input/output facilities in ACL2
Major Section:  ACL2 Documentation

  (channel state)
  (open-input-channel "foo.lisp" :object state)
  (mv-let (eofp obj state)
          (read-object channel state)
             (let ((state (close-input-channel channel state)))
                   (mv final-ans state))..)))
Also see file-reading-example.

For advanced ways to control printing, see print-control.

For a discussion of formatted printing, see fmt.

To control ACL2 abbreviation (``evisceration'') of objects before printing them, see set-evisc-tuple, see without-evisc, and see set-iprint.

To redirect output to a file, see output-to-file.

Some Related Topics

ACL2 supports input and output facilities equivalent to a subset of those found in Common Lisp. ACL2 does not support random access to files or bidirectional streams. In Common Lisp, input and output are to or from objects of type stream. In ACL2, input and output are to or from objects called ``channels,'' which are actually symbols. Although a channel is a symbol, one may think of it intuitively as corresponding to a Common Lisp stream. Channels are in one of two ACL2 packages, "ACL2-INPUT-CHANNEL" and "ACL2-OUTPUT-CHANNEL". When one ``opens'' a file one gets back a channel whose symbol-name is the file name passed to ``open,'' postfixed with -n, where n is a counter that is incremented every time an open or close occurs.

There are three channels which are open from the beginning and which cannot be closed:

All three of these are really Common Lisp's *standard-input* or *standard-output*, appropriately.

For convenience, three global variables are bound to these rather tedious channel names:

Common Lisp permits one to open a stream for several different kinds of io, e.g. character or byte. ACL2 permits an additional type called ``object''. In ACL2 an ``io-type'' is a keyword, either :character, :byte, or :object. When one opens a file, one specifies a type, which determines the kind of io operations that can be done on the channel returned. The types :character and :byte are familiar. Type :object is an abstraction not found in Common Lisp. An :object file is a file of Lisp objects. One uses read-object to read from :object files and print-object$ (or print-object$-ser) to print to :object files. (The reading and printing are really done with the Common Lisp read and print functions. For those familiar with read, we note that the recursive-p argument is nil.) The function read-object-suppress is logically the same as read-object except that read-object-suppress throws away the second returned value, i.e. the value that would normally be read, simply returning (mv eof state); under the hood, read-object-suppress avoids errors, for example those caused by encountering symbols in packages unknown to ACL2.

File-names are strings. ACL2 does not support the Common Lisp type pathname. However, for the file-name argument of the output-related functions listed below, ACL2 supports a special value, :STRING. For this value, the channel connects (by way of a Common Lisp output string stream) to a string rather than to a file: as characters are written to the channel they can be retrieved by using get-output-stream-string$.

Here are the names, formals and output descriptions of the ACL2 io functions.

Input Functions:
  (open-input-channel (file-name io-type state) (mv channel state))
  (open-input-channel-p (channel io-type state) boolean)
  (close-input-channel (channel state) state)
  (read-char$ (channel state) (mv char/nil state)) ; nil for EOF
  (peek-char$ (channel state) boolean)
  (read-byte$ (channel state) (mv byte/nil state)) ; nil for EOF
  (read-object (channel state) (mv eof-read-flg obj-read state))
  (read-object-suppress (channel state) (mv eof-read-flg state))

Output Functions:
  (open-output-channel  (file-name io-type state) (mv channel state))
  (open-output-channel! (file-name io-type state) (mv channel state))
  (open-output-channel-p (channel io-type state) boolean)
  (close-output-channel (channel state) state)
  (princ$ (obj channel state) state)
  (write-byte$ (byte channel state) state)
  (print-object$ (obj channel state) state)
  (print-object$-ser (obj serialize-character channel state) state)
  (fms  (string alist channel state evisc-tuple) state)
  (fms! (string alist channel state evisc-tuple) state)
  (fmt  (string alist channel state evisc-tuple) (mv col state))
  (fmt! (string alist channel state evisc-tuple) (mv col state))
  (fmt1 (string alist col channel state evisc-tuple) (mv col state))
  (fmt1! (string alist col channel state evisc-tuple) (mv col state))
  (cw (string arg0 arg1 ... argn) nil)
  (get-output-stream-string$ (channel state
                              &optional (close-p 't)
                                        (ctx ''get-output-stream-string$))
                             (mv erp string state))
The ``formatting'' functions are particularly useful; see fmt and see cw. In particular, cw prints to a ``comment window'' and does not involve the ACL2 state, so many may find it easier to use than fmt and its variants. The functions fms!, fmt!, and fmt1! are the same as their respective functions without the ``!,'' except that the ``!'' functions are guaranteed to print forms that can be read back in (at a slight readability cost).

