OCaml Weekly News

Hi, I would like to share my recent article about GADTs and state machines: GADTs and state machine

It's another introduction about GADTs and it explains a bit what I did for robur.io. Eenjoy it and happy hacking!

I found https://github.com/oxidizing/sihl/blob/c6786f25424c1b9f40ce656e908bd31515f1cd09/sihl/src/core_log.ml#L18 and wonder what a primitive way to log to a file would be.

I need to keep stdout clean and not show any log message under all circumstances.

I do a cgi and stdout is the response – logging has to go to a separate file. Not even stderr as I want debug logs not to taint the webserver error log in case. And I would like to funnel logging through Logs.

I don't know about logs but it should be relatively easy to keep an open file handle and print log messages there.

https://opam.ocaml.org/packages/logs/ - I like the loglevel approach. But maybe I will do without and pass around the channel, yes.

I believe logs support logging to a file via Format. See https://erratique.ch/software/logs/doc/Logs/index.html#val-format_reporter

I’ve found logs very ergonomic and easy to work with. I tend to pull it in via Bos, which has a very nice interface to OS interactions. Opening the Bos_setup module also does default logs configuration, and I find all quite painless and pleasant.

thanks @yawaramin @beajeanm @shonfeder, I took a middle ground and went along the lines of https://opam.ocaml.org/packages/logs/ (using the loglevels and logging call style) but base writing almost directly on out_channel. (I need a log rotation on top)

I was struggling with lost messages however – the logfile remained empty until I flushed after each log message.

Is that known behaviour that writing to a channel (with Printf.fprintf) doesn't necessarily end up in the file? Even when closed quickly.

You must Printf.printf with "%!" at the end of your format string, to be sure that the log is flushed to file.

That's what I do in dolog: https://github.com/UnixJunkie/dolog

Tiny update: Timedesc 0.9.0 has been released, moving sexplib dependency into timedesc-sexp and moved from mparser to angstrom for some date time text parsers since angstrom is a strict necessity for some binary (de)serialization already.

This overall means Timedesc is about as slim as it can get as a date time handling lib, depending only on: seq, angstrom, result, and ptime (ptime is not a strict dependency, but it's nice to have timedesc <-> ptime convertors).

Is this a lib that targets to process dates and time in a programmatically way? (this is what the provided example make me think) Or is it also supposed to be used to print something readable for a user else than a programmer?

If there is no end-user goal in this lib, please just ignore my message, and sorry to make you lose some time.

In the other case if you consider printing for end users, it's maybe worth to mention that there is the DateLocale-ocaml module that is available and which provides the name for the months, and days for more than 200 languages. It also provides abbreviated versions for both months and days, which are often used.

The ocaml-community/calendar was not designed with localisation in mind, it just does String.sub d 0 3 to provide short names, which will not work with languages that need UTF8.

There is this PR that is still waiting for some review since 2 years to make it compatible with localisation: ocaml-community/calendar/pull/33.

(At least the patch is available there for someone who could be interested.)

I don't know if it could interest some one but I see that the example outputs a list of dates, that look like some kind of logs. In case some one would like to visualise it in a way similar than the unix command cal you can just create empty files where the file name follows the pattern YYYY-MM-DD like for example "dir/2022-08-06.txt", you will then be able to visualise it in the console with detri.

Is this a lib that targets to process dates and time in a programmatically way? (this is what the provided example make me think) Or is it also supposed to be used to print something readable for a user else than a programmer?

Development has been primarily focused on former, mostly because solving it properly was already (very) involved.

Now that Timedesc has stabilised, the latter reads like a very nice next TODO to match feature parity of other date time libs.

In the other case if you consider printing for end users, it’s maybe worth to mention that there is the DateLocale-ocaml module that is available and which provides the name for the months, and days for more than 200 languages. It also provides abbreviated versions for both months and days, which are often used.

Looks neat! I believe there have been requests of locale sensitive pretty printing/conversion functions, so I definitely would be interested in incorporating your work (if that was the intention).

I don’t know if it could interest some one but I see that the example outputs a list of dates, that look like some kind of logs. In case some one would like to visualise it in a way similar than the unix command `cal` you can just create empty files where the file name follows the pattern YYYY-MM-DD like for example “dir/2022-08-06.txt”, you will then be able to visualise it in the console with detri.

I was interested in something like this for another small utility cmd I've written, neat!

It would be nice to add OCaml Scientific Computing to the list on https://ocaml.org/books.

This is a great book and it would be a shame not to promote it. Maybe anyone responsible for the website reads this or can point me in the right direction.

I'm not in any way affiliated with the authors of this book.

Personally, I often use the monadic Result type together with a polymorphic variant for the actual errors. This makes dealing with errors from different "levels" of my software (library, command-line tool, and GUI) quite comfortable (and type-safe!). @keleshev has written a nice blog post on this: https://keleshev.com/composable-error-handling-in-ocaml with a recent follow up: https://keleshev.com/advanced-error-handling-in-ocaml

Here are links from many OCaml blogs aggregated at the ocaml.org blog.

• Irmin in the Browser