std.jgrandson

[Andrea Fontana via Digitalmars-d]

On Sunday, 3 August 2014 at 07:16:05 UTC, Andrei Alexandrescu 
wrote:
> We need a better json library at Facebook. I'd discussed with 
> Sönke the possibility of taking vibe.d's json to std but he 
> said it needs some more work. So I took std.jgrandson to proof 
> of concept state and hence ready for destruction:
>
> http://erdani.com/d/jgrandson.d
> http://erdani.com/d/phobos-prerelease/std_jgrandson.html
>
> Here are a few differences compared to vibe.d's library. I 
> think these are desirable to have in that library as well:
>
> * Parsing strings is decoupled into tokenization (which is lazy 
> and only needs an input range) and parsing proper. Tokenization 
> is lazy, which allows users to create their own advanced (e.g. 
> partial/lazy) parsing if needed. The parser itself is eager.
>
> * There's no decoding of strings.
>
> * The representation is built on Algebraic, with the advantages 
> that it benefits from all of its primitives. Implementation is 
> also very compact because Algebraic obviates a bunch of 
> boilerplate. Subsequent improvements to Algebraic will also 
> reflect themselves into improvements to std.jgrandson.
>
> * The JSON value (called std.jgrandson.Value) has no named 
> member variables or methods except for __payload. This is so 
> there's no clash between dynamic properties exposed via 
> opDispatch.
>
> Well that's about it. What would it take for this to become a 
> Phobos proposal? Destroy.
>
>
> Andrei

On my bson library I found very useful to have some methods to 
know if a field exists or not, and to get a "defaulted" value. 
Something like:

auto assume(T)(Value v, T default = T.init);

Another good method could be something like xpath to get a deep 
value:

Value v = value["/path/to/sub/object"];

Moreover in my library I actually have three different methods to 
read a value:

T get(T)() // Exception if value is not a T or not valid or value 
doesn't exist
T to(T)()  // Try to convert value to T using to!string. 
Exception if doesn't exists or not valid

BsonField!T as(T)(lazy T default = T.init)  // Always return a 
value

BsonField!T is an "alias this"-ed struct with two fields: T value 
and bool error(). T value is the aliased field, and error() tells 
you if value is defaulted (because of an error: field not exists 
or can't convert to T)

So I can write something like this:

int myvalue = json["/that/deep/property"].as!int;

or

auto myvalue = json["/that/deep/property"].as!int(10);

if (myvalue.error) writeln("Property doesn't exists, I'm using 
default value);

writeln("Property value: ", myvalue);

I hope this can be useful...