OCaml Weekly News

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Here is the latest OCaml Weekly News, for the week of September 10 to 17, 2019.

Table of Contents

OCaml 4.09.0+beta2

Florian Angeletti announced

The release of OCaml 4.09.0 is approaching. We have created a second beta version to help you adapt your softwares to the new features ahead of the release.

This new beta integrates in particular the fixes from 4.08.1 that were not yet part of the first beta due to the overlap between the release of 4.08.1 and the first beta.

If you tried to test the first beta, and were stopped by the lack of working dune and ocamlfind, those issues has been fixed.

The source code is available at these addresses:

The compiler can also be installed as an OPAM switch with one of the following commands.

opam switch create ocaml-variants.4.09.0+beta2 --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git


opam switch create ocaml-variants.4.09.0+beta2+<VARIANT> --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

where you replace <VARIANT> with one of these:

  • afl
  • default-unsafe-string
  • flambda
  • fp
  • fp+flambda

We want to know about all bugs. Please report them here: https://github.com/ocaml/ocaml/issues

Happy hacking

— Florian "octachron" Angeletti, for the OCaml team

OCaml 4.09.0

(Changes that can break existing programs are marked with a "*")

  • Code generation and optimizations:
    • #2278: Remove native code generation support for 32-bit Intel macOS, iOS and other Darwin targets. (Mark Shinwell, review by Nicolas Ojeda Bar and Xavier Leroy)
    • #8507: Shorten symbol names of anonymous functions in Flambda mode (the directory portions are now hidden) (Mark Shinwell, review by Nicolás Ojeda Bär)
    • #7931, #1904: Add FreeBSD/aarch64 support (Greg V, review by Sébastien Hinderer, Stephen Dolan, Damien Doligez and Xavier Leroy)
    • #8547: Optimize matches that are an affine function of the input. (Stefan Muenzel, review by Alain Frisch, Gabriel Scherer)
    • #8681, #8699, #8712: Fix code generation with nested let rec of functions. (Stephen Dolan, Leo White, Gabriel Scherer and Pierre Chambart, review by Gabriel Scherer, reports by Alexey Solovyev and Jonathan French)
  • Compiler user-interface and warnings:
    • * #2276: Remove support for compiler plugins and hooks (also adds [Dynlink.unsafe_get_global_value]) (Mark Shinwell, Xavier Clerc, review by Nicolás Ojeda Bär, Florian Angeletti, David Allsopp and Xavier Leroy)
    • #2301: Hint on type error on int literal (Jules Aguillon, review by Nicolás Ojeda Bär , Florian Angeletti, Gabriel Scherer and Armaël Guéneau)
    • #2309: New options -with-runtime and -without-runtime in ocamlopt/ocamlc that control the inclusion of the runtime system in the generated program. (Lucas Pluvinage, review by Daniel Bünzli, Damien Doligez, David Allsopp and Florian Angeletti)
    • #2314: Remove support for gprof profiling. (Mark Shinwell, review by Xavier Clerc and Stephen Dolan)
    • #3819, #8546 more explanations and tests for illegal permutation (Florian Angeletti, review by Gabriel Scherer)
    • #8537: fix the -runtime-variant option for bytecode (Damien Doligez, review by David Allsopp)
    • #8541: Correctly print multi-lines locations (Louis Roché, review by Gabriel Scherer)
    • #8579: Better error message for private constructors of an extensible variant type (Guillaume Bury, review by many fine eyes)
  • Compiler distribution build system:
    • #2267: merge generation of header programs, also fixing parallel build on Cygwin. (David Allsopp, review by Sébastien Hinderer)
    • #8514: Use boot/ocamlc.opt for building, if available. (Stephen Dolan, review by Gabriel Scherer)