When one enters ACL2 with (lp), input and output are taken from *standard-oi* to *standard-co*. Because these are synonyms for *standard-input* and *standard-output*, one can drive ACL2 io off of arbitrary Common Lisp streams, bound to *standard-input* and *standard-output* before entry to ACL2.

The macro get-output-stream-string$ returns the string accumulated into the given channel. By default, a call of this macro closes the supplied output channel. However, a third argument is optional (default t), and if it evaluates to nil then the channel remains open. The fourth argument is an optional context, which generally evaluates to a symbol, for error reporting. The following example illustrates.

ACL2 !>
   (channel state)
   (open-output-channel :string :object state)
   (pprogn (print-object$-ser 17 nil channel state)
           (print-object$-ser '(a b (c d)) nil channel state)
             ((str1 (get-output-stream-string$
                     channel state
                     nil))) ; keep the channel open
             (pprogn (print-object$-ser 23 nil channel state)
                     (print-object$-ser '((e f)) nil channel state)
                     (er-let* ; close the channel
                       ((str2 (get-output-stream-string$ channel state)))
                       (value (cons str1 str2)))))))
(A B (C D))" . "
((E F))")
ACL2 !>
Also see printing-to-strings for a discussion of formatted printing functions such as fmt-to-string that do not take a channel or state argument and return a string.

By default, symbols are printed in upper case when vertical bars are not required, as specified by Common Lisp. See set-print-case for how to get ACL2 to print symbols in lower case.

By default, numbers are printed in radix 10 (base 10). See set-print-base for how to get ACL2 to print numbers in radix 2, 8, or 16.

To see the guard of an IO function, or indeed any function, see args or call the function guard; but some built-in functions (including some IO functions) will print the result using the variable STATE-STATE. While that is logically correct, if you want to execute the guard then you should replace that variable by STATE and also replace each built-in function symbol of the form xxx-p1 by corresponding function symbol xxx-p. Consider the following example.

ACL2 !>:args princ$

Function         PRINC$
Formals:         (X CHANNEL STATE-STATE)
Signature:       (PRINC$ * * STATE)
                 => STATE
Guard:           (AND (OR (ACL2-NUMBERP X)
                          (CHARACTERP X)
                          (STRINGP X)
                          (SYMBOLP X))
                      (STATE-P1 STATE-STATE)
                      (SYMBOLP CHANNEL)
                      (OPEN-OUTPUT-CHANNEL-P1 CHANNEL
                                              :CHARACTER STATE-STATE))
Guards Verified: T
Defun-Mode:      :logic
Documentation available via :DOC
ACL2 !>(untranslate (guard 'princ$ nil (w state)) t (w state))
         (CHARACTERP X)
         (STRINGP X)
         (SYMBOLP X))
                             :CHARACTER STATE-STATE))
ACL2 !>
If you want to execute the guard for princ$, then according to the suggestion above, you should consider the guard for (princ$ x channel state) to be as follows.
         (CHARACTERP X)
         (STRINGP X)
         (SYMBOLP X))
For example, we can check the guard for a given value and channel as follows.
ACL2 !>(let ((x 3) (channel *standard-co*))
         (AND (OR (ACL2-NUMBERP X)
                  (CHARACTERP X)
                  (STRINGP X)
                  (SYMBOLP X))
              (STATE-P STATE)
              (SYMBOLP CHANNEL)
ACL2 !>

Comment for advanced users: Function open-output-channel! is identical as a function to open-output-channel, except that the former may be called even during make-event expansion and clause-processor hints, but requires that there is an active trust tag (see defttag).

Finally, we note that the community book books/misc/file-io.lisp contains useful file io functions whose definitions illustrate some of the features described above.