EVISC-TUPLE.html -- ACL2 Version 6.3

## EVISC-TUPLE

control suppression of details when printing

    Major Section:  IO

ACL2 output is generally printed in full. However, ACL2 can be directed to abbreviate, or ``eviscerate'', objects before printing them. To ``eviscerate'' an object we replace certain substructures within it by strings that are printed in their stead. Such replacement is made relative to a so-called ``evisc-tuple'', which has four components: `(evisc-tuple print-level print-length alist hiding-cars)` is the same as the value of `(list alist print-level print-length hiding-cars)`, and the components are used as follows (with priority order as discussed below). The alist component is used to replace any substructure occurring as a key by the corresponding string. The print-level and print-length are analogous to Common Lisp variables `*print-level*` and `*print-length*`, respectively, and cause replacement of substructures deeper than print-level by ``#`' and those longer than print-length by ``...`'. Finally, any `consp` `x` that starts with one of the symbols in `hiding-cars` is printed as `<hidden>`.

The following example illustrates the use of an evisc-tuple that limits the print-level to 3 -- only three descents into list structures are permitted before replacing a subexpression by ``#`' -- and limits the print-length to 4 -- only the first four elements of any list structure will be printed before replacing its tail by ``...`'.

    ACL2 !>(fms "~x0~%"
                (list (cons #\0 '((a b ((c d)) e f g) u v w x y)))
                *standard-co*
                state
                (evisc-tuple 3 4 nil nil))

    ((A B (#) E ...) U V W ...)
    <state>
    ACL2 !>

Notice that it is impossible to read the printed value back into ACL2, since there is no way for the ACL2 reader to interpret ``#`' or ``...`'. To solve this problem, see set-iprint.

In the above example we pass an evisc-tuple explicitly to a printing function, in this case, `fms` (see fmt). But ACL2 also does its own printing, for example during a proof attempt. There are global evisc-tuples that control ACL2's printing; see set-evisc-tuple and see without-evisc.