I totally disagree. Functional programming teaches practices that are helpful regardless of what paradigm you use. Pure functions by definition adhere to dependency injection and single responsibility principle. While most programming languages don't enforce immutability, being aware of mutation is a generally helpful skill. There's a reason that lambda have become table stakes for new programming languages, and that's because composing functionality is a generally useful feature. I have never been paid to write in a functional language, but learning from functional languages has always improved my general programming skills.
Doing FP makes you a better programmer because you're writing programs. I'm taking the null hypothesis that after a year of writing functional code you would have improved just as much as you would have writing non-functional code. I have literally never seen anything to demonstrate otherwise other than hand waving and personal anecdotes.
> Pure functions by definition adhere to dependency injection and single responsibility principle.
Dependency injection and SRP are other man-made constructs with dubious utility in the same vein as functional programming.
> There's a reason that lambda have become table stakes for new programming languages, and that's because composing functionality is a generally useful feature
Lambda in functional programming is supposed to be a primitive you use to do everything ahem y combinators ahem. In Java 8/C++11/Swift, the lambdas you speak of are used only as embedded subroutines.
Functional programming selects for better than average programmers to begin with--the "programmer's programmer" that writes code for fun and probably visits this site. You're unlikely to convince to learn Haskell the person who writes enterprise .NET, never used anything but Windows, and who never opened a text editor after work. The functional programmers were already good before they became functional programmers. Then you get the cargo cult type of intrigue. "X writes really good code. X also uses $FP_LANG! Be more like X!"
