[raju]

Let me start by saying (as someone who has written a few technical books of his own)—Congratulations!

I am sure you (assuming this is your first book) are learning that this is a labor of love, and I wish you the very best in this endeavor. You should be proud!

I was exposed to "data oriented programming" thanks to Clojure—wherein maps/sets are the constructs used to pass data (as plain data) around, with simple functions that work with the data, as opposed to the traditional OO (hello ORM) that mangles data to fit some weird hierarchy.

Java's recent innovations certainly make this a lot easier, and I am glad someone is looking at propagating a much needed message.

I will take a look at the book, but I wish you the very best.

[mrbonner]

I am also very interested in how this work in practice. With OOP at least you know the shape of your data structure as opposed to the hash map as a mere container type.

[geophile]

I am an OOP programmer going back to the late 80s (including the cfront days of C++), and a serious user of Python since 2007.

In Python, I sometimes try data-oriented programming, using lists and dicts to structure data. And I find that it does not work well. Once I get two or more levels of nesting, I find it far too easy to get confused about which level I'm on, which is not helped by Python's lack of strong typing. In these situations, I often introduce objects that wrap the map or dict, and have methods that make sense for that level. In other words, the objects can be viewed as providing clear documentation for the whole nested structure, and how it can be navigated.

[goostavos]

>Once I get two or more levels of nesting, I find it far too easy to get confused about which level I'm on

Author here, I agree with you. I have the working memory of a small pigeon.

The flavor of data orientation we cover in the book leverages strongly typed representations of data (as opposed to using hash maps everywhere). So you'll always know what's shape it's in (and the compiler enforces it!). We spend a lot of time exploring the role that the type system can play in our programming and how we represent data.

[joshlemer]

Given the strongly typed flavour of data oriented programming, I wonder if you have any thoughts on the "proliferation of types" problem. How to avoid, especially in a nominally typed language like Java, an explosion of aggregate types for every context where there may be a slight change in what fields are present, what their types are, and which ones are optional. Basically, Rich Hickey's Maybe Not talk.

    record Make(makeId, name)
    record Model(modelId, name)
    
    record Car(make, model, year)
    record Car(makeId, modelId, year)
    record Car(make, model)
    record Car(makeId, modelId)
    record Car(make, year)
    record Car(makeId, year)
    record Car(make, model, year, colour)
    record Car(makeId, modelId, year, colour)
    record Car(year, colour)
    
    ....

[PaulHoule]

Hickey is great at trash-talking other languages. In the case of Car you might build a set of builders where you write

   Car.builder().make(“Buick”).model(“LeSabre”).build()

Or in a sane world code generate a bunch of constructors.

In the field of ontology (say OWL and RDF) there is a very different viewpoint about ‘Classes’ in the objects gain classes as they gain attributes. :Taylor_Swift is a :Person because she has a :birthDate, :birthPlace and such but was not initially a :Musician until she :playsInstrument, :recordedTrack, :performedConcert and such. Most languages have object systems like Java or C++ where a Person can’t start out as not a Musician but become one later like the way they can in real life.

Notably in a system like the the terrible asymmetry of where does an attribute really belong is resolved, as in real life you don’t have to say it is primary that Taylor Swift recorded the Album Fearless or that Fearless was recorded by Taylor Swift.

It’s a really fascinating question in my mind how you create a ‘meta object facility’ that puts a more powerful object system on your fingers in a language like Java or Python, for instance you can have something like

   taylorSwift.as(Musician.class)

which returns something that implements the Musician.class interface if

   taylorSwift.isA(Musician.class)

where

   TaylorSwift instanceof MetaObject.class

[romwell]

Well, that's what C++ templates were made for.

White your code to work on Musicians, pass Taylor Swift in.

If she's not a musician, your code won't compile.

[cutler]

builder() .... build() Rich Hickey got something right. This is about as far from the idea behind DOP as it gets.

[kentosi]

I haven't yet had the luxury to experiment with the latest version of Java, but this is one of the reasons why I wish Java introduced named parameters the say way kotlin and scala do.

Eg:

  data class Make(makeId: String, name: String)
  data class Model(modelId: String, name: String)

  data class Car(make: Make, model: Model, year: String, ...)

Now you can go ahead and order the params whichever way you wish so long as you're explicitly naming them:

  val v1 = Car(make = myMake1, model = myModel1, year = "2023", ...)
  val v1 = Car(model = myModel1, make = myMake1, year = "2023", ...)

