LD-POST-EVAL-PRINT.html -- ACL2 Version 6.3

## LD-POST-EVAL-PRINT

determines whether and how `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 a multiple value, `ld` prints the list containing all the values (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` and also `ld-error-triples` is `t`, then `ld` prints the result but treats ``error triples'' specially. An ``error triple'' (see error-triples) is a result, `(mv erp val state)`, that consists of two ordinary values and `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). For more information about error triples, see programming-with-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.