It’s been discussed here quite a bit already, but I’ll add a few more points.
Leiningen is basically an entire declarative, framework-style build tool. It is based around the unifying concept of the “project” (project.clj) and merging “profiles”. Plugins are written to operate within this framework to perform miscellaneous build tasks.
deps.edn seems to be designed with the more targeted goal of being classpath building tool that let’s you configure it in various ways via aliases. It manages dependencies associated with constructing these classpaths .
In doing this, it facilitates builds being composed of separate tools that are targeted more specifically to the various sorts of tasks you need to do to build and deploy something.
I agree that deps.edn seems more aligned with many popular clj libs in the ecosystem in that it only attempt to do an isolated part of the overall build, and therefore favors composing the pieces however you want, ie. using “simpler tools”.
However, when it comes to build tools there are tradeoffs that I think others may be able to relate to here as being similar to the (old) debates of Maven vs Ant in the Java ecosystem.
When you actually end up configuring a production-like build & deploy cycle for a project, you will eventually (likely) end up:
- composing a lot of tools somewhat ad hoc w/deps.edn and friends,
- or you can use lein with a bunch of plugins all conforming to the same declarative framework approach.
The deps.edn + other tools ad hoc composition could be thought of as more “imperative” in style. Also, it also may be more difficult to apply the same pattern to many projects - like what a framework specializes in doing.
The lein approach is monolithic, but has configuration be defined in a more “declarative” way. This can lead to more patterned reuse, at the expense of less flexibility at times & a need to possibly understand the framework system more upfront.
This Stack Overflow post (along with quite a few others if you search around) provides insight that I believe relates to this topic https://stackoverflow.com/questions/14955597/imperative-vs-declarative-build-systems
That said, you can actually write a lein plugin that uses these “simpler” composable tools, such as deps.edn, figwheel-main, etc. eg. It’s already been done for deps.edn as described Combining tools.deps with Leiningen (note: I’m not sure how much this has been used still).
Lastly, from personal experience, I’ve experimented a little with the composing-simple-tools path (eg. deps.edn) vs the monolith framework (eg. lein) to try to understand the pro’s con’s of each.
A few takeaways:
a) Simpler tools can be nice since each part is more isolated & clear where a step occurs. It can be easier to immediately understand & debug issues.
With Leiningen, you have to understand the framework more when debugging issues. If you understand it well, it becomes easier, but there is more overhead to learn there. Debugging can still more of a chore since things happen often in the “framework stack”, which can be difficult to walk through.
b) Within a organization you may end up with many projects that are quite similar in structure and build/deploy process. I’ve found the ability to share build tasks less ad hoc and more repeatable with Leiningen where you can have a bit more dynamism in the definition of the project.clj as well as write/use plugins to deal with the individual tasks you want configured.
When composing individual tools, it may be redundant or manual (like using scripts) to communicate common “variables” across the build, such as project name + version + asset locations (web), etc.
When unifying around Leiningen projects and using plugins, the project is the “source of truth” you can use to communicate across all the tasks.
I’ve seen some deps.edn composition setups that ended up using several bash scripts to “glue”/“wire” thing together. This was mentioned already here I see too
This concerns me a bit given the difficulty of writing portable bash scripts, as well as the need to understand more ad hoc build steps per project in a language like bash (for someone who is proficient/loves bash more than me, this may not be a concern).
c) It seems to me that there may be projects that are better suited to the piecemeal composition approach to the framework approach, or vice versa. There may be times when the framework is very useful due to it’s common patterns and shared “glue”/“wiring” infrastructure.
There may be times when the framework is more of a complexity problem than a useful tool - such as when the lack of flexibility becomes too much within a project’s build setup.
 In addition to this point, it provides it’s own dependency resolution infrastructure and includes more features than the maven/aether/pomegranate libs used by Leiningen - such as pulling dependencies from git sha’s. This was mentioned in other posts here.