[Ironlink]

Once withers land, I think you could approximate this by letting your record class have a zero argument constructor which sets every field to some blank value, and then fill the fields using `with`.

  var x = new Car() with { make = "Volvo"; year = "2023"; };

If you want the Car constructor to enforce constraints, you could use this pattern in a separate Builder record:

  record Car(String make, String year) {
    Car {
      Objects.requireNonNull(make);
      Objects.requireNonNull(year);
    }

    record Builder(String make, String year) {
      Builder() {
        this(null, null);
      }
      Car build() {
        return new Car(make, year);
      }
    }
  }

  var x = new Car.Builder() with { make = "Volvo"; year = "2023"; }.build();

Obviously syntax TBD.

[javanonymous]

I think named parameters would be a great addition

For now, I use Lombok's @Builder annotation. It makes it much easier to create and copy a record, where non-assigned attributes are set to default.

Example:

   var bmw = Car.builder().make("BMW").build()

It also has a practical toBuilder() syntax that creates a copy of the original record, with some attributes changed

   var other = bmw.toBuilder().year(2024).build()

[goostavos]

I have a long convoluted answer to this.

I love that talk (and most of Rich's stuff). I consider myself a Clojure fanboy that got converted to the dark side of strong static typing.

I think, to some degree, he actually answers that question as part of his talk (in between beating up nominal types). Optionality often pops up in place of understanding (or representing) that data has a context. If you model your program so that it has "15 maybe sheep," then... you'll have 15 "maybe sheep" you've got to deal with.

The possible combinations of all data types that could be made is very different from the subset that actually express themselves in our programs. Meaning, the actual "explosion" is fairly constrained in practice because (most) businesses can't function under combinatorial pressures. There's some stuff that matters, and some stuff that doesn't. We only have to apply typing rigor to the stuff that matters.

Where I do find type explosions tedious and annoying is not in expressing every possible combination, but in trying to express the slow accretion of information. (I think he talks about this in one of his talks, too). Invoice, then InvoiceWithCustomer, then InvoiceWithCustomerAndId, etc... the world that microservices have doomed us to representing.

I don't know a good way to model that without intersection types or something like Rows in purescript. In Java, it's a pain point for sure.

[1propionyl]

My sense is that what's needed is a generalization of the kinds of features offered by TypeScript for mapping types to new types (e.g. Partial<T>) "arithmetically".

For example I often really directly want to express is "T but minus/plus this field" with the transformations that attach or detach fields automated.

In an ideal world I would like to define what a "base" domain object is shaped like, and then express the differences from it I care about (optionalizing, adding, removing, etc).

For example, I might have a Widget that must always have an ID but when I am creating a new Widget I could just write "Widget - {.id}" rather than have to define an entire WidgetCreateDTO or some such.

[jakjak123]

Hopefully your domain is sane enough that you can read nearly all the data you are going to use up front, then pass it on to your pure functions. Speaking from a Java perspective.

[chipdart]

> Given the strongly typed flavour of data oriented programming, I wonder if you have any thoughts on the "proliferation of types" problem.

Not a problem.

You're just making your life needlessly hard and blaming Java for the problems you're creating for yourself.

This represents, coincidentally, the bulk of the problems pinned on Java.

Everywhere else the problem you described is a variant of an anti-pattern and code smell widely known as telescoping constructor pattern.

The problems caused by telescoping constructors have a bunch of known cures:

- builder pattern (Lombok supports this, by the way),

- the parameter object pattern (builder pattern's poor cousin)

- semantically-appropriate factory methods.

The whole reason behind domain models taking the center stage when developing a software project is that you build your whole project around a small set of types with the necessary and sufficient expressiveness to represent your problem domain.

Also, "explosion of aggregate types" can only become a problem if for some reason you don't introduce support for type conversion when introducing specialized types.

[kaba0]

I have thoroughly enjoyed that Hickey talk, but I think he has a very system-oriented view/take - which is very important and shows his experience - but it is also common to have control over the whole universe for our program.

In the interconnected system view, data schemas can change without notice, and the program should be backwards and forwards compatible to a reasonable degree to avoid being brittle.

This is not a problem when we control the whole universe.

I find that Haskell-esque type systems (strongly typed with frequent use of algebraic data types to represent every possible state in _that_ universe) work better for the latter, but are not the best fit for the former, and they often have to add some escape hatches at the boundaries.

Java itself is in a weird cross of this two - it has a reasonably strong type system nowadays, but it’s also a very dynamic runtime where one can easily create their own class at runtime and load it, reflect on it, etc.

So all in all — are you making that Car as part of your universe where you control everything, and it won’t change in unexpected ways? Make a record, potentially with nullable/Optional/Maybe types for the fields, if that makes sense.

