[Caml-list] Attn: Development Editor, Latest OCaml Weekly News

Alan Schmitt
2022-07-26T17:54:30Z

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Hello

Here is the latest OCaml Weekly News, for the week of July 19 to 26,
2022.

Table of Contents
─────────────────

Help w. my first GADT : unwrapping Sqlite3.Data.t
DocuLib 3.1.2 and MetaDB 1.0.2 now on OPAM
dune 3.4.0
OCaml 5.0, first normal alpha release
Other OCaml News
Old CWN

Help w. my first GADT : unwrapping Sqlite3.Data.t
═════════════════════════════════════════════════

Archive:
<https://discuss.ocaml.org/t/help-w-my-first-gadt-unwrapping-sqlite3-data-t/10202/1>

Philippe Strauss asked
──────────────────────

I would like to convert sqlite3-ocaml returns from Sqlite3.Data.t
array to plain ocaml types in a tuple. I guess unwrapping the Data.t
can be done using a GADT, here's my very very first attempt:

┌────
│ (* simulate Sqlite3.Data.t *)
│
│ type t =
│ | NONE
│ | NULL
│ | INT of int64
│ | FLOAT of float
│ | TEXT of string
│ | BLOB of string ;;
│
│ (* a simple GADT to unwrap Sqlite3.Data.t *)
│
│ type _ dbval =
│ | INT : int64 -> int64 dbval
│ | FLOAT : float -> float dbval
│ | TEXT : string -> string dbval
│ | BLOB : string -> string dbval
│ | NONE | NULL ;;
│
│ let unwrap_data : type a. a dbval -> a = fun dbval ->
│ match dbval with
│ | INT x -> x
│ | FLOAT x -> x
│ | TEXT str -> str
│ | BLOB str -> str ;;
│
│ let tuple_of_array4 (arr: t array) =
│ assert (Array.length arr = 4) ;
│ (unwrap_data arr.(0), unwrap_data arr.(1), unwrap_data arr.(2), unwrap_data arr.(3)) ;;
└────

Compilation fails with this typing error:

┌────
│ File "database.ml", line 233, characters 17-24:
│ 233 | (unwrap_data arr.(0), unwrap_data arr.(1), unwrap_data arr.(2), unwrap_data arr.(3)) ;;
│ ^^^^^^^
│ Error: This expression has type t but an expression was expected of type
│ 'a dbval
└────

What am I doing wrong? I need to make type t compatible with type 'a
dbval. Thanks in advance.

octachron replied
─────────────────

You cannot make the type `t' and `'a dbval' compatible, there are
different types.

A very important point to keep in mind with GADTs is that one cannot
create type-level information from dynamical values. In other words,
there are no functions of type ~ x : t -> f(x) dbval~that will infer
the type of its return from the value of its argument in OCaml.

Thus the type of the final result must come from your code source
rather than from the dynamical data. For instance, you can define
constructor from the type `t' to the right `dbval' type:
┌────
│ exception Type_error
│
│ let int: t -> _ dbval = function
│ | INT x -> INT x
│ | _ -> raise Type_error
│
│ let float: t -> _ dbval = function
│ | FLOAT x -> FLOAT x
│ | _ -> raise Type_error
└────
Then if you know the type of the tuple, you can write it as:
┌────
│ let tuple_of_array4 (arr: t array) =
│ assert (Array.length arr = 4) ;
│ int arr.(0), int arr.(1), int arr.(2), int arr.(3)
└────
or possibly as
┌────
│ let int4 = int, int, int, int
│ let tuple (a,b,c,d) arr =
│ assert (Array.length arr = 4) ;
│ a arr.(0), b arr.(1), c arr.(2), d arr.(3)
└────
There are more complex alternatives based on type witness, that allow
to implement a form of static matching over the dynamical type of
data, but the core idea that the types are always present in the
source code in some way is the same.

Philippe Strauss then said
──────────────────────────

Oh I didn't noticed it would be dynamical typing! I'm too used to ppx
(and previously camlp4) written db abstraction layer!

I'm simply replacing sqlexpr by plain sqlite3-ocaml in some existing
code of mine. sqlexpr quick doco:

<https://github.com/mfp/ocaml-sqlexpr>

But I can live with a Data.t array!

Yawar Amin then added
─────────────────────

Everybody has their favourite way of wrapping SQLite. Here's mine (no
PPX): <https://github.com/yawaramin/ocaml_sql_query>

It has a little data translation layer to convert from `Data.t' array
to the desired return type.

