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

[Alan Schmitt <alan.schmitt@polytechnique.org>]

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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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