If it represents some outside data that you don’t control, then you might only care about a subset of the fields: create a record type for that subset and use a converter from e.g. json to that record type, and the converter will save you from new fields. If everything is important then your best bet is basically what Clojure/JSONObject/etc do, just have a String-keyed map.

(Note: structural types can help here, and I believe OCaml has row polymorphism?)

[cutler]

There's always clojure.spec.

[geophile]

This discussion sounds like there is confusion about the Car abstraction.

Make and model vs. makeId and modelId: Pick one. Are Make and Model referenced by Cars or not? There seems a slight risk of the Banana/Monkey/Jungle problem here, so maybe stick with ids, and then rely on functions that lookup makes and models given ids. I think it's workable either way.

As for all the optional stuff (color, year, ...): What exactly is the problem? If Cars don't always have all of these properties then it would be foolish of Car users to just do myCar.colour, for example. Test for presence of an optional property, or use something like Optional<T> (which amounts to a language supported testing for presence). Doesn't any solution work out pretty much the same? When I have had this problem, I have not done a proliferation of types (even in an inheritance hierarchy) -- that seems overly complicated and brittle.

[nicoty]

I'm not familiar with Java. Does it have no notion of structural types at all? If it does, maybe you could wrap those fields in `Car` with `Maybe`/`Option` (I’m not sure what the equivalent is in Java) so you get something like `Car(Maybe Make, Maybe Model, Maybe Year, Maybe Colour)`?

[vips7L]

Records are structural types. Null restricted types are in draft: https://openjdk.org/jeps/8303099

[spullara]

yes and it is called Optional (rather than Maybe)

[cbsmith]

That one is pretty simple. You have a Car object with four fields. The types of the fields are, respectively Optional<Make>, Optional<Model>, Optional<Year>, and Optional<Colour>.

Hickey makes it sound worse than it is.

[chii]

so now when you have a function that takes in a Car object, you have no idea what fields those objects might have, because it's all optional! Which means the checks for the validity of each field end up spreading out to every function.

[js2]

> Python's lack of strong typing

I see people conflate strong/weak and static/dynamic quite often. Python is strong[1]/dynamic, with optional static typing through annotations and a type checker (mypy, pyright, etc).

Perhaps the easiest way to add static types to data is with pydantic. Here's an example of using pydantic to type-check data provided via an external yaml configuration file:

https://news.ycombinator.com/item?id=41508243

[1] strong/weak are not strictly defined, as compared to dynamic/static, but Python is absolutely on the strong end of the scale. You'll get a runtime TypeError if you try to add a number to a string, for example, compared to say JavaScript which will happily provide a typically meaningless "wat?"-style result.

[jstimpfle]

In some significant ways, it's not strong at all. It's stronger than Javascript but it's difficult not to be. Python is a duck typing language for the most part.

[js2]

Duck typing is an aspect of it being dynamically typed, not whether it is strong/weak. But strong/weak is not formally defined, so if duck typing disqualifies it for you, so be it.

https://langdev.stackexchange.com/questions/3741/how-are-str...

[cbsmith]

I always think of Python as having "fairly strong" typing, because you can override the type of objects by just assigning to __class__.

[htuyfjsdkfw]

Duck typing doesn't exist. What you refer to as duck typing is the inherit nature of dynamic typing.

[fire_lake]

The Python ecosystem is not built around types.

For example, you will find functions where the runtime value of parameters will change the return type (e.g. you get a list of things instead of one thing). So unless we want to throw out huge amounts of Python libraries (and the libraries are absolutely the best thing Python has going for it) then we have to accept that it’s not a very good statically type language experience.

The JS community on the other hand had adopted TypeScript very widely. JS libraries are often designed with typing in mind, so despite being weakly typed, the static type experience is actually very good.

[senand]

I don't disagree. However, often, when I use a library, I use it within a small function that I control, which I can then type again. Of course, if libraries change e.g. the type they return over time (which they shouldn't also according to Rich), you often only notice if you have a test (which you should have anyway).

Moreover, for many libraries there are types- libraries that add types to their interface, and more and more libraries have types to begin with.

Anyway just wanted to share that for me at least it's in practice not so bad as you make it sound if you follow some good processes.

[js2]

YMMV. I have over two decades of experience with Python and about a decade with JS though it's all backend work. I use both in my day job, but write in Python more frequently. I've found the transition to Python static typing much more seamless and easier to adopt than TS.

Amusingly, I can't call any time where I'd had to deal with differently typed return values in Python, but just recently had to fix some legacy JS code that was doing that (a function that was returning null, scalar, or array depending upon how many values it got in response to a SQL query).

