ld prints the result of evaluation
Major Section: MISCELLANEOUS
Ld-post-eval-print is an ld special (see ld). The accessor is
(ld-post-eval-print state) and the updater is
(set-ld-post-eval-print val state). Ld-post-eval-print must be
either t, nil, or :command-conventions. The initial value of
ld-post-eval-print is :command-conventions.
The general-purpose ACL2 read-eval-print loop, ld, reads forms from
standard-oi, evaluates them and prints the result to standard-co.
However, there are various flags that control ld's behavior and
ld-post-eval-print is one of them. If this global variable is t, ld
prints the result. In the case of a form that produces multiple
values, ld prints the list containing them all (which, logically
speaking, is what the form returned). If ld-post-eval-print is nil,
ld does not print the values. This is most useful when ld is used
to load a previously processed file.
Finally, if ld-post-eval-print is :command-conventions then ld
prints the result but treats ``error triples'' specially. An
``error triple'' is a result, (mv erp val state), that consists of
three values, the third of which is state. Many ACL2 functions use
such triples to signal errors. The convention is that if erp (the
first value) is nil, then the function is returning val (the second
value) as its conventional single result and possibly side-effecting
state (as with some output). If erp is t, then an error has been
caused, val is irrelevant and the error message has been printed in
the returned state. Example ACL2 functions that follow this
convention include defun and in-package. If such ``error
producing'' functions are evaluated while ld-post-eval-print is
set to t, then you would see them producing lists of length 3. This
is disconcerting to users accustomed to Common Lisp (where these
functions produce single results but sometimes cause errors or
side-effect state).
When ld-post-eval-print is :command-conventions and a form produces
an error triple (mv erp val state) as its value, ld prints nothing
if erp is non-nil and otherwise ld prints just val. Because it is a
misrepresentation to suggest that just one result was returned, ld
prints the value of the global variable 'triple-print-prefix before
printing val. 'triple-print-prefix is initially " ", which means
that when non-erroneous error triples are being abbreviated to val,
val appears one space off the left margin instead of on the margin.
In addition, when ld-post-eval-print is :command-conventions and the
value component of an error triple is the keyword :invisible then ld
prints nothing. This is the way certain commands (e.g., :pc) appear
to return no value.
By printing nothing when an error has been signalled, ld makes it
appear that the error (whose message has already appeared in state)
has ``thrown'' the computation back to load without returning a
value. By printing just val otherwise, we suppress the fact that
state has possibly been changed.