DocuLib 3.1.2 and MetaDB 1.0.2 now on OPAM
══════════════════════════════════════════

Archive:
<https://discuss.ocaml.org/t/doculib-3-1-2-and-metadb-1-0-2-now-on-opam/10204/1>

nguermond announced
───────────────────

I'm pleased to announce the release of `doculib' and `metadb', now
available on OPAM.

*DocuLib* is a GUI for document management, particularly for all the
textbooks and articles you've accumulated but know you'll never read
:thinking:. The idea of DocuLib is to keep track of metadata of files
stored across multiple libraries on your file system in such a way
that you can move, reorganize, or rename a file without losing your
metadata. You can additionally lookup metadata on `openlibrary.org' or
`semanticscholar.org'. DocuLib will also warn about missing and
duplicate files. Stored metadata presently includes author, title,
year, tags, and DOI/ISBN.

<https://global.discourse-cdn.com/standard11/uploads/ocaml/original/2X/f/fa064cd32bce6e52722d30047d8e0ef21fa09684.png>

For more screenshots and details:
<https://github.com/nguermond/doculib>

*Metadb* is the JSON database for manipulating file metadata
underlying DocuLib, in hopes that it may be useful somewhere
else. Data is stored in the following way:
┌────
│ path/to/library
│ |- .metadata
│ |- ./foo.txt.json
│ |- ./blah/bar.pdf.json
│ |- ./foobar.pdf.json
│ |- ./foo.txt
│ |- ./blah/bar.pdf
│ |- ./foobar.pdf
└────
For documentation: <https://github.com/nguermond/metadb>

dune 3.4.0
══════════

Archive: <https://discuss.ocaml.org/t/ann-dune-3-4-0/10211/1>

Etienne Millon announced
────────────────────────

On behalf of the dune team, I’m pleased to announce the release of
version 3.4.0.

Bug fixes, a couple new features, better hints and error messages - I
won't restate what's in the changelog below. Thanks to everyone
involved in this release!

• Make `dune describe' correctly handle overlapping implementations
for virtual libraries (#5971, fixes #5747, @esope)

• Building the `@check' alias should make sure the libraries and
executables don't have dependency cycles (#5892, @rgrinberg)

• [ctypes] Add support for the `errno' parameter using the
`errno_policy' field in the ctypes settings. (#5827, @droyo)

• Fix `dune coq top' when it is invoked on files from a subdirectory
of the directory containing the associated stanza (#5784, fixes
#5552, @ejgallego, @rlepigre, @Alizter)

• Fix hint when an invalid module name is found. (#5922, fixes #5273,
@emillon)

• The `(cat)' action now supports several files. (#5928, fixes #5795,
@emillon)

• Dune no longer uses shimmed `META' files for OCaml 5.x, solely using
the ones installed by the compiler. (#5916, @dra27)

• Fix handling of the `(deps)' field in `(test)' stanzas when there is
an `.expected' file. (#5952, #5951, fixes #5950, @emillon)

• Ignore insignificant filesystem events. This stops RPC in watch mode
from flashing errors on insignificant file system events such as
changes in the `.git/' directory. (#5953, @rgrinberg)

• Fix parsing more error messages emitted by the OCaml compiler. In
particular, messages where the excerpt line number started with a
blank character were skipped. (#5981, @rgrinberg)

• env stanza: warn if some rules are ignored because they appear after
a wildcard rule. (#5898, fixes #5886, @emillon)

• On Windows, XDG_CACHE_HOME is taken to be the
`FOLDERID_InternetCache' if unset, and XDG_CONFIG_HOME and
XDG_DATA_HOME are both taken to be `FOLDERID_LocalAppData' if unset.
(#5943, fixes #5808, @nojb)

Etienne Millon then added
─────────────────────────

This broke 32-bit cygwin installations, so 3.4.1 was released with a
fix.

OCaml 5.0, first normal alpha release
═════════════════════════════════════

Archive:
<https://discuss.ocaml.org/t/ocaml-5-0-first-normal-alpha-release/10216/1>

octachron announced
───────────────────

The stabilisation of OCaml 5.0 has been progressing well during the
last month. We have thus released a first normal alpha release of
OCaml 5.0.0 to help fellow hackers join us early in our bug hunting
and opam ecosystem fixing fun (see below for the installation
instructions).

You can follow the progress in stabilising the opam ecosystem on

<https://github.com/ocaml/opam-repository/issues/21526>

If you find any bugs, please report them here:

<https://github.com/ocaml/ocaml/issues>

Compared to the zeroth alpha release, this alpha release restores the
support for the bytecode debugger, and integrates a change of type in
the FFI API that might trigger some warnings in FFI code.

We also have a change in the installed files: the compiler distributes
now its own META files rather than relying on either findlib or dune
to provide those files. This should simplify the tasks of both tools
in future version.

