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Here is the latest OCaml Weekly News, for the week of December 16 to 23, 2014.

  1. New release of Menhir (20141215)
  2. fold missing in string/bytes
  3. ppx_deriving 1.1, ppx_deriving_yojson 2.1
  4. LexiFi is hiring
  5. Other OCaml News

New release of Menhir (20141215)


François Pottier announced:
I have recently started making a series of changes in Menhir, inspired by
the work of Frédéric Bour on Merlin (a smart emacs-mode for OCaml, which
uses a modified version of Menhir). Thanks to Frédéric for his stimulating
ideas, and thanks to Gabriel Scherer for not letting me sleep until I
promised I would do something about them! :-)

I have made a first release a couple days ago. The relevant chunk of the
CHANGES file is appended below. In summary, a new incremental API is
available; Menhir now requires ocaml 4.02; and a couple of obscure features
(--error-recovery and $previouserror) have been removed in the interest of
speed and simplicity.

The incremental API means that you can take a snapshot of the parser's state,
essentially at no cost, at any time. It also means that the parser no longer
drives the lexer; you drive the lexer, and you provide tokens to the parser
when it requests them. This can be convenient if the lexer is in a monad (the
Lwt monad, for instance).

More changes are planned. The type "env" exposed by the new incremental API is
currently opaque. We are planning to offer a range of functions that allow
inspecting and building values of type "env". This should allow the user to
implement new error handling and error recovery strategies outside of Menhir.

The new release is available now as a .tar.gz archive:

It is also available via opam ("opam install menhir").


François Pottier

New incremental API (in --table mode only), inspired by Frédéric Bour.

Menhir now reports an error if one of the start symbols produces
either the empty language or the singleton language {epsilon}.

Although some people out there actually define a start symbol that recognizes
{epsilon} (and use it as a way of initializing or re-initializing some global
state), this is considered bad style. Furthermore, by ruling out this case, we
are able to simplify the table back-end a little bit.

A speed improvement in the code back-end.

Menhir now requires OCaml 4.02 (instead of 3.09).

Removed support for the $previouserror keyword.
Removed support for --error-recovery mode.
Gerd Stolpmann then said:
Thanks for this, it is great news. A couple of months ago I tried to
develop a monadic parser for the IMAP protocol, which has a weird
grammar and needs some strange interactions between lexing and parsing.
Lacking a parser generator I started to write the parser manually, but
it never made good progress. It looks like Menhir can now be used for
this case, and I'll try it.
Nicolas Ojeda Bar then replied:
I wrote an IMAP client library in pure OCaml
(  It can handle the full RFC 3501
grammar.  This library itself is not yet ready for prime-time, but I
spent quite a bit of time thinking about the question of parsing since
it is such a big part of the protocol.

While the incremental parsing in the new Menhir is great I do not
think it will help to parse IMAP because the token types of IMAP
grammar are highly context dependent.  By this I mean that there are
many overlapping token definitions that are valid in different parts
of the grammar.  While this could be hacked around (by changing the
lexer used in dummy non-terminals) the complexity of doing so quickly
gets out of hand.  In my view this almost requires using a scannerless
parser, which rules out ocamlyacc/menhir-type parsers.

A parser generator that could handle the IMAP grammar nicely is Dypgen
(which generates GLR parsers).  You can almost copy the IMAP grammar
verbatim from RFC 3501.  However it can not be made incremental.  This
means that you need to have all the input available before starting
the parse.  Since the amount of data is potentially very big, this
rules using Dypgen for anything else than toy applications.

In my library I use a monadic combinator parser
When programming monadically you are reifying the continuation at
every `bind` point so you get the incremental bit for free (I think
this goes by the fancy name of `iteratees` nowadays).  On the other
hand it suffers from poor time/space performance common to this type
of parser.  Also, while combinator parsers can handle arbitrary
backtracking (and you end up paying for this), IMAP itself requires
very little, so it would seem that the full flexibility of combinators
are not needed.

