D JSON (WAT?!)

[John Colvin via Digitalmars-d-learn]

On Thursday, 24 July 2014 at 15:54:21 UTC, Pavel wrote:
> On Thursday, 24 July 2014 at 15:48:32 UTC, Edwin van Leeuwen 
> wrote:
>> On Thursday, 24 July 2014 at 15:42:58 UTC, Pavel wrote:
>>> On Thursday, 24 July 2014 at 15:38:06 UTC, John Colvin wrote:
>>>> On Thursday, 24 July 2014 at 15:32:29 UTC, John Colvin wrote:
>>>>> On Thursday, 24 July 2014 at 15:15:37 UTC, Pavel wrote:
>>>>>> Ok, let me start with the sample code:
>>>>>>
>>>>>> import std.stdio;
>>>>>> import std.json;
>>>>>>
>>>>>> void main() {
>>>>>> scope(failure) writeln("FaILED!!");
>>>>>> string jsonStr = `{ "name": "1", "type": "r" }`;
>>>>>> auto parsed = parseJSON(jsonStr);
>>>>>> string s = parsed["fail"].str;
>>>>>> writeln(s == "");
>>>>>> writeln(s is null);
>>>>>> writeln(s);
>>>>>> }
>>>>>>
>>>>>> Running "rdmd app.d" doesn't produce any output.
>>>>>> Can anyone explain such a behavior???
>>>>>>
>>>>>>
>>>>>> PS: Running dmd v2.065 on Linux x64.
>>>>>
>>>>> It's a bug in std.json (you should get a segfault, not no 
>>>>> output at all)
>>>>>
>>>>> It is fixed now and I'm pretty sure it will be in 2.066
>>>>>
>>>>> std.json has been improved a lot, but I would still 
>>>>> recommend using http://vibed.org/api/vibe.data.json/ instead
>>>>
>>>> perhaps "bug" is too strong a word, but it was a deficiency 
>>>> that is now corrected. You will get an exception thrown now 
>>>> and everything should work how you expect.
>>>
>>> Maybe. But still it's not the way I expect, any time you 
>>> check for non-existing property you must consider exception, 
>>> which is very heavy to deal with in such a situation. I'd 
>>> rather expect to get null, whenever I try to fetch 
>>> non-existing property, and not an exception.
>>
>> You can turn your json object into an AA object and then use 
>> in to check for existence (I know it is not very intuitive):
>>
>> JSONValue[string] jsonAA = parsed.object;
>> if ( "fail" in jsonAA )
>>  s = jsonAA["fail"].str;
>>
>>
>>
>>>
>>> That's purely my point, and I don't claim to be right in this 
>>> way. It's up to Phobos maintainers to decide how to reprent 
>>> JSON parsing results.
>
> Guess what, here's a new snippet:
>
> import std.stdio;
> import std.json;
>
> void main() {
>   scope(failure) writeln("FaILED!!");
>   string jsonStr = `{ "name": "1", "type": "r" }`;
>   auto parsed = parseJSON(jsonStr).object;
>   writeln("fail" in parsed);
> }
>
> Output is:
> null
>
> WAT?!
>
> Ofcourse, writing like:
>
> writeln(cast(bool)("fail" in parsed));
>
> Produces "false"... but why on earth boolean expression would 
> output null?
>
> PS: Sorry, for such an emotional boom, I'm so frustrated right 
> now.

The in operator in d returns a pointer to the element found or 
null.
This might be a little confusing at first* but it makes sense for 
efficiency reasons. There is often a lot overlap between the work 
needed to test for presence and the work needed to fetch an 
element.

For types where fetching elements is a significant extra cost you 
can always define a `bool contains(K key) { /* ... */ }` so a 
user can avoid this cost when appropriate.


*although to be honest it's just a different reading:
`b in c` can be read as "the b in c" or "is b in c?"