$0.02.

[zahlman]

>For example, you will find functions where the runtime value of parameters will change the return type (e.g. you get a list of things instead of one thing).

I have long argued that such interfaces are doing it wrong. That's what "Special cases aren't special enough to break the rules." in the Zen is supposed to warn about, to my understanding.

[thelastparadise]

You're being pydantic =)

[lelanthran]

> You'll get a runtime TypeError if you try to add a number to a string, for example,

A popular but nonsensical myth:

    $ cat t.py
    def foo(myval):
       return myval * 12
    
    print (foo("a"))
    $ python3 t.py
    aaaaaaaaaaaa

In real-world usage, Python's "typing" is about as helpful as Javascript's "typing". Plain old C has stronger typing guarantees than Python/PHP/etc.

[zahlman]

Defining an operation between two different types is not at all the same thing as enabling implicit conversions. Notice for example that "1" * 2 gives "11", and not "2" nor 2. Interpreting multiplication of a string by an integer as "repeat the string that many times" doesn't require any kind of conversion (the integer is simply a counter for a repeated concatenation process). Interpreting addition as "append the base-10 representation of the integer" certainly does. (Consider: why base 10?)

You have a point that strong vs weak typing is not a binary and that different languages can enable a varying amount of implicit conversions in whatever context (not to mention reinterpretation of the underlying memory). But from ~20 years of experience, Python's type system is nothing like JavaScript's - and it's definitely helpful to those who understand it and don't fight against it.

In my experience it's typically people from languages like Haskell that can't see the difference.

[Tainnor]

that's just operator overloading and it exists in many statically typed languages too

[lelanthran]

> that's just operator overloading and it exists in many statically typed languages too

My point is that Python's "typing" guarantees allow a caller to call a function with the wrong type, and get back a wrong answer and/or silently lose data.

Strong typing is pointless if the language is unable to actually prevent common footguns, like passing in the incorrect type.

I'm moving more and more to the opinion that arguing about the spectrum of strong <-> weak typing is stupid, because type utility is on the spectrum of static <-> dynamic, with dynamic being full of footguns.

[cbsmith]

Living this dream in Python right now (inherited a code base that used nasty nesting of lists & dicts). You don't strictly need to do OOP to solve the problem, but it really does help to have a data model. Using dataclasses to map out the data structures makes the code so much more readible, and the support for type hints in Python is good enough that you can even debug problems with the type system.

[clepto]

I see a lot of people mentioning Pydantic here, but you should take a look into TypedDict. It provides a type structure ontop of a plain dictionary, and sounds like exactly what you’d want, and is a built-in that you don’t need a dependency for.

Mypy for example can also see the types of the dictionary are supposed to be when you use it just like a normal dictionary.

[mejutoco]

I recommend you use pydantic for type annotations. Alternatively, dataclasses. Then you pair it with typeguards @typechecked annotation and the types will be checked at runtime for each method/function. You can use mypy to check it at "compile time".

Having clear data types without oop is possible, even in python.

[sodapopcan]

Python's not really built for that AFAIK, though. In languages built for it, you can type your dicts/hashes/maps/whatever and its easier to see what they are/know where the functions that operate on them live. I'm most familiar with Elixir which has structs which are simply specialized map (analogous to dict in Python) where their "type" is the name of the module they belong to. There can only be one struct per module, In this sense is easy to know exactly where its functions live and is almost like a class with the very key difference that modules are not stateful.

[cbsmith]

> In languages built for it, you can type your dicts/hashes/maps/whatever and its easier to see what they are/know where the functions that operate on them live.

I think I must be misunderstanding what you mean by that, because I can very much do that in Python.

[sodapopcan]

I think I misunderstand OP's problem then.

[cbsmith]

Their problem stems from the scenario where you don't type them. You just leave them as generic lists & dicts.

[chipdart]

> (...) by Python's lack of strong typing. In these situations, I often (...)

Python does support strong typing, albeit optional, with type annotations and tools like mypy.

If a problem is caused by the lack of strong typing, why not use strong typing?

[baq]

static.

python was always strongly typed: you could not do 2 + '3', ever. nowadays mypy/pyright will tell you that before runtime, hence static.

[chipdart]

I stand corrected.

[FpUser]

>"In these situations, I often introduce objects that wrap the map or dict, and have methods that make sense for that level."

I've been doing the same thing since the end of the 80s as well starting with Turbo/Borland Pascal, C++, and later any other language that supports OOP.

[ninetyninenine]

Python now has type hints which can be used with an external type checker in the IDE. You'd probably type it with a class.

