What does :provided mean in profiles.clj?

Question

Luminus right now is creating a profiles.clj with this content:

{:provided {:env {;;when set the application start the nREPL server on load
                  :nrepl-port "7001"
                  :database-url "jdbc:mysql://localhost:3306/mysqlkorma_dev?user=db_user_name_here&password=db_user_password_here"}}}

What does :provided do here? In environ's documentation it seems to point to having two entries, one for dev and one for test https://github.com/weavejester/environ.

Answer 1

TL;DR: The provided profile is used in profiles.clj as an alternative to the dev profile, because if dev is used there it would overwrite the entire dev profile specified in project.clj.

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The most common use of :provided is to specify dependencies that should be available during jar creation, but will be provided by the runtime environment. But I think here is used as a way to prevent the :env configured in profiles.clj (which is intended to not be committed into your source code repository) to overwrite the :env configured in project.clj.

Luminus would have used the :dev profile instead of :provided in profiles.clj, if it wasn't by the fact that they already put stuff in an :env entry in the :dev profile in project.clj which would be overwritten by what's in profiles.clj.

See this example repo. If you run it right away, without any change (with :provided in profiles.clj) the output will be:

› lein run
Hello, world
Db config: some:db://localhost

If you change :provided to :dev in profiles.clj, the output changes to:

› lein run
Hello, nil
Db config: some:db://localhost

They didn't get merged, but the :env in profiles.clj overwrote the :env in profile.clj

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EDIT: I just found out that not only the :env entry would be overwritten if :dev was used in profiles.clj. The entire :dev profile would be overwritten. This is explained in the profiles documentation:

Remember that if a profile with the same name is specified in multiple locations, only the profile with the highest "priority" is picked – no merging is done. The "priority" is – from highest to lowest – profiles.clj, project.clj, user-wide profiles, and finally system-wide profiles.

So using :provided in profiles.clj is a little hack around the merging strategy of leiningen profiles.

It has at least one downside: if you need to define a :provided profile in project.clj to specify dependencies that will be available in the runtime environment it would be overwritten by the one defined in profiles.clj.