Lwt_io, Printf and Format

[Drup]

There are two issues with Lwt_io:

• It needs unix. but the printf part should not.
• All the printf functions are basically broken. They rely on Printf instead of Format, which has become the standard in OCaml pretty printing, and they do not handle the %a very well.

My current idea is to:

• Split Lwt_io in two parts, one unix-independent (Lwt_format ?)
• Deprecate Lwt_io.print*
• Reimplement all the printing functions in term of formats. There are some concerns with blocking, but I have some ideas to solve them.

I think this process would respect #293 and allows us to improve the state of lwt-based printing.

[aantron]

I always had the impression that a lot of the functions in Lwt_io should be in a module that corresponds to the I/O functions of Pervasives and does not depend on Lwt_unix – not just the formatting functions.

[Drup]

@aantron maybe we should create new functions in Lwt_unix for the stuff that is unix-dependent in Lwt_io, deprecate them, and move Lwt_io in lwt after the deprecation ?

[aantron]

Something to that effect. I am not sure if Lwt_unix is necessarily the right destination, but we can use a deprecation to warn about removal of functions from Lwt_io.

[hcarty]

@Drup Lwt_fmt maybe? For the Format-focused code?

[Drup]

I was wondering if my idea would work, so here is an implementation: https://gist.github.com/Drup/2698c69fd70cf526ba3164b135177bcd

[aantron]

The biggest part of this was done by @Drup in #548.