  • Internal/compiler-libs changes:
    • #1579: Add a separate types for clambda primitives (Pierre Chambart, review by Vincent Laviron and Mark Shinwell)
    • #1965: remove loop constructors in Cmm and Mach (Vincent Laviron)
    • #1973: fix compilation of catches with multiple handlers (Vincent Laviron)
    • #2190: fix pretty printing (using Pprintast) of "lazy …" patterns and "fun (type t) -> …" expressions. (Nicolás Ojeda Bär, review by Gabriel Scherer)
    • #2228, #8545: refactoring the handling of .cmi files by moving the logic from Env to a new module Persistent_env (Gabriel Scherer, review by Jérémie Dimino and Thomas Refis)
    • #2229: Env: remove prefix_idents cache (Thomas Refis, review by Frédéric Bour and Gabriel Scherer)
    • #2237, #8582: Reorder linearisation of Trywith to avoid a call instruction (Vincent Laviron and Greta Yorsh, additional review by Mark Shinwell; fix in #8582 by Mark Shinwell, Xavier Leroy and Anil Madhavapeddy)
    • #2265: Add bytecomp/opcodes.mli (Mark Shinwell, review by Nicolas Ojeda Bar)
    • #2268: Improve packing mechanism used for building compilerlibs modules into the Dynlink libraries (Mark Shinwell, Stephen Dolan, review by David Allsopp)
    • #2277: Use newtype names as type variable names (Matthew Ryan)
    • #2280: Don't make more Clambda constants after starting Cmmgen (Mark Shinwell, review by Vincent Laviron)
    • #2281: Move some middle-end files around (Mark Shinwell)
    • #2283: Add [is_prefix] and [find_and_chop_longest_common_prefix] to [Misc.Stdlib.List] (Mark Shinwell, review by Alain Frisch and Stephen Dolan)
    • #2284: Add various utility functions to [Misc] and remove functions from [Misc.Stdlib.Option] that are now in [Stdlib.Option] (Mark Shinwell, review by Thomas Refis)
    • #2286: Functorise [Consistbl] (Mark Shinwell, review by Gabriel Radanne)
    • #2291: Add [Compute_ranges] pass (Mark Shinwell, review by Vincent Laviron)
    • #2292: Add [Proc.frame_required] and [Proc.prologue_required]. Move tail recursion label creation to [Linearize]. Correctly position [Lprologue] relative to [Iname_for_debugger] operations. (Mark Shinwell, review by Vincent Laviron)
    • #2308: More debugging information on [Cmm] terms (Mark Shinwell, review by Stephen Dolan)
    • #7878, #8542: Replaced TypedtreeIter with tast_iterator (Isaac "Izzy" Avram, review by Gabriel Scherer and Nicolás Ojeda Bär)
    • #8598: Replace "not is_nonexpansive" by "maybe_expansive". (Thomas Refis, review by David Allsopp, Florian Angeletti, Gabriel Radanne, Gabriel Scherer and Xavier Leroy)
  • Runtime system:
    • #1725, #2279: Deprecate Obj.set_tag and Obj.truncate (Stephen Dolan, review by Gabriel Scherer, Damien Doligez and Xavier Leroy)
    • #2075, #7729: rename _T macro used to support Unicode in the (Windows) runtime in order to avoid compiler warning (Nicolás Ojeda Bär, review by Gabriel Scherer and David Allsopp)
    • * #2240: Constify "identifier" in struct custom_operations (Cedric Cellier, review by Xavier Leroy)
    • #2250: Remove extra integer sign-extension in compare functions (Stefan Muenzel, review by Xavier Leroy)
    • * #2293: Constify "caml_named_value" (Stephen Dolan, review by Xavier Leroy)
    • #8607: Remove obsolete macros for pre-2002 MSVC support (Stephen Dolan, review by Nicolás Ojeda Bär and David Allsopp)
    • #8656: Fix a bug in [caml_modify_generational_global_root] (Jacques-Henri Jourdan, review by Gabriel Scherer)
    • #8787, #8788: avoid integer overflow in caml_output_value_to_bytes (Jeremy Yallop, report by Marcello Seri)
  • Standard library:
    • #2262: take precision (.<n>) and flags ('+' and ' ') into account in printf %F (Pierre Roux, review by Gabriel Scherer)
