ignore null value

[co-dh]

add an option to ignore key whose value is null, e.g.
{a : null } equal to {}

[thsutton]

@co-dh Can explain what you mean? Do you mean when generating a patch using the diff function? Or when applying a patch with the patch function?

[co-dh]

It's about using diff function.
I have a piece of code that reading from a json using aeson and write it back to test.
An optional field like a :: Maybe String will parsed as { a : null } if there is no value.
so if I use toJson , I got { a : null } , instead of {}. It would be desirable when you diff two objects o1 and o2, if a key k exist in o2 but not o1, with value null, you may consider them the same. e.g.
o1 = { a: 1 }, o2 = {a: 1, k : null }, you can conside o1 == o2 with the option.

[co-dh]

https://hackage.haskell.org/package/aeson-0.11.1.4/docs/Data-Aeson-TH.html
I think what I need is something like omitNothingFields in above link.

[co-dh]

Here is what I mean

import qualified Data.HashMap.Strict as H
import qualified Data.Vector as V
import Data.Aeson ((.:), (.:?), eitherDecode, FromJSON(..), Value(..), encode, toJSON)


stripNull :: Value -> Value
stripNull (Object hash) =  Object $ H.map stripNull $ H.filter (/= Null) hash
stripNull (Array x)  = Array $ V.map stripNull x
stripNull (String x) = String x
stripNull (Number x) = Number x
stripNull (Bool x) = Bool x
stripNull Null = Null

[thsutton]

You want results like this?

diff {"a":"hello", "b":"world"} {"a":null,"b":"world"} === [{"op":"remove", "path":"/a"}]
diff {"a":null,"b":"world"} {"a":"hello","b":"world"} === [{"op":"add", "path":"/a", "value": "hello"}]
diff {"a":null, "b":"world"} {"b":"world"} === []

I'm focusing on refactoring the code at the moment. I'll think about how this could fit into the library.

In the mean time, you'll have to use your stripNull function:

import Data.Function (on)
import Data.Aeson.Diff (diff)

diffIgnoringNull :: Value -> Value -> Patch
diffIgnoringNull = diff `on` stripNull

[co-dh]

I'm not sure about

diff {"a":"hello", "b":"world"} {"a":null,"b":"world"} === [{"op":"remove", "path":"/a"}]
diff {"a":null,"b":"world"} {"a":"hello","b":"world"} === [{"op":"add", "path":"/a", "value": "hello"}]

But I want
diff {"a":null, "b":"world"} {"b":"world"} === [] when the option stripNull is on.

The on function is something I don't now, thanks.

[thsutton]

OK. I'll look at implementing this option for the next release.