It seems that the "scannerless, incremental" space in parser
technology is not very developed (in OCaml; maybe other languages is
different ?).  One intriguing possibility that I am exploring (but
this is a much longer term project) is to use PEG parsers for this.
It would be easy to write an IMAP parser with PEG. The main problem is
that the usual implementation of PEG parsers uses memoization to keep
parsing time from blowing up exponentially on the length of input (
(in OCaml the Aurochs parser generator is of this type).  This
requires storage proportional to input length, which is a problem when
parsing large amounts of data.  But for grammars that don't need a lot
of backtracking (like IMAP) the memoization is not actually required,
so I think there is an interesting space to explore between all these

In any case, I would love to hear about your experience if you do try
to parse IMAP using Menhir.
Daniel Bünzli replied to Nicolas:
For a long time I played with combinator parsers but I never got to
the point of being satisfied with the result. I also played a little
bit with iteratees but couldn't get the performance I wanted with the
full model in all its compositional glory.

Nowadays I simply write my streaming lexers/parsers manually in CPS
and you can drive them in non-blocking mode with a single, fixed size,
buffer. See here [1] for an interface and an implementation on a toy

If you get the lowest continuation decoding bits correctly (e.g. don't
bind on each byte) you can actually get good time and space
performance. Once you have handled that (mainly the painful bits about
read overlapping two input buffers) you get very nice parsing
flexibility, e.g. for things like text encoding discovery where you
need to patch the continuation with the appropriate character
decoder. If you care for your users you can also get very good error
reporting and error recovery capabilities by applying knowledge
specific to the decoded protocol. That's the way Uutf [2], Jsonm [3]
(on top of Uutf) and Dicomm [4] are programmed.



Simon Cruanes then said:
I saw this on mirage's wiki:

Apart from using bigarrays for performance, the idea of using a ppx to
make the combinators more efficient looks interesting.

I also plan to try to mix recursive descent with a monadic "refill"
function for simple parsers (S-expressions, Bencode, etc.) It will be
interesting to compare that with Daniel's manual CPS.
Gerd Stolpmann also replied to Nicolas:
thanks for pointing me to your project. Maybe I'm just going to use it
(well, I'd need it to make it compatible with Ocamlnet, and I also need
TLS and SASL). Regarding the question of whether Menhir is usable: 

Part of the difficulty are the BLOBs that can occur anywhere. Here is an
example from the RFC (C = sent by client, S = sent by server, i.e. what
we need to parse):

C:    a004 fetch 12 body[header]
S:    * 12 FETCH (BODY[HEADER] {342}
S:    Date: Wed, 17 Jul 1996 02:23:25 -0700 (PDT)
S:    From: Terry Gray <>
S:    Subject: IMAP4rev1 WG mtg summary and minutes
S:    To:
S:    cc: minutes@CNRI.Reston.VA.US, John Klensin <KLENSIN@MIT.EDU>
S:    Message-Id: <>
S:    MIME-Version: 1.0
S:    )
S:    a004 OK FETCH completed

The {342} means that a BLOB with 342 bytes follows. This could be
completely harmless if we could recognize such tokens in the lexer, and
switch the lexer to a BLOB lexer before the tokens reach the parser.
However, this is actually not as easy, as the grammar also allows such
strings at other places where they do not prefix BLOBs.

My idea was that the lexer marks such tokens as "maybe-prefixes", and we
initially only parse to that point. These maybe-prefixes are always
followed by a line ending, so either the response ends and the parser
finishes (meaning that the maybe-prefix isn't a BLOB prefix), or the
parser needs more tokens, in which case it is either a BLOB prefix
followed by a BLOB or completely invalid. I haven't checked yet: could
we inspect Menhir's state and see whether the only possible
interpretation is a BLOB prefix? I.e. whether it is in the middle of a

blob := blob-prefix blob-data

and there is no other way of interpreting the tokens? This would make
the decision somewhat safer how such context-dependent tokens are
Francois Pottier then said:
At this moment, it is not possible to inspect Menhir's state. In the future,
we are planning to allow it. However, I don't know if it would help with this

An idea that comes to mind is: Menhir's new incremental interface actually
allows you to fork the parser. When you find {342} in the input stream, you
could try two alternatives (viewing it as a blob token, and not viewing it as
a non-blob token) in parallel. If you can be certain that one of the two
alternatives will quickly die, then this approach might be tenable. It is
actually a form of poor man's GLR.
oleg also replied to Nicolas:
Regarding incremental parsing of protocols like IMAP: I have
successfully (as in successfully deployed in production and being used
around the clock, since about 2010 or so) used iteratees for
incremental parsing of full XML, including CDATA, parsed entities and
namespaces. The full XML is actually quite difficult to parse: for
example, parsed entity references like &amp; are not recognized within
CDATA blocks; the content of attributes has its own whitespace
handling rules. The parser is used for handling sometimes quite large
XML documents. The parser is incremental and so can work in constant

