the ACL2 read-eval-print loop, file loader, and command processor
Major Section:  OTHER

(LD standard-oi              ; open obj in channel, stringp file name
                             ; to open and close, or list of forms

; Optional keyword arguments: :standard-co ... ; open char out or file to open and close :proofs-co ... ; open char out or file to open and close :current-package ... ; known package name :ld-skip-proofsp ... ; nil, 'include-book, or t ; (see ld-skip-proofsp) :ld-redefinition-action ; nil or '(:a . :b) :ld-prompt ... ; nil, t, or some prompt printer fn :ld-keyword-aliases ... ; an alist pairing keywords to parse info :ld-pre-eval-filter ... ; :all, :query, or some new name :ld-pre-eval-print ... ; nil, t, or :never :ld-post-eval-print ... ; nil, t, or :command-conventions :ld-evisc-tuple ... ; nil or '(alist nil nil level length) :ld-error-triples ... ; nil or t :ld-error-action ... ; :return (default), :continue or :error :ld-query-control-alist ; alist supplying default responses :ld-verbose ...) ; nil or t

Ld is the top-level ACL2 read-eval-print loop. (When you call lp, a little initialization is done in raw Common Lisp and then ld is called.) ld is also a general-purpose ACL2 file loader and a command interpreter. Ld is actually a macro that expands to a function call involving state. Ld returns an ``error/value/state'' triple as explained below.

The arguments to ld all happen to be global variables in state. For example, 'current-package and 'ld-verbose are global variables, which may be accessed via (@ current-package) and (@ ld-verbose). When ld is called, it ``binds'' these variables. By ``binds'' we actually mean the variables are globally set but restored to their old values on exit. Because ld provides the illusion of state global variables being bound, they are called ``ld specials'' (after the Lisp convention of calling a variable ``special'' if it is referenced freely after having been bound).

Note that all arguments but the first are passed via keyword. Any variable not explicitly given a value in a call retains its pre-call value, with the exception of :ld-error-action, which defaults to :return if not explicitly specified.

Just as an example to drive the point home: If current-package is "ACL2" and you typed

(ld *standard-oi* :current-package "MY-PKG")
you would find yourself in (an inner) read-eval-print loop in which the current-package was "MY-PKG". You could operate there as long as you wished, changing the current package at will. But when you typed :q you would return to the outer read-eval-print loop where the current package would still be "ACL2".

Roughly speaking, ld repeatedly reads a form from standard-oi, evaluates it, and prints its result to standard-co. It does this until the form evaluates to an error triple whose value component is :q or until the input channel or list is emptied. However, ld has many bells and whistles controlled by the ld specials. Each such special is documented individually. For example, see the documentation for standard-oi, current-package, ld-pre-eval-print, etc.

A more precise description of ld is as follows. In the description below we use the ld specials as variables, e.g., we say ``a form is read from standard-oi.'' By this usage we refer to the current value of the named state global variable, e.g., we mean ``a form is read from the current value of 'standard-oi.'' This technicality has an important implication: If while interacting with ld you change the value of one of the ld specials, e.g., 'standard-oi, you will change the behavior of ld, e.g., subsequent input will be taken from the new value.

Three ld specials are treated as channels: standard-oi is treated as an object input channel and is the source of forms evaluated by ld; standard-co and proofs-co are treated as character output channels and various flavors of output are printed to them. However, the supplied values of these specials need not actually be channels; several special cases are recognized. If the supplied value of one of these is in fact an open channel of the appropriate type, that channel is used and is not closed by ld. If the supplied value of one of these specials is a string, the string is treated as a file name in (essentially) Unix syntax (see pathname) and a channel of the appropriate type is opened to/from that file. Any channel opened by ld during the binding of the ld specials is automatically closed by ld upon termination. If standard-co and proofs-co are equal strings, only one channel to that file is opened and is used for both.

Several other alternatives are allowed for standard-oi. If standard-oi is a true list then it is taken as the list of forms to be processed. If standard-oi is a list ending in an open channel, then ld processes the forms in the list and then reads and processes the forms from the channel. Analogously, if standard-oi is a list ending a string, an object channel from the named file is opened and ld processes the forms in the list followed by the forms in the file. That channel is closed upon termination of ld.

