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• Parser

Unread-token

Unread a token.

Signature

(unread-token parstate) → new-parstate

Arguments

parstate — Guard (parstatep parstate).

Returns

new-parstate — Type (parstatep new-parstate), given (parstatep parstate).

We move the token from the sequence of read tokens to the sequence of unread tokens.

It is an internal error if tokens-read is 0. It means that the calling code is wrong. In this case, after raising the hard error, logically we still increment tokens-read so that the theorem about parsize holds unconditionally.

Definitions and Theorems

Function: unread-token

(defun unread-token (parstate)
 (declare (xargs :stobjs (parstate)))
 (declare (xargs :guard (parstatep parstate)))
 (let ((__function__ 'unread-token))
  (declare (ignorable __function__))
  (b*
   ((parstate.tokens-read (parstate->tokens-read parstate))
    (parstate.tokens-unread (parstate->tokens-unread parstate))
    (parstate.size (parstate->size parstate))
    ((unless (> parstate.tokens-read 0))
     (raise "Internal error: no token to unread.")
     (b*
      ((parstate
         (update-parstate->tokens-unread (1+ parstate.tokens-unread)
                                         parstate))
       (parstate (update-parstate->size (1+ parstate.size)
                                        parstate)))
      parstate))
    (parstate
         (update-parstate->tokens-unread (1+ parstate.tokens-unread)
                                         parstate))
    (parstate
         (update-parstate->tokens-read (1- parstate.tokens-read)
                                       parstate))
    (parstate (update-parstate->size (1+ parstate.size)
                                     parstate)))
   parstate)))

Theorem: parstatep-of-unread-token

(defthm parstatep-of-unread-token
  (implies (parstatep parstate)
           (b* ((new-parstate (unread-token parstate)))
             (parstatep new-parstate)))
  :rule-classes :rewrite)

Theorem: parsize-of-unread-token

(defthm parsize-of-unread-token
  (b* ((?new-parstate (unread-token parstate)))
    (equal (parsize new-parstate)
           (1+ (parsize parstate)))))