[Haskell-cafe] Lenses: Changing/creating values deep inside JSON or HashMap

[David Fox]

You can use the "non" lens for this:

λ> view (at "a" . non "") (Data.Map.fromList [("a", "x"), ("b", "y")])
"x"
λ> view (at "c" . non "") (Data.Map.fromList [("a", "x"), ("b", "y")])
""
λ> set (at "c" . non "") "z" (Data.Map.fromList [("a", "x"), ("b", "y")])
fromList [("a","x"),("b","y"),("c","z")]

So it will return the non argument when an entry doesn't exist, and it will
create values using the non argument when you try to set an entry that
doesn't exist.


On Tue, Dec 20, 2016 at 12:02 PM, Jan von Löwenstein <
jan.loewenstein at gmail.com> wrote:

> Thanks for looking into this. Apparently I did not yet express my question
> properly.
>
> In short:
> Given a deeply nested data structure (e.g. Json with nested objects) I
> want to set a leaf value without considering which parent and or sibling
> nodes already exist.
>
> More concrete, given the following Json fragment (out of a configuration
> file):
> "disks": {
>        "system": "/dev/xvda",
>        "ephemeral": "/dev/sdb",
>        "persistent": {
>            "<disk identifier>" : { "path" : "/container/mounted/volume" }
>        }
>      },
>
> I want to add a persistent disk `my-disk` with a given path
> `/yet/another/volume` and I don't want to handle
>
>    - there is no node 'my-disk'
>    - there is no node 'persistent'
>    - there is no node 'disks'
>
> Given a function
>   defaultsToEmpty :: Maybe (Value) -> Maybe (Value)
>   defaultsToEmpty Nothing = Just $ Object HashMap.empty
>   defaultsToEmpty m = m
> I can do
>   json & key "disks" %~ defaultsToEmpty
>           & key "disks".key "persistent" %~ defaultsToEmpty
>           & key "disks".key "persistent".key diskId %~ defaultsToEmpty
> but this doesn't look nice and was wondering if lenses can do better.
>
>
>
> David McBride <toad3k at gmail.com> schrieb am Di., 20. Dez. 2016 um
> 16:59 Uhr:
>
>> Sorry, replying to list:
>>
>>
>> Would that not just be
>>
>> json & key "outer" .~ object ["inner", "new-value"]
>>
>> On Tue, Dec 20, 2016 at 8:26 AM, Jan von Löwenstein <
>> jan.loewenstein at gmail.com> wrote:
>>
>> I guess I haven't explained well what I am looking for.
>>
>> I want to set values deep inside a json structure, no matter if the full
>> structure already exists.
>>
>> Go into "outer", if it doesn't exist make it an empty object.
>> From there go into "inner" and set it to "new-value".
>>
>> The first part should be arbitrarily deep.
>>
>> Best
>> Jan
>>
>> Benjamin Edwards <edwards.benj at gmail.com> schrieb am Di., 20. Dez. 2016
>> um 13:42 Uhr:
>>
>> If the desired function doesn't inspect the input then why not just use
>> const <<thing you want>> ?
>>
>> As to the lenses part: If you use a prism and it doesn't match, then it's
>> a noop.
>>
>> On Tue, 20 Dec 2016 at 09:38 Jan von Löwenstein <
>> jan.loewenstein at gmail.com> wrote:
>>
>> Hi,
>>
>> I want to have code that produces the following result no matter if input
>> A or B was given:
>>
>> Result:
>> {
>>   "outer" : {
>>                   "inner" : "new-value"
>>                 }
>> }
>>
>> A:
>> {
>>   "outer" : {
>>                   "inner" : "old-value"
>>                 }
>> }
>>
>> B:
>> {}
>>
>> `json & key "outer"._Object.key "inner" .~ "new-value"` didn't work.
>>
>> Is what I want even possible with lenses?
>>
>> Best
>> Jan
>>
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