Note there are still some changes expected in the Effect module before
the next candidate release. Generally, both the Effect and Domain
modules are still experimental and might change API even during the
beta releases.

If you are interested by the ongoing list of bug fixes, the updated
change log for OCaml 5.0.0 is available at:

<https://github.com/ocaml/ocaml/blob/5.0/Changes>

A short summary of the changes since the zeroth alpha release is also
available below.

Installation instructions
╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌

The base compiler can be installed as an opam switch with the
following commands on opam 2.1:
┌────
│ opam update
│ opam switch create 5.0.0~alpha1
└────
For previous version of opam, the switch creation command line is
slightly more verbose:
┌────
│ opam update
│ opam switch create 5.0.0~alpha1 --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git
└────
If you want to test this version, it is strongly advised to install
the alpha opam repository

<https://github.com/kit-ty-kate/opam-alpha-repository>

with
┌────
│ opam repo add alpha git+https://github.com/kit-ty-kate/opam-alpha-repository.git
└────
You can check that the alpha repository has been correctly installed
with

┌────
│ $ opam repo
│
│ <><> Repository configuration for switch 5.0.0~alpha1 <><><><><><><><><><><><><>
│ 1 alpha git+https://github.com/kit-ty-kate/opam-alpha-repository.git
│ 2 default https://opam.ocaml.org
└────

This alpha repository contains various fixes in the process of being
upstreamed which vastly increases the number of opam packages
currently compatible with OCaml 5.0.0 .

If you want to tweak the configuration of the compiler, you can switch
to the option variant with:
┌────
│ opam update
│ opam switch create <switch_name> ocaml-variants.5.0.0~alpha1+options <option_list>
└────

where `option_list' is a comma separated list of `ocaml-option-*'
packages. For instance, for a flambda and no-flat-float-array switch:
┌────
│ opam switch create 5.0.0~alpha1+flambda+nffa ocaml-variants.5.0.0~alpha1+options ocaml-option-flambda
│ ocaml-option-no-flat-float-array
└────
The command line above is slightly more complicated for opam version
anterior to 2.1:
┌────
│ opam update
│ opam switch create <switch_name> --packages=ocaml-variants.5.0.0~alpha1+options,<option_list>
│ --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git
└────

In both cases, all available options can be listed with `opam search
ocaml-option'.

The source code for the alpha is also available at these addresses:

• <https://github.com/ocaml/ocaml/archive/5.0.0-alpha1.tar.gz>
• <https://caml.inria.fr/pub/distrib/ocaml-5.0/ocaml-5.0.0~alpha1.tar.gz>

Changes since the zeroth alpha release:
╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌

Runtime system:
┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄

• [#11400]: Runtime events counters fixes Fixes mismatch between OCaml
and C APIs, removes events from 4.x that are not present in the 5.0
GC and adds some missing probes. (Sadiq Jaffer, review by Gabriel
Scherer, Florian Angeletti)

• [#11368]: Runtime events buffer size OCAMLRUNPARAMS fix The runtime
events buffer size can now be set via the 'e' OCAMLRUNPARAM. This
is previously mistakenly enabled/disabled tracing instead. (Sadiq
Jaffer, review by KC Sivaramakrishnan, David Allsopp, Damien
Doligez)

• [#11304]: Fix data race on Windows file descriptors (Olivier Nicole
and Xavier Leroy, review by Xavier Leroy, David Allsopp, and Sadiq
Jaffer)

• *breaking change* [#11337]: pass 'flags' metadata to root scanners,
to optimize stack scanning in the bytecode interpreter. Changes the
interface of user-provided root-scanning hooks. (Gabriel Scherer,
review by Xavier Leroy, Guillaume Munch-Maccagnoni, Sadiq Jaffer
and Tom Kelly)

• [#11144]: Restore frame-pointers support for amd64 (Fabrice Buoro,
review by Frederic Bour and KC Sivaramakrishnan)

• *breaking change* [#11255]: in the C interface, `&Field(v, i)' now
has type `volatile value *' instead of `value *' in OCaml 4. This
makes the memory model for mixed OCaml/C code better defined, but
can cause warnings or type errors in user C code. (KC
Sivaramakrishnan, review by Xavier Leroy, Gabriel Scherer and
Guillaume Munch-Maccagnoni, additional discussions with Stephen
Dolan and Luc Maranget)

[#11400] <https://github.com/ocaml/ocaml/issues/11400>

[#11368] <https://github.com/ocaml/ocaml/issues/11368>

[#11304] <https://github.com/ocaml/ocaml/issues/11304>

[#11337] <https://github.com/ocaml/ocaml/issues/11337>

[#11144] <https://github.com/ocaml/ocaml/issues/11144>

[#11255] <https://github.com/ocaml/ocaml/issues/11255>

Standard library:
┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄

• [#10867], +[#11345]: Remove deprecated values: …, the infix operator
(.[ ]<-). (Nicolás Ojeda Bär, review by Damien Doligez)

