It would be nice if the language required such functions, and the modules in which they reside, to be flagged. (I don't know if Haskell does something like this with mutation, or not)

It also seems that an "actor model" would be a good way to encapsulate the updates in an otherwise FP program by having a loop/reduce/fold wrapped around the mutable data responding to request-events and generating responses. This allows the other pure/immutable/idempotent type of code to remain isolated from it.

Also, I've never needed an unsorted dictionary, and parallelism is actually great in Haskell. http://chimera.labs.oreilly.com/books/1230000000929/index.ht...

"Why is Haskell used so little in the industry?" - http://flyingfrogblog.blogspot.de/2010/05/why-is-haskell-use...

Go and grab some popcorn before you move on to the comment section...

It's from 2010, I'd be interested in an update, just out of mild curiosity.

There's plenty of words and claims are made but benchmarks or code are nowhere to be seen. Why should anyone take these claims at face value?

> Furthermore, most functional programming languages (OCaml, Haskell, Scala) are incapable of expressing a fast generic mutable hash table because they lack the killer combo of: reified generics, value types and a fast GC write barrier.

Sounds plausible? Maybe. Incapable is a strong word. I'm not an expert on mutable hash-tables so I don't know for sure. If I was making a claim that you can't implement mutable hash table without 2 square feet of badger fur and a pound of whale fat would you believe me? I would like to see a citation.

I have written a lot of Haskell and there was never a situation when I said to myself "if only Data.HashMap (unordered-containers) was faster". Just make sure you're using the right tool for the job (which might not be Haskell).

The author doesn't seem to write any code himself but instead links to code that other people have written and then makes claims about F# being better (I can't find the F# solution to parallel quicksort from one of the linked posts). I see very little value in that.

So the first 7 points are true because pure functional programming, by definition does not support mutable data structures very well. It is what it says on the tin.

If you go on stack overflow you'll find no shortage of ill-informed, unhelpful input about imperative programming languages. I think it's very strange to call a social phenomenon that is universal in software that affects every language community is a disadvantage of something as broad as "purely functional programming". If you want good discussion, go on the mailing lists or the IRC channels. #haskell is a very good channel with a lot of active professionals who love answering questions.

I am a professional Haskell programmer, AMA.

I have settled on a style of doing "functional-like" programming in Python and C++. It's more about the high level architecture than low level coding details.

It means being very paranoid and rigorous about state -- but still having great tools to express it! For example: Using ZERO mutable globals, and passing state in as a parameter to functions. Using immutable/persistent data structures. These techniques are done very naturally and effectively in Python and C++. You just have to be disciplined.

To me there's no real advantage to expressing something like "split a string by a delimiter" in a purely functional style. Either way, you have a trivial referentially transparent function you can reuse without causing complexity in your program. You might as well do the obvious imperative thing.

However there IS a benefit to threading state explicitly throughout the application, and encapsulating it in OBJECTS (yes objects).

For me, the thing that sealed the deal against functional languages for "real work" was trying to write a production quality sh parser.

I went down a long path of trying to do this first in OCaml and then in Lisp. The VERY first thing that hits you over the head -- lexing -- is heavily and inherently stateful. ocamllex and ocamlyacc to me are evidence of this paucity and poverty of purely functional solutions. They're just transliterating solutions from C. Well I might as well use C then.

Actually, I decided to use C++, which was like my 5th choice as language. Aside from headers and compile times, it's a good choice. I use "functions and data" (a la Rich Hickey).

Except my functions and data are both CLASSES. Data objects are basically structs, except they can do things like print themselves and answer simple queries based on their values, which helps readability (e.g. 1 liners, like word.AsFuncName() ).

Function objects are simply classes with configuration passed to constructors. That usually have a single method, but multiple methods are also often useful. Calling this method is basically equivalent to calling a curried function. But this is supremely useful for both compilers and servers, because often you have config/params that is constant once you reach main(), and then you have params that vary per request or per file processed. Many functions depend on both, and it's cleaner to separate the two kinds of params.

So both "functions and data" are effectively and usefully implemented as classes. The key is to make some classes like functions, and some classes like data. And have more of a bipartite dependency graph, where functions depend on data, and data depends on functions.

When all your classes are an equal mix of data and behavior, that's when they start getting "hard-coded" weird-shaped dependencies, and your program turns into fragile spaghetti. Functions and data are a useful architectural technique, and to me it doesn't have that much to do with Clojure or Lisp, although Hickey is certainly a great advocate.