    • #6148, #8596: optimize some buffer operations (Damien Doligez, reports by John Whitington and Alain Frisch, review by Jeremy Yallop and Gabriel Scherer)
  • Other libraries:
    • #2112: Fix Thread.yield unfairness with busy threads yielding to each other. (Andrew Hunter, review by Jacques-Henri Jourdan, Spiros Eliopoulos, Stephen Weeks, & Mark Shinwell)
    • #7903, #2306: Make Thread.delay interruptible by signals again (Xavier Leroy, review by Jacques-Henri Jourdan and Edwin Török)
    • #2248: Unix alloc_sockaddr: Fix read of uninitialized memory for an unbound Unix socket. Add support for receiving abstract (Linux) socket paths. (Tim Cuthbertson, review by Sébastien Hinderer and Jérémie Dimino)
    • #2289: Delete the vmthreads library. This library was deprecated in 4.08.0. (Jérémie Dimino)
    • #2318: Delete the graphics library. This library is now available as a separate "graphics" package in opam. Its new home is: https://github.com/ocaml/graphics (Jérémie Dimino, review by Nicolas Ojeda Bar, Xavier Leroy and Sébastien Hinderer)
  • Tools:
    • #2221: ocamldep will now correctly allow a .ml file in an include directory that appears first in the search order to shadow a .mli appearing in a later include directory. (Nicolás Ojeda Bär, review by Florian Angeletti)
  • Manual and documentation:
    • #8757: Rename Pervasives to Stdlib in core library documentation. (Ian Zimmerman, review by David Allsopp)
    • #7584, #8538: Document .cmt* files in the "overview" of ocaml{c,opt} (Oxana Kostikova, rewiew by Florian Angeletti)
    • #8515: manual, precise constraints on reexported types (Florian Angeletti, review by Gabriel Scherer)
  • Bug fixes:
    • #7156, #8594: make top level use custom printers if they are available (Andrew Litteken, report by Martin Jambon, review by Nicolás Ojeda Bär, Thomas Refis, Armaël Guéneau, Gabriel Scherer, David Allsopp)
    • #3249: ocamlmklib should reject .cmxa files (Xavier Leroy)
    • #7937, #2287: fix uncaught Unify exception when looking for type declaration (Florian Angeletti, review by Jacques Garrigue)
    • #2296: Fix parsing of hexadecimal floats with underscores in the exponent. (Hugo Heuzard and Xavier Leroy, review by Gabriel Scherer)
    • #8610, #8613: toplevel printing, consistent deduplicated name for types (Florian Angeletti, review by Thomas Refis and Gabriel Scherer, reported by Xavier Clerc)
    • #8635, #8636: Fix a bad side-effect of the -allow-approx option of ocamldep. It used to turn some errors into successes (Jérémie Dimino)
    • #8701, #8725: Variance of constrained parameters causes principality issues (Jacques Garrigue, report by Leo White, review by Gabriel Scherer)
    • #8777(partial): fix position information in some polymorphic variant error messages about missing tags (Florian Angeletti, review by Thomas Refis)
    • #8779, more cautious variance computation to avoid missing cmis (Florian Angeletti, report by Antonio Nuno Monteiro, review by Leo White)
    • #8800: Fix soundness bug in extension constructor inclusion (Leo White, review by Jacques Garrigue)
    • #8810: Env.lookup_module: don't allow creating loops (Thomas Refis, report by Leo White, review by Jacques Garrigue)
    • #8848: Fix x86 stack probe CFI information in caml_c_call and caml_call_gc (Tom Kelly, review by Xavier Leroy)
    • #8864, #8865: Fix native compilation of left shift by (word_size - 1) (Vincent Laviron, report by Murilo Giacometti Rocha, review by Xavier Leroy)
    • #8862, #8871: subst: preserve scopes (Thomas Refis, report by Leo White, review by Jacques Garrigue)
    • #8921, #8924: Fix stack overflow with Flambda (Vincent Laviron, review by Pierre Chambart and Leo White, report by Aleksandr Kuzmenko)

