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

Its funny how with lens, you first try something and it doesn't work. Then you work on it a lot, and when you finally get it working it looks exactly like the first thing you tried. Mysterious. On Thu, Dec 22, 2016 at 12:00 AM, Jan von Löwenstein < jan.loewenstein@gmail.com> wrote:

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 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@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 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@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 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@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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