std.jgrandson

[Andrea Fontana via Digitalmars-d]

On Monday, 4 August 2014 at 16:58:12 UTC, Andrei Alexandrescu 
wrote:
> On 8/4/14, 12:47 AM, Andrea Fontana wrote:
>> 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);
>
> Nice. Probably "get" would be better to be in keep with 
> built-in hashtables.

I wrote assume just to use proposed syntax :)

>> Another good method could be something like xpath to get a 
>> deep value:
>>
>> Value v = value["/path/to/sub/object"];
>
> Cool. Is it unlikely that a value contains an actual slash? If 
> so would be value["path"]["to"]["sub"]["object"] more precise?

Key with a slash (or dot?) inside is not common at all. Never 
seen on json data.

In many languages there're libraries to bind json to struct or 
objects so usually people doesn't use strange chars inside key. 
If needed you can still use old good method to read a single 
field.

value["path"]["to"]["object"] was my first choice but i didn't 
like it.

First: it create a lot of temporary objects.

Second: it is easier to implement using a single string (also on 
assignment)

I gave it a try with value["path", "to", "index"] but it's not 
confortable if you need to generate your path from code.


>> 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...
>
> Sure is, thanks. Listen, would you want to volunteer a 
> std.data.json proposal?
>

What does it mean? :)

>
> Andrei