[SomeCallMeTim]

The hoops you have to jump through in a pure FP approach can make the problem 100x harder, though.

Some problems are just more inherently imperative and stateful. I use FP code when it makes sense, and I use imperative code when it makes sense.

There Is No Silver Bullet.

[cassowary]

really? It's been a while since I've programmed in Haskell, but something like this is about right:

   data Data = Data { thing :: Int, that :: Boolean }}

   addToThing :: Int -> State Data Int
   addToThing n = do 
     x <- get thing
     modify (data -> data {thing: x + n})
     return x

It corresponds to this Typescript code:

   class Data {
     constructor(
        private thing: number, 
        private that: Number
     ) {}

     addToThing(n: number) {
        const old = this.thing
        this.thing = old + n
        return old
     }
   }

Except for the fact that you only need to see mutation if you want to, and you certainly can't use the mutation unless you want it.

To me it doesn't really look like jumping through hoops, but we're up front about the mutation so no-one's ever going to get a surprise.

There's no silver bullet, sure. But I don't think pure FP is really all that complex. It's mostly just different.

[SomeCallMeTim]

> But I don't think pure FP is really all that complex. It's mostly just different.

I can't help but think that people who say this, yourself included, just don't deal with the problems that I face on a regular basis.

I write a lot of games and game engine code. I might have 60 dynamic objects in the game, each of which with its own behaviors, state and physics. Object oriented code means that the functionality for each object has both inherited and specialized behavior.

And when you're working with those dynamic objects, the "information hiding" aspect of OO programming can be very useful. Some objects have a member variable; others have an accessor that pulls the value out of a child class. I use that to good effect in my current game.

Or I might be parsing a map that I need to iterate over a rectangle in the map. But sometimes it's a hex map; ever tried to iterate over a hex map? Saying map.iterateRect(iterator, x,y,w,h) or equivalent, and not having to know whether the map is rectangular or hexagonal is very nice.

Functional programming is great when you have a lot of generic functions that you can apply to lots of different kinds of data. Object oriented programming ... is good for UI and game development, and anything else where the functions you're writing are very tailored to the data, where polymorphism can map well to the problem, and where you have lots of variations of kinds of data, but fewer (or less complex, or more specialized) things to do with the data.

[pka]

I think the point still stands: FP is different.

None of the things you listed are particularly hard in an FP context. Classes/traits and parametric polymorphism will allow you to do these things just fine (Haskell code):

    class RectIterable map where
      iterateRect :: map -> (Point -> a) -> Rectangle -> [a]

    // define map data structures
    data TileMap = ...
    data HexMap  = ...

    // make them instances of the RectIterable class
    instance RectIterable TileMap where
      iterateRect = ...

    instance RectIterable HexMap where
      iterateRect = ...

> Object oriented programming ... is good for UI

I don't know, but React begs to differ. A pure function (state -> UI) is basically one of the best UI programming paradigms we've come up with so far, the other probably being FRP.

[SomeCallMeTim]

> React begs to differ.

React is good for certain classes of UI, granted. I use another FRP-style library (not React) to render UI in my current game, and it works well -- except that it doesn't mesh well with me trying to control it from an OO game, but I've created an interface that works.

But imagine creating Adobe Photoshop, Maya, or Microsoft Word entirely in React, and tell me again that FP would be the ideal choice.

For any particular dialog that pops up? Absolutely.

For the entire app? You'd end up with a "god object" that would make the app unmaintainable. OO gives you compartmentalization that you don't get with FP.

FP is great for problems up to a certain complexity. After that it falls apart. Thing is, 98% of apps don't come close to that level of complexity, so there are lots of people who never even work on an app that won't work well with FP.

In the RectIterable example you gave, I'm afraid I don't know enough Haskell to comment on it intelligently.

Is "instance" adding an interface to TileMap/HexMap so that either can be passed in as a parameter that requires a RectIterable? Can other interfaces be added that way to TileMap/HexMap?

In my editor, can I say something like myTileMap.iterateRect after doing this, or do I need to know that iterateRect exists and remember exactly what it's called?

The last point is salient: Discoverability is a huge part of the UI of a programming language, and memorizing all of the exact function names that are appropriate for a data structure is not fun.

[pka]

> For the entire app? You'd end up with a "god object" that would make the app unmaintainable. OO gives you compartmentalization that you don't get with FP.

Well, not quite; take a look at the Elm architecture [0]. It's basically what Redux gives you, but more explicit.

