let ((str "TEST")
>       (pos 0)
>       (prev nil))
>   (read (lambda (&optional ch)
>           (cond
>            (ch (push ch prev))
>            (prev (pop prev))
>            ((< pos (length str))
>             (prog1 (aref str pos)
>               (setq pos (1+ pos))))))))
>
> Above returns "TEST".
>



-- 
Mihai Bazon,
http://lisperator.net/