couchdb-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Damien Katz <>
Subject Re: cjson.erl
Date Mon, 07 Jul 2008 17:25:28 GMT
So the sad history of cjson.erl I started with the erlang json library  
I found on the website (which now appears to be a dead link),  
and used that for a while. For reasons I cannot remember (bugs or  
performance), I switched to using the mochiweb json library. However,  
it used slightly different conventions for using Erlang terms to  
represent the Json. For one thing, objects were {struct, [...]}, while  
the library used {obj, [...]}. I think there was one other  
thing, but I can't remember now.

Anyway, rather than change all my code to use the new convention, I  
change the mochi library it to use the conventions and  
changed the name to cjson.erl (for reason I again cannot remember).  
Some of the comments in the library are likely wrong because of this.

Now switching libraries is easy, but switching the Erlang  
respresentation of json objects is not. However I'd be glad to switch  
over CouchDB to using a different Erlang representation of json, if  
there is a "blessed" Erlang format. Otherwise, I'll need practical  
reasons for doing so. Performance is one such reason.

One thing I'm no happy about is the idea of representing strings using  
binaries. From a code asthetics point of view, it uglifies the source  
dramatically, but I think it might also cause lots of extra  
conversions between binary strings and normal list strings used in  
most Erlang libraries and APIs. If the memory and performance  
improvements will have to be big to make up for the extra complexities  
in the source.


On Jul 7, 2008, at 12:24 PM, Jan Lehnardt wrote:

> Heya,
> Joe Armstrong tries to get the Erlang community to agree
> on a single JSON library that fits everybody's needs. The
> biggest players here (according to Joe I guess) are
> MochiMedia and ourselves.
> Hence the dialogue I quote below:
> Begin forwarded message:
>> From: "Joe Armstrong"
>> Date: July 7, 2008 10:51:07 AM GMT+02:00
>> To: "Jan Lehnardt" <>
>> Cc: "Bob Ippolito"
>> Subject: cjson.erl
>> Hi Jan,
>> [CC'd to Bob Ippolito (Glad to see the facebook stuff taking off -
>> great work :-)) ]
>> I've been staring at cjson.erl ...
>> The comments say it's derived from mochijson.erl.
>> In the mochiweb there are two json representations
>> mochijson2.erl  and mochijson.erl
>> I think the "2" is the better one :-)
>> I think it would be a good idea if you could come to some agreement
>> with the mochiweb people as to the best representation of
>> JSON terms in ERlang and both go out with a single library.
>> cjson.erl lacks a type declaration in the documentation - which it  
>> needs
>> (reading the code is hopeless)
>> mochijson2.erl has this type declaration
>> %% @type json_string() = atom | binary()
>> %% @type json_number() = integer() | float()
>> %% @type json_array() = [json_term()]
>> %% @type json_object() = {struct, [{json_string(), json_term()}]}
>> %% @type json_term() = json_string() | json_number() | json_array() |
>> %%                     json_object()
>> I'm not sure about the additional "struct" tag - nor the additional
>> atom tag in json_string
>> How about ...
>> @type json_object = {[json_tag::binary(), json_term()]}
>> @type json_string() = binary()
>> this makes the erlang term map to JSON in an unambigous manner and
>> the compiler should be able to generate faster code, since
>> unpack(Json) when is_binary(J) -> ...
>> will only have disjoint branches.
>> I think that:
>> lists should *only* be used for json_arrays
>> binary should *only* be used for json_strings
>> json objs should be *only* be tuples (of pairs) {{Tag,Val}, 
>> {Tag,Val},...}
>> (possibly {Tag1,Val1,Tag2,Val2,....} might be better???)
>> I think it would be a good idea to isolate this problem - agree
>> (having done some
>> measurements, on the fastest and *prettiest* way to do this) -  
>> jointly change
>> your code bases (at the same time) and then tell the world - then  
>> issue ONE
>> library.
>> Just for fun I've downloaded the wikipedia using the ideas in
>> (I want to converts the XML representation of the wikipedia into JSON
>> and inject it into coutchDB
>> and serve it up with mochiweb - I need to write a rendering engine to
>> convert wiki markup to HTML
>> (this is said to be tricky since there is no spec :-)
>> This should be a good test of coutchDB and mochiweb)
>> Cheers
>> /Joe Armstrong
> And Bob's reply:
>> From: "Bob Ippolito"
>> Date: July 7, 2008 6:12:32 PM GMT+02:00
>> To: "Joe Armstrong"
>> Cc: "Jan Lehnardt" <>
>> Subject: Re: cjson.erl
>> {struct, ...} is what the library that ships with Yaws does, which is
>> why I used that. Using just {[{Key, Value}]} looks fine to me also  
>> and
>> should be do-able without breaking compatibility immediately.
>> The reason atoms are accepted is only for encoding purposes, not for
>> decoding. There is an unambiguous format from JSON -> Erlang but for
>> Erlang -> JSON some conveniences are allowed for practical reasons.
>> I'm fine with the {struct, ...} -> {...} change that Joe proposed
>> because I can do that in a backwards compatible way.
>> -bob
> What is our take on this? :) Damien?
> I'll forward our discussions back to Joe and Bob (in case they don't
> read this list).
> Cheers
> Jan

View raw message