The idea is, that yes, while you have a god object in the top-most component, child components don't know anything about it; they operate on their piece of state completely independently. This makes UI elements perfectly composable and reusable, since in the end, a UI element is just a pure function and nothing more.

Not to mention other nice side-effects (sorry :)) of pure UI architectures, like time-travelling debugging, sane logging and so on.

> FP is great for problems up to a certain complexity. After that it falls apart.

I disagree completely. FP, and moreover pure FP, is especially well suited for complex problems. Pure functions are testable (if you haven't heard of QuickCheck / generative testing, you've missed out) and as I said above, composable.

If anything, I've never seen a big OO program made out of truly reusable components. Because classes and methods are often impure, before calling a method one must make sure that the correct environment is set up in the right way (if you've ever forgotten calling initSubsystem() before subsytemDoSomething() you know what I mean).

One can rip out a pure function from one codebase and reuse it, as-is, no modification necessary, in another one without any problems. How often does this happen in a big OO project?

> Is "instance" adding an interface to TileMap/HexMap so that either can be passed in as a parameter that requires a RectIterable? Can other interfaces be added that way to TileMap/HexMap?

Yes, and yes. In Java, when you have a class like this:

    class A implements B, C {
    ...
    }

in some library, you can't add D to the list of implemented interfaces.

In Haskell, you can:

    instance D A where
      ...

Haskell assumes an "open world" - i.e. everybody can add instances to a data type even after it has been defined already.

I can't give you a complete overview of the differences between ad-hoc and parametric polymorphism, but there are plenty of resources online if you are interested.

> In my editor, can I say something like myTileMap.iterateRect after doing this, or do I need to know that iterateRect exists and remember exactly what it's called?

I can only speak for Haskell here. It's a static language, so things like autocompletion, show type at cursor, jump to definition etc are not a problem. The tooling could be a whole lot better, no doubt, but it's not bad. The REPL helps a lot, too.

[0] https://guide.elm-lang.org/architecture/

[SomeCallMeTim]

> Haskell assumes an "open world" - i.e. everybody can add instances to a data type even after it has been defined already.

TypeScript, which is my current OO language, will use signatures to match to an interface, so it's even more open: You can create an object with the right members and pass it in to a function expecting an interface without even explicitly identifying it as "implementing" the interface.

> If anything, I've never seen a big OO program made out of truly reusable components.

Agreed. The point of using OO in a giant complex program isn't just the reuse, which you point out is exaggerated. The point of using OO in a giant complex program is the isolation you get, so that a team of 1000 people can develop the app in a reasonable way.

I think the cost of FP as the complexity of a program goes up is that the cognitive load to think about the structure of the program goes up exponentially faster than OO. Debugging is a huge issue when things get complicated, and the last I heard Haskell debugging is still in its infancy. A debugger where you just can step through the code is really, really important for some kinds of analysis.

It's all Turing Complete, so you can do anything in FP that you can do in OO. It just feels harder to decompose problems into FP units than OO units, at least at a larger architectural level. Not everything is easily isolated: Sometimes you need a control over here to poke into the state of a control over there, and you don't want that information in some "god object" at the top level; you want a direct connection and mutability.

I have heard good things about Elm, and will probably give it a try at some point. But my 30+ years of programming experience have given me a (possibly incorrect) strong intuition that FP just won't work well for some problems. Can they apply to them? Sure.

> [...] pure FP, is especially well suited for complex problems.

Which is why it's so popular for solving complex problems? Looking at a list of Haskell's uses in industry [1] and applications [2] I don't see a single example of Haskell used in what I would call a "complex problem". Lots of machine learning, parsing/compiling, and transformation tools. Some games written by amateur game developers -- getting a game working is about 10% of the problem. A very few more complex apps that I've never heard of, but nothing that hits the complexity of even a Microsoft Word or Adobe Photoshop, much less a 3d editor like Maya. There's an order of magnitude more complexity in an app like those.

Haskell is 27 years old. Over the last ten years it's been incredibly popular. Totally enough time for at least a few high profile applications to have been created. Where are they?

Maybe Haskell would be a better platform, but it's just harder for many programmers to think pure-FP. But even that's a real limitation: If you're putting more cognitive load on the developer, then you're just changing the problems, not preventing them.

[1] https://wiki.haskell.org/Haskell_in_industry

[2] https://wiki.haskell.org/Libraries_and_tools