The remaining ld specials are handled more simply and generally have to be bound to one of a finite number of tokens described in the :doc entries for each ld special. Should any ld special be supplied an inappropriate value, an error message is printed.

Next, if ld-verbose is t, ld prints the message ``ACL2 loading name'' where name names the file or channel from which forms are being read. At the conclusion of ld, it will print ``Finished loading name'' if ld-verbose is t.

Finally, ld repeatedly executes the ACL2 read-eval-print step, which may be described as follows. A prompt is printed to standard-co if ld-prompt is non-nil. The format of the prompt is determined by ld-prompt. If it is t, the default ACL2 prompt is used. If it is any other non-nil value then it is treated as an ACL2 function that will print the desired prompt. See ld-prompt. In the exceptional case where ld's input is coming from the terminal (*standard-oi*) but its output is going to a different sink (i.e., standard-co is not *standard-co*), we also print the prompt to the terminal.

Ld then reads a form from standard-oi. If the object read is a keyword, ld constructs a ``keyword command form'' by possibly reading several more objects. See keyword-commands. This construction process is sensitive to the value of ld-keyword-aliases. See ld-keyword-aliases. Otherwise, the object read is treated as the command form.

Ld next decides whether to evaluate or skip this form, depending on ld-pre-eval-filter. Initially, the filter must be either :all, :query, or a new name. If it is :all, it means all forms are evaluated. If it is :query, it means each form that is read is displayed and the user is queried. Otherwise, the filter is a name and each form that is read is evaluated as long as the name remains new, but if the name is ever introduced then no more forms are read and ld terminates. See ld-pre-eval-filter.

If the form is to be evaluated, first prints the form to standard-co, if ld-pre-eval-print is t. With this feature, ld can process an input file or form list and construct a script of the session that appears as though each form was typed in. See ld-pre-eval-print.

Ld then evaluates the form, with state bound to the current state. The result is some list of multiple values. If a state is among the values, then ld uses that state as the subsequent current state.

Depending on ld-error-triples, ld may interpret the result as an ``error.'' See ld-error-triples. We first discuss ld's behavior if no error signal is detected (either because none was sent or because ld is ignoring them as per ld-error-triples).

In the case of a non-erroneous result, ld does two things: First, if the logical world in the now current state is different than the world before execution of the form, ld adds to the world a ``command landmark'' containing the form evaluated. See command-descriptor. Second, ld prints the result to standard-co, according to ld-post-eval-print. If ld-post-eval-print is nil, no result is printed. If it is t, all of the results are printed as a list of multiple values. Otherwise, it is :command-conventions and only the non-erroneous ``value'' component of the result is printed. See ld-post-eval-print.

Whenever ld prints anything (whether the input form, a query, or some results) it ``eviscerates'' it if ld-evisc-tuple is non-nil. Essentially, evisceration is a generalization of Common Lisp's use of *print-level* and *print-length* to hide large substructures. See ld-evisc-tuple.

We now return to the case of a form whose evaluation signals an error. In this case, ld first restores the ACL2 logical world to what it was just before the erroneous form was evaluated. Thus, a form that partially changes the world (i.e., begins to store properties) and then signals an error, has no effect on the world. You may see this happen on commands that execute several events (e.g., an encapsulate or a progn of several defuns): even though the output makes it appear that the initial events were executed, if an error is signalled by a later event the entire block of events is discarded.

After rolling back, ld takes an action determined by ld-error-action. If the action is :continue, ld merely iterates the read-eval-print step. Note that nothing suggestive of the value of the ``erroneous'' form is printed. If the action is :return, ld terminates normally. If the action is :error, ld terminates signalling an error to its caller. If its caller is in fact another instance of ld and that instance is watching out for error signals, the entire world created by the erroneous inner ld will be discarded by the outer ld.

Ld returns an error triple, (mv erp val state). Erp is t or nil indicating whether an error is being signalled. If no error is signalled, val is the ``reason'' ld terminated and is one of :exit (meaning :q was read), :eof (meaning the input source was exhausted), :error (meaning an error occurred but has been supressed) or :filter (meaning the ld-pre-eval-filter terminated ld).