( xn ... x1 n -- flag )

Attempt to restore the input source specification to the state described by x1 through xn. flag is true if the input source specification cannot be so restored.

An ambiguous condition exists if the input source represented by the arguments is not the same as the current input source.



ruvavatar of ruv [274] Address of the input buffer after RESTORE-INPUTRequest for clarification2023-02-07 10:03:28

The section Input buffers says:

The address and length returned by SOURCE, the string returned by PARSE, and directly computed input-buffer addresses are valid only until the text interpreter does I/O to refill the input buffer or the input source is changed.

So, refill may change the the address of the input buffer. And the following line:

:noname ( -- )  source drop  refill drop  postpone \  source drop  <> . ;  execute

may print -1 as well as 0.

So a question arises: may restore-input change the address of the input buffer so it will distinct from the address before save-input?

I think, it may, since restore-input can do I/O. Hence, we should conclude that a particular state of the input buffer does not include the address where this buffer is located.

And it means that the fragment:

:noname ( -- ) source drop  save-input  refill drop  restore-input abort" cannot restore input"  source drop  <> .  ;  execute

may print -1 as well as 0.

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ruvavatar of ruv [275] About behavior or RESTORE-INPUTComment2023-02-07 16:09:01

It can be not obvious on the first glance from the specifications that:

  • restore-input is not obligated to be paired with save-input one-to-one.
  • restore-input can be applied one time, several times (more than 1), or even never to the same input source specification.

The input source specification, which save-input returns, is a kind of a position in the input source, and a program may use this position zero, one, or more times within this input source. And this position should be a valid parameter all the time while the program has access to the corresponding input source.

After performing restore-input, the input buffer content should be the same as it was on the time of getting the position.

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