• [#11309], [#11424], [#11427]: Add
Domain.recommended_domain_count. (Christiano Haesbaert, Konstantin
Belousov, review by David Allsopp, KC Sivaramakrishnan, Gabriel
Scherer, Nicolas Ojeda Bar)

[#10867] <https://github.com/ocaml/ocaml/issues/10867>

[#11345] <https://github.com/ocaml/ocaml/issues/11345>

[#11309] <https://github.com/ocaml/ocaml/issues/11309>

[#11424] <https://github.com/ocaml/ocaml/issues/11424>

[#11427] <https://github.com/ocaml/ocaml/issues/11427>

Tools:
┄┄┄┄┄┄

• [#11065]: Port the bytecode debugger to 5.0, adding support for
effect handlers. (Damien Doligez and fabbing, review by fabbing and
Xavier Leroy)

• [#11382]: OCamlmktop use a new initialization module
"OCamlmktop_init" to preserve backward-compatibility with
user-module provided modules that install toplevel printers.
(Florian Angeletti, review by Gabriel Scherer and David Allsopp)

[#11065] <https://github.com/ocaml/ocaml/issues/11065>

[#11382] <https://github.com/ocaml/ocaml/issues/11382>

Installation:
┄┄┄┄┄┄┄┄┄┄┄┄┄

• [#11007], [#11399]: META files for the stdlib, compiler-libs and
other libraries (unix, dynlink, str, runtime_events, threads,
ocamldoc) are now installed along with the compiler. (David Allsopp,
Florian Angeletti, Nicolás Ojeda Bär and Sébastien Hinderer, review
by Daniel Bünzli, Kate Deplaix, Anil Madhavapeddy and Gabriel
Scherer)

[#11007] <https://github.com/ocaml/ocaml/issues/11007>

[#11399] <https://github.com/ocaml/ocaml/issues/11399>

Bug fixes:
┄┄┄┄┄┄┄┄┄┄

• [#10768], [#11340]: Fix typechecking regression when combining first
class modules and GADTs. (Jacques Garrigue, report by François
Thiré, review by Matthew Ryan)

• [#10790]: don't drop variance and injectivity annotations when
pretty printing `with' constraints (for example, `with type +!'a t =
...'). (Florian Angeletti, report by Luke Maurer, review by Matthew
Ryan and Gabriel Scherer)

• [#11289], [#11405]: fix some leaks on systhread termination (Fabrice
Buoro, Enguerrand Decorne, Gabriel Scherer, review by Xavier Leroy
and Florian Angeletti, report by Romain Beauxis)

• [#11314], [#11416]: fix non-informative error message for module
inclusion (Florian Angeletti, report by Thierry Martinez, review by
Gabriel Scherer)

• [#11358], [#11379]: Refactor the initialization of bytecode
threading, This avoids a "dangling pointer" warning of GCC
12.1. (Xavier Leroy, report by Armaël Guéneau, review by Gabriel
Scherer)

• [#11387], module type with constraints no longer crash the compiler
in presence of both shadowing warnings and the `-bin-annot' compiler
flag. (Florian Angeletti, report by Christophe Raffalli, review by
Gabriel Scherer)

[#10768] <https://github.com/ocaml/ocaml/issues/10768>

[#11340] <https://github.com/ocaml/ocaml/issues/11340>

[#10790] <https://github.com/ocaml/ocaml/issues/10790>

[#11289] <https://github.com/ocaml/ocaml/issues/11289>

[#11405] <https://github.com/ocaml/ocaml/issues/11405>

[#11314] <https://github.com/ocaml/ocaml/issues/11314>

[#11416] <https://github.com/ocaml/ocaml/issues/11416>

[#11358] <https://github.com/ocaml/ocaml/issues/11358>

[#11379] <https://github.com/ocaml/ocaml/issues/11379>

[#11387] <https://github.com/ocaml/ocaml/issues/11387>

Other OCaml News
════════════════

From the ocaml.org blog
───────────────────────

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

• [Tarides is on the Wavestone Radar!]

[the ocaml.org blog] <https://ocaml.org/blog/>

[Tarides is on the Wavestone Radar!]
<https://tarides.com/blog/2022-07-19-tarides-is-on-the-wavestone-radar>

Old CWN
═══════

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[Alan Schmitt]

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[Alan Schmitt] <https://alan.petitepomme.net/>

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