Edit: I kid, but most of the languages I just mentioned have very fast native compilers, easy ways of invoking low-level interop, functional-style libraries if you want them, and (except for Python) C-like syntax making it easy to cut-and-paste.
If we limit the discussion to ML languages, they are generally very nice e.g. for datastructure transforms (including interpreters and compilers). Static typing with type inference makes implementations short, and, if written in good style, are correct when they compile.
There are various other reasons why one would not want to use them. For me it's platform stability and availability - while I enjoy functional languages in principle - I yet seem to choose C++ more often than not for my hobby projects.
I would say learning self-described fp-languages have made me a better programmer who values static typing more than before.
FP paradigm has indeed didactic value and the experience gained in a functional language may translate into a plus of productivity ...in other languages. When you have to get things done, the functional language's embedded arm-twisting (well, mind-twisting actually) doesn't help! That's when you'll either abandon the (idea of) "pure" functional language for a multi-paradigm language (that will allow you to keep a mainly functional style with limited deviations when the situation warrants it) or you'll keep pushing yourself and throw the blame on you for not being good enough with a language that wasn't designed from very beginning for much anything but the FP standpoint (for the sake of it).
I see FP languages being able to excel at the kind of projects with large, maintainable codebases that evolve over a decade and are shared between dozens of developers. Service oriented architectures serve the same goal -- isolating behavior and state locally, and being explicit about mutation and state changes over time.
"R, at its heart, is a functional programming (FP) language. This means that it provides many tools for the creation and manipulation of functions. In particular, R has what’s known as first class functions. You can do anything with functions that you can do with vectors: you can assign them to variables, store them in lists, pass them as arguments to other functions, create them inside functions, and even return them as the result of a function."
Weak example is Wikipedia list of Functional Programming Languages list R as one. https://en.wikipedia.org/wiki/Functional_programming#R
R describes itself as a Functional programming Language:
The language syntax has a superficial similarity with C, but the semantics are of the FPL (functional programming language) variety with stronger affinities with Lisp and APL. In particular, it allows “computing on the language”, which in turn makes it possible to write functions that take expressions as input, something that is often useful for statistical modeling and graphics. https://cran.r-project.org/doc/manuals/r-release/R-lang.html
Academic Evidence: "The Journal Trends in Functional Programming" also list R as a functional programming language.
R is an environment and functional programming language for statistical data analysis and visualization. Largely unknown to the functional programming community, it is popular and influential in many empirical sciences.
http://link.springer.com/chapter/10.1007%2F978-3-642-40447-4...
The issue might be it isn't Haskel so it is not 100% functional but there are more Functional Programming Languages then Haskel.
PS This is why so many programs in R are poorly written because people try to force OO into R when they could do things Functional. That is also why my eyes bleed when I see people looping through everything in their R scripts.
Yes, you can write R code in a functional style in places when convenient, but one floor down it's still running that loop you didn't want to see. It's a language based on mutable values and sequential execution and looping, not pure function calls and recursion. The syntax itself is very much sequential, unlike stereotypical FPLs. Even the lexical scoping design relies fundamentally on mutable environments. So yes, people do seem to enjoy proudly declaring R a FPL citing a few features, but as I said, those features make R as much a FPL as Python.
That was very condescending and I am sure don't know who Hadley Wickham is? I wouldn't call his understanding "Shallow."