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

[Jan von Löwenstein]

Thanks a lot.

I finally figured out how to use `non` properly.

I tried hard to get it working with `key` from Data.Aeson.Lens.
Without a Maybe in the signature of `key` I didn't get `non` to work. Took
some time to realize this.

Now I switched to using `at` and everything is fine (i.e. compiles).

David Fox <dsf at seereason.com> schrieb am Di., 20. Dez. 2016 um 22:59 Uhr:

> 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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