3 ms·

> The moment you start ripping cases as distinct types out of the sum-type, you create the ability to side-step exhaustiveness and sum-types become useless in m

> The moment you start ripping cases as distinct types out of the sum-type, you create the ability to side-step exhaustiveness and sum-types become useless in making invalid program states unrepresentable. Quite the opposite, that gives me the ability to explicitly express what kinds of values I might return. With your shape example, you cannot express in the type system "this function won't return a point". But with sum type as sealed inheritance hierarchy I can. > C#/Java don't actually have sum-types. > They're pretty much the same Not sure about C#, but in Java if you write `sealed` correctly you won't need the catch-all throw. If they're not actual sum types but are pretty much the same, what good does the "actually" do?

tomsmeding 1y ago
> Not sure about C#, but in Java if you write `sealed` correctly you won't need the catch-all throw. Will the compiler check that you have handled all the cases still? (Genuinely unsure — not a Java programmer)
- ackfoobar 1y ago
  Yes https://openjdk.org/jeps/409#Sealed-classes-and-pattern-matching https://openjdk.org/jeps/409#Sealed-classes-and-pattern-matc... > with pattern matching for switch (JEP 406)the compiler can confirm that every permitted subclass of Shape is covered, so no default clause or other total pattern is needed. The compiler will, moreover, issue an error message if any of the three cases is missing
- brabel 1y ago
  Yes, that's the whole purpose of marking an interface/class `sealed`.
sunnydiskincali 1y ago
> With your shape example, you cannot express in the type system "this function won't return a point". Sure you can, that's just subtyping. If it returns a value that's not a point, the domain has changed from the shape type and you should probably indicate that. structure Shape = struct datatype shape = Circle of real | Rectangle of real * real | Point end structure Bound = struct datatype shape = Circle of real | Rectangle of real * real end This is doing things quick and dirty. For this trivial example it's fine, and I think a good example of why making sum-types low friction is a good idea. It completely changes how you solve problems when they're fire and forget like this. That's not to say it's the only way to solve this problem, though. And for heavy-duty problems, you typically write something like this using higher-kinded polymorphism: signature SHAPE_TYPE = sig datatype shape = Circle of real | Rectangle of real * real | Point val Circle : real -> shape val Rectangle : real * real -> shape val Point : shape end functor FullShape () : SHAPE_TYPE = struct datatype shape = Circle of real | Rectangle of real * real | Point val Circle = Circle val Rectangle = Rectangle val Point = Point end functor RemovePoint (S : SHAPE_TYPE) :> sig type shape val Circle : real -> shape val Rectangle : real * real -> shape end = struct type shape = S.shape val Circle = S.Circle val Rectangle = S.Rectangle end structure Shape = FullShape() structure Bound = RemovePoint(Shape) This is extremely overkill for the example, but it also demonstrates a power you're not getting out of C# or Java without usage of reflection. This is closer to the system of inheritance, but it's a bit better designed. The added benefit here over reflection is that the same principle of "invalid program states are unrepresentable" applies here as well, because it's the exact same system being used. You'll also note that even though it's a fair bit closer conceptually to classes, the sum-type is still distinct. Anyways, in both cases, this is now just: DoesNotReturnPoint : Shape.shape -> Bound.shape Haskell has actual GADTs and proper higher kinded polymorphism, and a few other features where this all looks very different and much terser. Newer languages bake subtyping into the grammar. > If they're not actual sum types but are pretty much the same, what good does the "actually" do? Conflation of two different things here. The examples given are syntactically similar, and they're both treating the constituent part of the grammar as a tagged union. The case isn't any cleaner was the point. However in the broader comparison between class hierarchies and sum-types? They're not similar at all. Classes can do some of the things that sum-types can do, but they're fundamentally different and encourage a completely different approach to problem-solving, conceptualization and project structure... in all but the most rudimentary examples. As I said, my 2nd example here is far closer to a class-hierarchy system than sum-types, though it's still very different. And again, underlining that because of the properties of sum-types, thanks to their specific formalization, they're capable of things class hierarchies aren't. Namely, enforcing valid program-states at a type-level. Somebody more familiar with object-oriented formalizations may be a better person to ask than me on why that is the case. It's a pretty complicated space to talk about, because these type systems deviate on a very basic and fundamental level. Shit just doesn't translate well, and it's easy to find false friends. Like how the Japanese word for "name" sounds like the English word, despite not being a loan word.
- ackfoobar 1y ago
  You wrote a lot of words to say very little. Anyway, to translate your example: sealed interface Shape permits Point, Bound {} final class Point implements Shape {} sealed interface Bound extends Shape permits Circle, Rectangle {} record Circle(double radius) implements Bound {} record Rectangle(double width, double height) implements Bound {} A `Rectangle` is both a `Bound` (weird name choice but whatever), and a `Shape`. Thanks to subtyping, no contortion needed. No need to use 7 more lines to create a separate, unrelated type. > the Japanese word for "name" sounds like the English word, despite not being a loan word. Great analogy, except for the fact that someone from the Java team explicitly said they're drawing inspirations from ML. https://news.ycombinator.com/item?id=24203363 https://news.ycombinator.com/item?id=24203363
  - sunnydiskincali 1y ago
    > You wrote a lot of words to say very little. Substantiate this. > weird name choice but whatever I don't think this kind of snarky potshot is in line with the commentary guidelines. Perhaps you could benefit from a refresher? https://news.ycombinator.com/newsguidelines.html#comments https://news.ycombinator.com/newsguidelines.html#comments > Thanks to subtyping, no contortion needed I see the same degree of contortion, actually. Far more noisy, at that. > No need to use 7 more lines to create a separate, unrelated type. You're still creating a type, because you understand that a sum-type with a different set of cases is fundamentally a different type. Just like a class with a different set of inheritance is a different type. And while it's very cute to compress it all into a single line, it's really not compelling in the context of readability and "write once, use many". Which is the point you were making, although it was on an entirely different part of the grammar. > Great analogy, except for the fact that someone from the Java team explicitly said they're drawing inspirations from ML. ML didn't invent ADTs, and I think you know it's more than disingenuous to imply the quotation means that the type-system in Java which hasn't undergone any fundamental changes in the history of the language (nor could it without drastically changing the grammar of the language and breaking the close relationship to the JVM) was lifted from ML.
    - 1y ago