I have also used iteratees to parse HTTP Log files, also
incrementally. The log files have an (unintended, I hope) complication:
the user-agent string (quoted in the log) may, according to RFC,
itself contain quotes. Since the embedded quotes are not escaped
(again, according to RFC), we may end up with quoted strings
containing unescaped quote characters. Parsing will require unbounded
look-ahead then. Iteratees can handle that -- and report errors
precisely and recover.

Incidentally, there are quite many iteratee libraries. Some, like
pipes, emphasize apparent simplicity and do no input buffering. The
performance is indeed pretty bad then. 

        I should also mention that a parser with a call-back interface
and the absence of visible side-effects can _automatically_ be made
incremental. The following web page describes incrementalization of
stdlib's Genlex lexer.
Dario Teixeira replied to the original announcement:
> More changes are planned. The type "env" exposed by the new incremental API is
> currently opaque. We are planning to offer a range of functions that allow
> inspecting and building values of type "env". This should allow the  user to
> implement new error handling and error recovery strategies outside of Menhir.

I have one very concrete parsing problem which can potentially be solved elegantly
by this new API.  In the Lambtex parser [1], I need to switch between lexers
mid-parsing (think handling verbatim-like blocks).  The current solution is very
hackish and complex, to the point that I've seriously considered switching to some
parser combinator approach.

Anyway, hopefully this particular use case will be solvable by the API once it
allows inspection of values of type "env"!

Best regards,
Dario Teixeira


fold missing in string/bytes


Jiten Pathy asked and David Allsopp replied:
> Any rationale, why the stdlib doesn't have fold on string/bytes?

The Standard Library has to remain maintainably small for Inria -
a line has to be drawn somewhere. There are alternatives available,
all of which have String folding:

Jane Street Core:

ExtLib & Batteries are intended to be drop-in replacements for the
Standard Library (so all existing standard library functions are
included); Core is not.

ppx_deriving 1.1, ppx_deriving_yojson 2.1


Peter Zotov announced:
I'm glad to announce the releases of ppx_deriving[1] and
ppx_deriving_yojson[2]. These are mainly bugfix releases.

ppx_deriving 1.1

  * New plugin: create.
  * Show, eq, ord: handle `_`.
  * Show, eq, ord, map, iter, fold: handle inheriting from a parametric
    polymorphic variant type.
  * Make Ppx_deriving.poly_{fun,arrow}_of_type_decl construct functions
    in correct order. This also fixes all derivers with types with
    more than one parameter.
  * Add Ppx_deriving.fold_{left,right}_type_decl.

ppx_deriving_yojson 2.1

  * Handle inheriting from a parametric polymorphic variant type.
  * Don't leak type variables.


LexiFi is hiring


Alain Frisch announced:
LexiFi is hiring!

We are looking for full time software developers, with a passion for
functional programming, ideally in OCaml, and for beautiful code.

Learn about how LexiFi uses OCaml:

No prior knowledge of finance is required.  If you are interested,
please send your resume to

We also offer quantitative developer positions for those with a
background in mathematical finance, and internship positions for
master or PhD students.

About LexiFi:

LexiFi is a financial software vendor based in Paris (Boulogne
Billancourt).  OCaml is our main development language, and we are
strongly involved in the evolution of OCaml.  We have created
approaches and tools, rooted in the theory of programming languages,
to radically simplify the development of financial applications.  Our
products are used by financial institutions and other software or
service providers that embed our technology (and sometimes become
OCaml enthusiasts as well).  For more information, visit

Other OCaml News

From the ocamlcore planet blog:
Thanks to Alp Mestan, we now include in the OCaml Weekly News the links to the
recent posts from the ocamlcore planet blog at

LBFGS 0.8.6 released:

Compiling regular expressions (II):

Compiling regular expressions (I):

LexiFi is hiring OCaml developers:

OCaml 4.01 for iOS 8 Simulator:

Become a BST Ninja - Genin Level:

OPAM 1.2 and Travis CI:

Old cwn

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