# Help an ocamlyacc beginner **URL:** \ **Category:** Learning\ **Created:** [September 28, 2019, 7:54pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459 "2019-09-28T19:54:45Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![phstrauss](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@phstrauss](https://discuss.ocaml.org/u/phstrauss)\ **Post date:** [September 28, 2019, 7:54pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/1 "2019-09-28T19:54:45Z")
Hello ocamlers! For a side project, I need a config file format both readable in ocaml and C or C++. For now I’ve settled on using Libconfig : [https://hyperrealm.github.io/libconfig/](https://hyperrealm.github.io/libconfig/) It’s a C built library for parsing a custom but very versatile config file format. The BNF grammar of this file format is documented at page 49 of the following .PDF : [http://www.strauss-engineering.ch/tmp/libconfig.pdf](http://www.strauss-engineering.ch/tmp/libconfig.pdf) BTW, under this URI : [http://www.strauss-engineering.ch/tmp/](http://www.strauss-engineering.ch/tmp/) you’ll find : - libconfig doco : libconfig.pdf - first attempt at a lexer: config\_lexer.mll - first attempt at a parser : config\_parser.mly (\*) - first attempt at ocaml typing of the parser result: configuration.mli - libconfig configuration files examples : \*.cfg - parser\_test.ml : a basic test for the parser, using a generic printing lib. - build\_parser.sh : a build script I can’t figure out how to type the parser result using a standard variant type. I’m totally noob about GADT’s and polymorphic variant BTW. Can someone help with the ocaml typing? Thanks in advance. (\*) the ocamlyacc parser is a subset of libconfig one’s: I’ve dropped the heterogenous list support on the ocaml side. best regards! ---
**Author:** ![octachron](https://avatars.discourse-cdn.com/v4/letter/o/49beb7/32.png) [@octachron](https://discuss.ocaml.org/u/octachron)\ **Post date:** [September 28, 2019, 8:14pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/2 "2019-09-28T20:14:17Z")
What is the issue more specifically? From your message, it sounds like you get a type checking error, which one? Normally, a big enough variant type should suffice for most parsing needs. ---
**Author:** ![SkySkimmer](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/skyskimmer/32/1672_2.png) [@SkySkimmer](https://discuss.ocaml.org/u/SkySkimmer)\ **Post date:** [September 28, 2019, 8:22pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/3 "2019-09-28T20:22:07Z")
The spec says `Group` is `setting list` not `value list`. Additional nitpicking: I would parse hex encoded values directly to their interpretation, also I prefer shorter names for types (no `_t` or `_kind_t`) and for the string value constructor. So I would do ```ocaml type scalar = | Bool of bool | Integer of int | Integer64 of int64 | Float of float | String of string type value = | Scalar of scalar | Array of scalar list | Group of setting list and setting = string * value type t = setting list ``` ---
**Author:** ![SkySkimmer](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/skyskimmer/32/1672_2.png) [@SkySkimmer](https://discuss.ocaml.org/u/SkySkimmer)\ **Post date:** [September 28, 2019, 8:23pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/4 "2019-09-28T20:23:12Z")
Also in the future instead of a directory url give a .zip url, it’s easier to download (my naive wget grabbed a bunch of extra stuff from your server before I killed it ;)) ---
**Author:** ![phstrauss](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@phstrauss](https://discuss.ocaml.org/u/phstrauss)\ **Post date:** [September 28, 2019, 8:29pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/5 "2019-09-28T20:29:30Z")
./build\_parser.sh 46 states, 1240 transitions, table size 5236 bytes 4 rules never reduced File “config\_parser.mly”, line 68, characters 36-51: Error: This expression has type Configuration.setting list but an expression was expected of type Configuration.value\_kind\_t list Type Configuration.setting = string \* Configuration.value\_kind\_t is not compatible with type Configuration.value\_kind\_t line 68 of the .mly: group: | LBRACE RBRACE { [] } line 68–\> | LBRACE setting\_list RBRACE { List.rev $2 } ; ---
**Author:** ![phstrauss](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@phstrauss](https://discuss.ocaml.org/u/phstrauss)\ **Post date:** [September 28, 2019, 10:28pm UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/6 "2019-09-28T22:28:10Z")
the previous msg from myself was for octachron. Thanks SkySkimmer, didn’t know the “type … _and_ …”, that was the piece missing. Now I’ve got something that build fine but totally mute, it output nothing yet, stay tuned for more adventure 🙂 ---
**Author:** ![Chet\_Murthy](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/chet_murthy/32/1501_2.png) [@Chet\_Murthy](https://discuss.ocaml.org/u/Chet_Murthy)\ **Post date:** [September 29, 2019, 12:22am UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/7 "2019-09-29T00:22:58Z")
I’m sure this answer isn’t going to be useful, but just in case: It’s possible that you’re spending a lot of effort, where if you were to choose a different config-file format, you could avoid almost all of it. [OTOH, perhaps you’re forced to use this format, in which case this comment is useless.] If you can choose another format, you might choose INI-files, or YAML, or even (esp!) JSON; in each of these cases, there’s well-developed libraries in most languages for this format, including in Ocaml. Just a thought. P.S. I find JSON to be really nice, and esp. b/c the ubiquity of the format means it’ll be vanishingly rare for a language to not have a JSON interface library. ---
**Author:** ![phstrauss](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@phstrauss](https://discuss.ocaml.org/u/phstrauss)\ **Post date:** [September 29, 2019, 11:20am UTC](https://discuss.ocaml.org/t/help-an-ocamlyacc-beginner/4459/8 "2019-09-29T11:20:42Z")
Sure JSON would do fine, but I want to limit the number of runtime lib dependencies somehow.