[ederamen]

Use Data Classes

[kccqzy]

Clojure has spec. That allows you to know a specification of what the data structure contains.

[akavi]

You can get strongly typed "shaped" data without objects[0], even in Java: Records[1].

~Unfortunately, I believe they're mutable (and cannot be made immutable).~ Edit: I was wrong, they're immutable.

[0]: I'm using "object" to mean "data bound to methods", since the concept of aggregate data in general long pre-date OOP (eg, C's structs)

[1]: https://docs.oracle.com/en/java/javase/17/language/records.h...

[taftster]

Java Records are immutable (by the most common definition). They don't have any means to update the record (via setters, etc.) after construction. That doesn't mean, for example, you can't store a reference to a mutable type (for example, a List or Map) in your record.

The frustration I have with Records is there is no good way to prevent direct construction of them. That is, the constructor is public, which prevents an easy way of enforcing an invariant during construction.

For example, let's say that you have a record with a Date type. There's no good way to prevent a user from creating the record with an invalid date, one that is out of a needed date range. Or maybe enforcing a field cannot be null or some combination of fields must meet requirements as a group.

The benefit I get from the classic Builder pattern is defeated with Records. I can't enforce checking of my fields before the construction of the record object itself. Presumably I would need to verify the object after construction, which is unfortunate.

[vips7L]

You can enforce some invariants during construction:

    record Point(int x, int y) {
        Point {
            if (x < 0) throw new IllegalArgumentException()
        }
    }

or if you want to assert something is not null:

    record Person(String name) {
        Person {
            requireNonNull(name);
        }
    }

[nogridbag]

I think records will be much more useful if https://openjdk.org/jeps/468 gets out of preview.

[elric]

> There's no good way to prevent a user from creating the record with an invalid date

That is factually incorrect.

You can do all of that validation in a record constructor, much like in a normal Java class constructor. There's a difference in syntax: you don't need to repeat the constructor arguments in parantheses, and don't have to perform the assignments yourself. These are tailored specifically for easy validation.

[tpmoney]

As mentioned by the other commenters, you should be able to run any validations or transformations on the data that you want in the canonical constructor, including re-assigning values (for example we've done defaults with `foo != null ? foo : new DefaultFoo()`). The only thing I think you can't do with a record is make the canonical constructor private and force users of your type to call factory methods that can return null instead of throwing an exception. You can provide those factory methods, but anyone can still call the constructor, so you have to do your hard checks in the constructor. On the other hand, no matter how many alternate constructors or factory methods you make, you're also guaranteed that every one of them eventually has to call the canonical constructor, so you don't need to spread your validation around either.

[akavi]

Can you make the Record class private to a module, and only export a static function that constructs them?

(I know very little about Java)

[kaba0]

To a degree, yes, that’s possible. But leaking a private type over module boundaries is bad form, so a better (though possibly over engineered solution) would be to have a separate public interface, implemented by the private record type, and the static function would have that interface as return type.

[enugu]

Why is it bad form to expose a record type only via custom functions and not its field accessors? Isn't this just like exposing a more usual object with its public functions and private functions remain inaccessible?

[snmx999]

You can create dedicated, already verified objects to pass on to your record. E.g. AllowedDate (extends Date).

[bedatadriven]

A record's fields are final, so records are immutable (though they can include immutable pointers to mutable objects)

[davedx]

With TypeScript you have types to tell you the shape of your data.

[goostavos]

Thanks for the kind words :)

>learning that this is a labor of love

I underestimated both the amount of labor and the amount of love that would be involved. There were more than a few "throw everything out and start over" events along the way to this milestone.

Clojure definitely had a huge impact on how I think about software. Similarly, Haskell and Idris have rearranged my brain. However, I still let Java be Java. The humble object is really tough to beat for managing many kinds of runtime concerns. The book advocates for strongly typed data and leveraging the type system as a tool for thinking.

>Java's recent innovations certainly make this a lot easier

Yeah, it's an exciting time! Java has evolved so much. Algebraic types, pattern matching, `with` expressions -- all kinds of goodies for dealing with data.

[jwr]

> Clojure definitely had a huge impact on how I think about software

I could be called a "Clojure programmer", because I make a living from an app written Clojure and ClojureScript. While I always appreciated the incredible JVM, I always looked at Java the language with disgust and contempt, interfacing with it only as was necessary, but recent work on Java makes it much more attractive. I was impressed by the functional interfaces, modern design with mostly static methods, JSR-310 (date and time) is absolutely great — overall, Java has improved a lot over the years.

It has come to the point where I gasp might consider writing some Java code :-)