SAVE-INPUT and RESTORE-INPUT allow the same degree of input source repositioning within a text file as is available with BLOCK input. SAVE-INPUT and RESTORE-INPUT "hide the details" of the operations necessary to accomplish this repositioning, and are used the same way with all input sources. This makes it easier for programs to reposition the input source, because they do not have to inspect several variables and take different action depending on the values of those variables.

SAVE-INPUT and RESTORE-INPUT are intended for repositioning within a single input source; for example, the following scenario is NOT allowed for a Standard Program:

This is incorrect because, at the time RESTORE-INPUT is executed, the input source is the string via EVALUATE, which is not the same input source that was in effect when SAVE-INPUT was executed.

The following code is allowed:

After EVALUATE returns, the input source specification is restored to its previous state, thus SAVE- INPUT and RESTORE-INPUT are called with the same input source in effect.

In the above examples, the EVALUATE phrase could have been replaced by a phrase involving INCLUDE-FILE and the same rules would apply.

The standard does not specify what happens if a program violates the above rules. A Standard System might check for the violation and return an exception indication from RESTORE-INPUT, or it might fail in an unpredictable way.

The return value from RESTORE-INPUT is primarily intended to report the case where the program attempts to restore the position of an input source whose position cannot be restored. The keyboard might be such an input source.

Nesting of SAVE-INPUT and RESTORE-INPUT is allowed. For example, the following situation works as expected:

In principle, RESTORE-INPUT could be implemented to "always fail", e.g.:

Such an implementation would not be useful in most cases. It would be preferable for a system to leave SAVE-INPUT and RESTORE-INPUT undefined, rather than to create a useless implementation. In the absence of the words, the application programmer could choose whether or not to create "dummy" implementations or to work-around the problem in some other way.

Examples of how an implementation might use the return values from SAVE-INPUT to accomplish the save/restore function:

These are examples only; a Standard Program may not assume any particular meaning for the individual stack items returned by SAVE-INPUT.