Support for deep object merging

[JakeStanger]

Currently object merging only takes the immediate children into account.

There may be utility in changing this, or adding additional syntax, to perform a 'deep merge', running the merge against every child object.

Issue originates from the below discussion, which compares the feature to Nix's 'attrset merge':

Discussed in https://github.com/orgs/corn-config/discussions/29

^{Originally posted by fbewivpjsbsby November 6, 2023}

1. What version of corn are you using?
   corn-cli 0.9.2
2. What is your operating system?
   Windows 10

Description

It is like nixos module, config will merge when you imports config, use // operators or use mkMerge.
Corn is override attrsets now.

Steps to reproduce

1. Write this in corn file:

let {
  $preferences_user_enable = {
    Value = true
    Status = "user"
  }

  // telemetry
  $policies_disable_telemetry = {
    DisableTelemetry = true
    DisableFirefoxStudies = true
  }
  // #TODO 'raw data' need git version corn
  // theme
  $policies_theme = {
    Preferences = {
      'toolkit.legacyUserProfileCustomizations.stylesheets' = $preferences_user_enable
      'layout.css.backdrop-filter.enabled' = $preferences_user_enable
      'widget.non-native-theme.use-theme-accent' = $preferences_user_enable
    }
  }

  $policies_ech = {
    // ech need enable DNS over HTTPS
    DNSOverHTTPS = {
      Enabled =  true
      ProviderURL = "https://1.1.1.1/dns-query"
      Locked = false
      ExcludedDomains = [""]
    }
    Preferences = {
      'network.dns.echconfig.enabled' = $preferences_user_enable
      'network.dns.http3_echconfig.enabled' = $preferences_user_enable
    }
  }
} in
{
  policies = {
    // telemetry
    ..$policies_disable_telemetry
    // ech
    ..$policies_ech
    // theme
    ..$policies_theme
  }
}

2. run corn [your corn file name].corn -t json It is override by $policies_theme:

{
  "policies": {
    "DisableTelemetry": true,
    "DisableFirefoxStudies": true,
    "DNSOverHTTPS": {
      "Enabled": true,
      "ProviderURL": "https://1.1.1.1/dns-query",
      "Locked": false,
      "ExcludedDomains": [
        ""
      ]
    },
    "Preferences": {
      "toolkit.legacyUserProfileCustomizations.stylesheets": {
        "Value": true,
        "Status": "user"
      },
      "layout.css.backdrop-filter.enabled": {
        "Value": true,
        "Status": "user"
      },
      "widget.non-native-theme.use-theme-accent": {
        "Value": true,
        "Status": "user"
      }
    }
  }
}

[jficz]

I'd suggest ... operator for deep merge and keep .. with current shallow merge behavior. Both have utility.

[JakeStanger]

I'm not a fan of ... because it's too similar to ... It'd be easy to miss, and it's not obvious what the difference is.

[jficz]

fail point. I couldn't find any existing prior art for deep merge operator (didn't dig too much) but I got a few more ideas in no particular order:

>.
>>
.>
+>
>+
*.
**
.*

I don't like any of those very much, most of them have an established meaning in other languages, except perhaps +>, .> and >.

[JakeStanger]

Perhaps it could be considered a "union merge" and copy the Typescript & union operator (or &&).

A nix-style // is also an option, but only if it's not confusing.

[jficz]

ha, I forgot about nix's //. Shame on me :)
Since corn is inspired by Nix in many ways (at least that's what it looks like to me), I think // is a good idea. I think any operator will be confusing to some and keeping some level of consistency with "close relatives" is imho the best compromise

[jficz]

yeah, except // is used for comments in cornlang

[JakeStanger]

Very good point, forgot about that. My preference would probably still be for a &

[jficz]

In this case I'd agree with & as opposed to && which is imho better typographically but at the same time more confusing logically. Happy to do the testing but code-wise not sure I can help much, rust isn't exactly a language I understand (at this point) :(