OCaml Weekly News

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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

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 =
| 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:


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

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.


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:

|- .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

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

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


If you find any bugs, please report them here:


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:


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



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

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>

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:

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)
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)
  • #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)
  • #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)
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)

Other OCaml News

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


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