I suppose this should use mark and reset?

I suppose this should use mark and reset?

I am not quite familiar with those. Where should I add them?

Looks like it just needs a NEWS item, something like:

* `startswith` now supports seekable `IO` streams ([#43055]).

Personally, I think this is kind of a strange API for an IO. For example, in the implementation here you need to support mark and reset but those are not actually needed to check if an IO "starts with" something. In the same vein, there are many other "stringy" type of functions (contains, endswith, etc) that one could theoretically call on an IO because you can always materialize a String from an `IO´ and then reset but that doesn't mean it is a good idea to add all these.

But I think it is best to keep IO to be used for reading/writing and then you just compose that with the string functions to get the desired behavior. That avoids many questions like what state the IO object should be in after this function is called and makes it up to the user to decide on a case by case basis.

As @schneiderfelipe pointed out in the original issue, it's a nice complement to peek — indeed, checking if the subsequent stream starts with something is the basic use case for peek, and this is both a generalization that and arguably nicer API for that kind of query.