It is easier to reason about functional programs because they maintain certain properties. These could be types, an absence of side effect, etc. This is the point of the entire article.
You get none of those benefits if your "multi paradigm" language allows you to break those guarantees.
For a multi paradigm language to derive benefits from functional programming, it should allow developers to explicitly write guarantees that can be enforced at compile time. This is a promising direction but, to my knowledge, it's not available in any of the languages you mentioned.
If you had a multi-paradigm language, you'd need not only just some way to break the rules easily and have that bubble up, you'd also want this rule-breaking behavior to be exposed up the chain from any libraries. You'd also have to enforce the benefits of immutability and such through social pressure in order for users to see large parts of the benefits of side-effect isolation.
A lot of the choices FP languages make that are less about side-effects (like strict typing systems) are also often an all-or-none proposition. Optional typing systems have very limited benefits if libraries aren't well-typed. The static analyzers can only do so much.
That's what I used to think, and in fact I still think like that somewhat, but do note it's explicitly mentioned as a red herring in John Hughes' well-known and often quoted paper "Why Functional Programming Matters".
From the paper:
> Such a catalogue of “advantages” [things that FP lacks or constraints] is all very well, but one must not be surprised if outsiders don’t take it too seriously. It says a lot about what functional programming isn’t (it has no assignment, no side effects, no flow of control) but not much about what it is. The functional programmer sounds rather like a medieval monk, denying himself the pleasures of life in the hope that it will make him virtuous. To those more interested in material benefits, these “advantages” are totally unconvincing.
> [...]
> Functional programmers argue that there are great material benefits — that a functional programmer is an order of magnitude more productive than his or her conventional counterpart, because functional programs are an order of magnitude shorter. Yet why should this be? The only faintly plausible reason one can suggest on the basis of these “advantages” is that conventional programs consist of 90% assignment statements, and in functional programs these can be omitted! This is plainly ridiculous. If omitting assignment statements brought such enormous benefits then Fortran programmers would have been doing it for twenty years. It is a logical impossibility to make a language more powerful by omitting features, no matter how bad they may be.
He then goes on to argue that the real benefit of FP languages is that they excel at modularity, a widely agreed upon trait of good software, and that they have an excellent mechanism for "gluing" stuff together, i.e. lazy evaluation.
(I've since read that some FP devs disagree about the importance Hughes placed on laziness).
Of course it's about modularity, but how do you get modularity? How do you safely abstract the internals of libraries? Types and lack of side effects!
It's also about correctness. Most of the time spent writing any complex bit of software is generally spent finding out what's wrong with it.
Even philosophically this position is weak. It's self-evident to me you can make something more powerful/better by removing things. You can make something better by removing risks. You can make an UI saner by removing the possibility of invalid interactions. You can make art better by removing superfluous details.
I still think that types and control of side effects is essential to (good) FP.
- You can make limbs more useful by removing degrees of freedom, simplifying the control task.[2]
Static languages such as Java and C# are not OK though. For one they lack features that are essential for statically typed languages. For example higher kinded types is an essential feature for FP. Without that you can't build libraries of reusable FP stuff. Comparing it with Scala for example you can't build Scalaz or the newer Cats or Shapeless [1].
The other problem is that FP programming languages come with FP libraries. You can't underestimate the importance of having good implementations for persistent collections be the default.
On having "no side effects", that's not what FP is about. After all, without side-effects you couldn't get any output out of a program. FP is about encoding those side-effects in types and abstractions you can reason about, or in other words it's about controlling side-effects. Haskell developers are fond of their IO monads. FRP is another way of doing that, being particularly good at dealing with user interfaces. And for example FRP-related libraries that were born by playing to the strengths of the mainstream languages we are dealing with are Facebook's React and Microsoft's Rx.
BTW, this is one of the best books on functional programming available today: http://www.manning.com/bjarnason/
[1] https://github.com/scalaz/scalaz, https://github.com/non/cats, https://github.com/milessabin/shapeless
My personal definition of 'functional' is based on this lack of side-effects (or, more abstractly, on modelling computation as a declaration of relations between sets, which is what mathematical functions are), but it gets fuzzy once you leave the realm of purely functional languages.
If you prefer another definition for pragmatic reasons, I'm fine with it, it's just not my choice.
And languages I listed (JavaScript, C#, Swift and Java) have all stolen large chunks of ideas pioneered by functional programming and put it into their latest release.
Procedures can be modeled as functions, state can be passed around, user interface hotness currently is also trending to functional / immutable (Elm, React).
> bit like doing object-orientated programming in C with macros and a lot of conventions.
If you are forced to use C and macros yeah it is no fun.
But if you have tail call optimizations, a good type system (even optional one like Erlang does), immutable data structure support, monitoring and tools (Erlang VM), you can build large systems that handle lots of mutable state in a sane way.
Doesn't matter what language or OO heavy framework (worked on Python, C++, C, C#, Java over the years) if there is lots of mutable state stored in class instances, it makes it very hard to jump in and understand a new large code base. While say something written in Erlang the state is explicitly passed around, it is minimized, and looking at a piece of code it easy to understand what happens vs say looking at somethingSomethingManagerInstance with 100 methods and 100 private data attributes, and understand when something breaks how it go to that broken state.
That's just my observation based on systems I worked over the years.
The author seems to believe that no side effects is the key property of a functional language. But for me most problems I deal with (user interface, various business domains, integration with other systems over network etc.) are filled with state and procedures. Sure you can deal with that in a purely functional way but to me it is a bit like doing object-orientated programming in C with macros and a lot of conventions. Why not simply use a programming language with suitable primitives? In particular now that multiparadigm languages are the most widely used in our industry.