For a non-infinite iterator (with a known end target), tqdm or a typical progress bar library can show the progress percentage, e.g.: 

 20%|###        | 200/1000 (00:03<00:09 98.3it/s)

For an infinite iterator, like in the case of this command-line (tqdm is also available as a CLI): 

% seq 999999 | tqdm

the progress library could just show something like: 

203495 (00:03<00:09 99988.3it/s)

or: 

 ??%|      ###  | 203495 (00:03<00:09 99988.3it/s)

where the progress bar is just animated with a running ### bar, like you would see in various UI.

A circular iterator wouldn't be problematic, the progress bar would just be reset and start again accordingly. But of course normally the user will not attach a circular iterator if he/she wants to display a progress bar for a process that ends after a certain time.

In reply to Re^4: Porting tqdm to Perl by perlancar
in thread Porting tqdm to Perl by perlancar

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