However I came to the conclusion that the latest improvemetns in C#, Java and C++ are already quite good for doing FP style programming.

Yes, I feel a bit dirty not being able to use more pure FP languages, but I have to work within the context that customers allow us to do.

Although it feels like blasphemy, C++14/7 can be seen almost as an impure Haskell with curly brackets.

As a matter of style, I still prefer using currying over "one trick pony" (single method/function) classes, though. Sometimes this means I have several "layers" of references to the original function, with varying numbers of parameters applied as each layer further specializes (adds configuration). I'm doing most of this type of work in Javascript, though, so YMMV.

(PS - also did some work with Lisp in Uni back in the 80s, and sundry imperative/oop/func ; static/dynamic things since)

I went down an almost identical path as you many years ago but found that after a certain point, the maintenance and just being away from the codebase for a couple of months made the time needed to make coherent mods that fit the design quite problematic.

Functional style is simply better and is applicable in many languages. Modern C++ is a great example. Throw away much of OOP clunkiness, use simple functions, win! (OOP has it's place but in small quantities and with moderation).

With respect to purity I'm on the sidelines. I think what's far more important is algebraic data-types, pattern matching and value-returning conditionals (values, not statements!). Rust has them and it is great and not pure.

For various reasons, I chose to use OCaml, and from my experience, the difficulty part is only true insofar as the learning curve goes. Once past that, FP-style problem-solving is spectacular in that the vast majority of time you spend goes not into the coding, but rather (a) thinking about what exactly your problem is, and (b) how to decompose and solve it. I can't tell you how many times in Matlab, people start writing code before they've even fleshed out (a) and (b), simply because there's so much drivel and overhead that one naturally tends to feel they're "getting things done" by jumping in and writing this code.

Once I had a sufficient grasp of OCaml concepts and syntax, I would find myself getting frustrated by how little code I might get done in a day. Upon stepping back and reflecting, that was because I had not worked my way through (a) and (b), and that is where the human thinking is really required. I have since been delighted at the utility of each line of code I write and am similarly annoyed by the low signal-to-noise of something like Matlab.

With respect to slower, I think my points above address "slowness" in terms of development time. In terms of runtime, it's very much a function of your choice of language/ecosystem and paradigms. If GC and typical functional overhead are a constraint in your problem domain, one can always bring over a functional style into modern languages like Rust, where performance and lack of GC are first-order design goals. But you can't tar all of FP with the label of "slow" any more than anyone can make a blanket statement that "solving matrix problems is slow". It just doesn't make sense.

Lastly, as others have pointed out, Jon Harrop's points refer to purely functional styles and languages. In the end, striving for functional purity may not make sense. To illustrate, one can easily drop into an imperative style where needed in OCaml, for example when doing I/O. I use this quite a bit. But make no mistake that adopting a functional approach where it makes sense can pay dividends.

I think the concept of immutability confuses people bit. It really clicked for me when I stopped thinking of it in terms of things not being able to change and instead in terms of every version of things having different names.

Functional programming from that perspective is like version control for your variables. It makes explicit, not only which variable you are accessing but which version of it.

The reason it seems like you need to copy a variable each time you modify is just to give each particular mutation a different name. Note that the compiler can optimize things. It doesn't need to keep every version in memory. If it sees that you are not going to reference a particular mutation, it might just physically overwrite it with the next mutation so that in the background "var a=i, var b=a+j", might run as something like "var b = i; b+=j";

It's clear that some other aspects of FP, such as first-order functions, closures, etc are very useful in practice.

Immutability does have its advantages in that it supplies strong guarantees about what your code does. It avoids spaghetti code where everything can and does mutate everything else.

Myself, I tend to use whenever its advantages outweigh its inconveniences. Immutability is especially handy at interface boundaries.

Art - as something subjective - also means opinions differ on difficulty and slowness. That's perhaps why downvotes - the statement like this presented as an objective fact draws enough scepticism from those who have different experience in the subject.

Slower - Optimize for developer time. Same reason most people write their backends in Python/Ruby instead of C++.

Unfortunately, it does have the challenges listed in the OP, so it seems to make most sense where either (a) it is ok to leave performance on the table in the interest of clarity or (b) you create a little sandbox in which you use functional programming for clarity, and something outside your sandbox compensates for the performance cost.

React and Om are examples of (b). They let you write pure functions that return the entire DOM. Then, outside that sandbox, a DOM differ makes it so you don't force the browser to re-render the entire DOM every time.

Also, there is a lot of cases where you need a total language with all its guarantees.