merge-configs doesn't allow explicit set of nil value

[dhryniewicz]

merge-configs function, deeper down, uses a pick-prioritized function that, among other scenarios, picks a non-nil value over a nil value. I suppose this is to deal with k-v pairs missing in one of the maps.
But there are scenarios when one might want to explicitly set value of something to nil (like for :logger for instance). And sadly this is not possible:

(duct.core/merge-configs
  {:duct.database.sql/hikaricp {:logger nil
                                :something-else :yay}}
  {:duct.database.sql/hikaricp
   ^:demote {:jdbc-url "123"
             :logger "999"}})
=> #:duct.database.sql{:hikaricp {:jdbc-url "123", :logger "999", :something-else :yay}}

Probably at one point Duct should be using a contains? function instead of nil? or some? at some point. I'm looking at these two lines.

Would you consider that a bug? merge-configs fn tests don't include it so I'm not sure 🤔

[dhryniewicz]

Now I see that it's somewhat a duplicate of this issue on weavejester/meta-merge

[dhryniewicz]

I just realized that that nil can be forced by using a replace meta key:

(duct.core/merge-configs
  {:duct.database.sql/hikaricp {:logger (duct.core.merge/replace nil)
                                :something-else :yay}}
  {:duct.database.sql/hikaricp
   ^:demote {:jdbc-url "123"
             :logger "999"}})
=> #:duct.database.sql{:hikaricp {:jdbc-url "123", :logger nil, :something-else :yay}}

But I think that if this is expected then maybe the tests should include it to avoid confusion in expectations?

[weavejester]

Sure; I don't mind another test. As for why this is the behaviour in the first place, meta-merge is mimicing the behaviour of Leiningen's profile merging.

[dhryniewicz]

Hmm... An update to what I said about the workaround using #duct/replace:
I don't have time to debug now but sometimes the hikaricp logger overwrite doesn't work despite the replace. Could it be because I the right hikaricp config comes not from explicit call but from duct.module/sql?

[weavejester]

Can you give me a sample code that demonstrates the behaviour you're seeing? Duct modules should prefer your configuration over the defaults they add.

[dhryniewicz]

(duct/prep-config
  {:duct.profile/base
   {:duct.database.sql/hikaricp {:logger (duct.core.merge/replace nil)}}
   :duct.module/sql {:database-url "123"}}
  profiles)
=>
{:duct.database.sql/hikaricp {:jdbc-url "123", :logger #integrant.core.Ref{:key :duct/logger}},
 :duct.migrator/ragtime {:database #integrant.core.Ref{:key :duct.database/sql},
                         :strategy :raise-error,
                         :logger #integrant.core.Ref{:key :duct/logger},
                         :migrations []}}