Make Lwt_seq.of_list lazier, Lwt_seq.to_list tail recursive

[samer--]

This PR makes a couple of small changes to make Lwt_seq.to_list and Lwt_seq.of_list work better with large lists.

of_list: current version builds a large term eagerly and can fail with a stack overflow or segmentation fault for large lists. This lazier version defers the creation of Lwt_seq.node terms until they are needed.

to_list: current version is not tail recursive and can fail with long sequences. This tail recursive version (using a difference list accumulator) seems able to handle longer sequences.

[raphael-proust]

Thanks!

I think there's one regression: if the first call to seq fails with an exception, then the call to to_list raises an exception (where it used to return a failed promise).

In practice this is probably not too big a deal because the the exception is caught if the call is on the right-on-side of a bind (or inside another closure/lambda). But for now I'd rather we keep this.

Can you add a call to Lwt.apply on the first call to the auxiliary function?

[samer--]

Thanks!

I think there's one regression: if the first call to seq fails with an exception, then the call to to_list raises an exception (where it used to return a failed promise).

In practice this is probably not too big a deal because the the exception is caught if the call is on the right-on-side of a bind (or inside another closure/lambda). But for now I'd rather we keep this.

Can you add a call to Lwt.apply on the first call to the auxiliary function?

Hi, thanks for reviewing my PR!
I did try to include a call to Lwt.apply when generating the first element of the sequence by using the partially applied Lwt.apply seq as the first argument to aux on line 295. Will this not have the desired effect?

[raphael-proust]

I did try […]

Ah you're right! I missed this because it's not in the same "shape" as some of the other functions where the recursive function is inlined and the first call has an extra apply.

I think it's all good to go then.

[raphael-proust]

Thanks for the contribution!

[samer--]

Thanks for the contribution!

Thank you for your time reviewing it and for Lwt! All the best.