Learning Clojure is like going from driving a car on the right side of the road, to driving a car on the left side of the road.
If you can't figure it out in under a month, you're either a hopeless driver, or just didn't really care to try to adapt to the slightly unfamiliar.
I've evangelized Clojure at my work. I've seen many people having to learn and try it given different context.
There's people who end up loving it just as much as me, and people who end up hating it. In all cases, the lovers were willing to break their habits and figure out how to adapt to Clojure's syntax and paradigms. They were eager and happy to learn something different. The haters were not, they felt compelled and forced to drive on a side of the road they weren't comfortable with, it scared them, caused them additional stress, and clearly they didn't care to be here, like they were dragged along a vacation to a place they had no interest for, and would have really just wanted to stay home instead, where things are easy and familiar.
The interesting part though, neither groups were slowed down, or failed to deliver their tasks and stories, because of Clojure. The overhead of having to learn Clojure is minimal. Its just that it's not painless, it's uncomfortable, and you need to think harder then you're used too, that's all.
So even though both groups end up successfully using Clojure professionally to deliver on tasks and stories. The ones who just never wanted to be there in the first place don't want to keep at it, they associated stress and difficulty with it. While the ones who enjoyed learning and being challenged want to use it everywhere and can't go back to Java.
I've learned that, if you give a lot of initial support to the people who aren't open to being put out of their comfort zone, especially help them setup a friendly IDE, show them how to use the REPL first and foremost, make sure they do use it, if they don't, remind them to use it, show them again how, address why they aren't, and help guide them a little through their first month, then you can push them over to the group that loves Clojure, or at least a neutral place.
I say neutral, because generally the latter group tends to have less passion about programming in general. This is a career for them, not a hobby. They may never fall in love with any language, there's just the one they're familiar and comfortable with, and the ones they're not.
Now, some positives to adopting Clojure is that if you are in a medium to large org, the passionate, curious to learn new things, programmers on other teams will be easy target to recruit to yours.
Initially, our manager was worried it would hurt our ability to find talent, but it has actually become our biggest selling point, allowing us to attract really motivated, passionate, curious and knowledgeable devs from other teams to move to ours, because they wanted the opportunity to learn and use Clojure.
Now, I've seen people in the latter group adopt Scala or Kotlin afterwards, this is because while Europe might be too far out your comfort zone, a trip down to Disneyland or Las Vegas where things are mostly the same, and you still feel safe and at home, while only being a little out left might still be enjoyable to them.
Also, languages like Scala and Kotlin can be treated like new versions of Java to some extent. It does make some of the more annoying parts of Java easier. So they're more like having bought a brand new car in a different category, like going to an SUV after a Sedan.
My observation on that is that for a very long time they do not learn about and use the more different parts of Kotlin and Scala, but because of how linear and smooth that curve can be, eventually they end up learning. So again, the learning was made easier and safer by just being amortized over a really long period of time, 6 months to 2 years.
So I've observed that people who go through 1 month of Clojure learn as much new paradigms as 6 months of Scala and Kotlin.
Not everyone is willing to push themselves hard from the start. Those that are, will pick up Clojure in no time and love it. Those that aren't will hate it, even though they managed to use it without issue. For those, you need to offer them really good coaching and support, to ease their pain. Or, you're better off with a FP language that has a more gradual curve like Kotlin or Scala or even Java 8 FP features.