chartjs: OCaml bindings for Chart.js charting library

Alex announced

I'm pleased to announce chartjs, OCaml bindings for Chart.js charting library and its popular extensions. The library relies on js_of_ocaml bindings for browser API and follows js_of_ocaml guides for binding a JavaScript library.

Line, bar, horizontal bar, pie and doughnut charts are currently supported. The bindings are not full yet, some chart types and plugins are to be added later.

Some auxiliary libraries are available providing bindings for popular Chart.js plugins:

  • chartjs-annotation
  • chartjs-streaming
  • chartjs-datalabels
  • chartjs-colorschemes

These libraries are also available via opam.

Any feedback is highly appreciated.

How does one print any type?

Pinocchio asked

I just want to print any type (like a print statement in python). I am just learning ocaml using the ocaml command interactively with my script basically. How do I do this?

Alex replied

You may also use the ppx_deriving.show plugin from the ppx_deriving library. This plugin is able to generate 'a -> string and Format.formatter -> 'a -> unit (where 'a is the type you want to print) functions which can then be used for value printing.

type t =
{  number : int
;  text : string
}[@@deriving show]

This will produce a show and pp functions of type t -> string and Format.formatter -> t -> unit, which can then be used as

let v = { number = 2; text = "Hello world"} in
print_endline (show v)


let v = { number = 2; text = "Hello world"} in
let s = Format.asprintf "%a" pp v in
print_endline s

Of course, you can always define such functions for the needed types by yourself.

As I can see, almost every library introducing a new type also provides pretty-printing (kinda pp in this example) functions out of the box.

progman also replied

The same basic printing as the toplevel can be had with:

http://github.com/progman1/genprintlib (general value printing in compiled code)

Available through the official opam repository: opam install genprint

Ivan Gotovchits also replied

There is no such facility in OCaml. OCaml is not an interpreted language with a dynamic type system. When a program is compiled all types are erased, so it is impossible in runtime to reflect a value to its type. And yes, as you've already pointed out, when you interact with OCaml using interactive toplevel there is some generic printing facility, but this is a very special case because you're not really running a program, but interactively compile it.

So, you have to accept this fact and learn how to program in OCaml using OCaml ways of doing things. First of all, learn how to use the Format.printf function. This is a generic function that takes the format specification and arguments and prints them, e.g.,

open Format

let () =
  printf "Hello, %s world\n%!" "cruel";
  printf "Hello, %d times!\n%!" 42

As you can see, the special format specifiers, like %s or %d specify the type of an argument. So %d expects one argument of type int, and %s expects one of string, there is also %c for char , %b for bool and so on.

There is also a generic %a specifier, which expects not one argument, but two – a function, that will tell how to print the argument, and the argument itself. It is used to print values of abstract types, hence the name %a. There is a convention, that modules that define abstract types also provide a function called pp which specifies how this value should be printed, e.g.,

let () = printf "Hello, abstract student %a\n%!" Student.pp s

Where the Student module might be defined as

struct Student : sig
   type t
   val create : string -> int -> t
   val pp : Format.formatter -> t -> unit
end = struct
   type t = {name : string; cls : int}
   let create name cls  = {name; cls}
   let pp ppf {name; cls} =
      Format.fprintf ppf "%s of %d" name cls

That is roughly how we print everything in OCaml :)

Interesting OCaml Articles

Ryan Slade also said

Interesting performance comparison with quite a few languages when writing a user space network driver:


Dune 2.0.0 coming soon!

Jérémie Dimino announced

Update: after thinking about this more and working more towards preparing Dune 2.0.0, we made the following changes compared to this post:

  • The source code of Dune itself will remain compatible with the last 3 released versions of OCaml rather than just the last one. i.e. if 4.08 is the last released version of OCaml when Dune 2.0.0 is released then Dune 2.0.0 will be guaranteed to build with OCaml 4.06, 4.07 and 4.08. We still plan to provide an easy way to install Dune 2.0.0 in older opam switches
  • Dune 2.0.0 will not be able to read jbuild files at all. Only dune upgrade will still be able to read them in order to upgrade them


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