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• Std/io
• Read-byte$

Std/io/read-byte$

Read one byte from an open :byte stream.

Signature: (read-byte$ channel state-state) returns (mv byte/nil state).

• channel is a symbol that must refer to an open :byte input channel; see open-input-channel.
• state is the ACL2 state.

This is a logic-mode function, but its logical definition is tricky; see logical-story-of-io. The main logical consequence is the updating of state.

Under the hood, we read a byte (i.e., a number between 0 and 255, inclusive) from the Lisp input stream that is associated with channel. If we reach the end of the file, nil is returned.

Definitions and Theorems

Theorem: state-p1-of-read-byte$

(defthm state-p1-of-read-byte$
        (implies (and (state-p1 state)
                      (symbolp channel)
                      (open-input-channel-p1 channel
                                             :byte state))
                 (state-p1 (mv-nth 1 (read-byte$ channel state)))))

Theorem: open-input-channel-p1-of-read-byte$

(defthm open-input-channel-p1-of-read-byte$
        (implies (and (state-p1 state)
                      (open-input-channel-p1 channel
                                             :byte state))
                 (open-input-channel-p1
                      channel
                      :byte (mv-nth 1 (read-byte$ channel state)))))

Theorem: bytep-of-read-byte$

(defthm
 bytep-of-read-byte$
 (implies
     (and (state-p1 state)
          (open-input-channel-p1 channel
                                 :byte state)
          (mv-nth 0 (read-byte$ channel state)))
     (and (bytep (mv-nth 0 (read-byte$ channel state)))
          (natp (mv-nth 0 (read-byte$ channel state)))
          (integerp (mv-nth 0 (read-byte$ channel state)))
          (unsigned-byte-p 8
                           (mv-nth 0 (read-byte$ channel state))))))

Theorem: bytep-of-read-byte$-type

(defthm bytep-of-read-byte$-type
        (implies (and (state-p1 state)
                      (open-input-channel-p1 channel
                                             :byte state))
                 (or (null (mv-nth 0 (read-byte$ channel state)))
                     (natp (mv-nth 0 (read-byte$ channel state)))))
        :rule-classes :type-prescription)

Theorem: bytep-of-read-byte$-linear

(defthm bytep-of-read-byte$-linear
        (implies (and (state-p1 state)
                      (open-input-channel-p1 channel
                                             :byte state))
                 (and (< (mv-nth 0 (read-byte$ channel state))
                         256)
                      (<= 0
                          (mv-nth 0 (read-byte$ channel state)))))
        :rule-classes :linear)

Theorem: no-bytes-after-read-byte$-null

(defthm
     no-bytes-after-read-byte$-null
     (let ((byte1 (mv-nth 0 (read-byte$ channel state)))
           (state2 (mv-nth 1 (read-byte$ channel state))))
          (implies (and (not byte1)
                        (state-p1 state)
                        (open-input-channel-p1 channel
                                               :byte state))
                   (not (mv-nth 0 (read-byte$ channel state2))))))

Theorem: state-preserved-by-read-byte$-when-eof

(defthm state-preserved-by-read-byte$-when-eof
        (implies (and (not (mv-nth 0 (read-byte$ channel state)))
                      (state-p1 state)
                      (open-input-channel-p1 channel
                                             :byte state))
                 (equal (mv-nth 1 (read-byte$ channel state))
                        state)))