tomsmeding 1y ago

> Not sure about C#, but in Java if you write `sealed` correctly you won't need the catch-all throw. Will the compiler check that you have handled all the cases still? (Genuinely unsure — not a Java programmer)

ackfoobar 1y ago
Yes https://openjdk.org/jeps/409#Sealed-classes-and-pattern-matching https://openjdk.org/jeps/409#Sealed-classes-and-pattern-matc... > with pattern matching for switch (JEP 406)the compiler can confirm that every permitted subclass of Shape is covered, so no default clause or other total pattern is needed. The compiler will, moreover, issue an error message if any of the three cases is missing
brabel 1y ago
Yes, that's the whole purpose of marking an interface/class `sealed`.

sunnydiskincali 1y ago

> With your shape example, you cannot express in the type system "this function won't return a point". Sure you can, that's just subtyping. If it returns a value that's not a point, the domain has changed from the shape type and you should probably indicate that. structure Shape = struct datatype shape = Circle of real | Rectangle of real * real | Point end structure Bound = struct datatype shape = Circle of real | Rectangle of real * real end This is doing things quick and dirty. For this trivial example it's fine, and I think a good example of why making sum-types low friction is a good idea. It completely changes how you solve problems when they're fire and forget like this. That's not to say it's the only way to solve this problem, though. And for heavy-duty problems, you typically write something like this using higher-kinded polymorphism: signature SHAPE_TYPE = sig datatype shape = Circle of real | Rectangle of real * real | Point val Circle : real -> shape val Rectangle : real * real -> shape val Point : shape end functor FullShape () : SHAPE_TYPE = struct datatype shape = Circle of real | Rectangle of real * real | Point val Circle = Circle val Rectangle = Rectangle val Point = Point end functor RemovePoint (S : SHAPE_TYPE) :> sig type shape val Circle : real -> shape val Rectangle : real * real -> shape end = struct type shape = S.shape val Circle = S.Circle val Rectangle = S.Rectangle end structure Shape = FullShape() structure Bound = RemovePoint(Shape) This is extremely overkill for the example, but it also demonstrates a power you're not getting out of C# or Java without usage of reflection. This is closer to the system of inheritance, but it's a bit better designed. The added benefit here over reflection is that the same principle of "invalid program states are unrepresentable" applies here as well, because it's the exact same system being used. You'll also note that even though it's a fair bit closer conceptually to classes, the sum-type is still distinct. Anyways, in both cases, this is now just: DoesNotReturnPoint : Shape.shape -> Bound.shape Haskell has actual GADTs and proper higher kinded polymorphism, and a few other features where this all looks very different and much terser. Newer languages bake subtyping into the grammar. > If they're not actual sum types but are pretty much the same, what good does the "actually" do? Conflation of two different things here. The examples given are syntactically similar, and they're both treating the constituent part of the grammar as a tagged union. The case isn't any cleaner was the point. However in the broader comparison between class hierarchies and sum-types? They're not similar at all. Classes can do some of the things that sum-types can do, but they're fundamentally different and encourage a completely different approach to problem-solving, conceptualization and project structure... in all but the most rudimentary examples. As I said, my 2nd example here is far closer to a class-hierarchy system than sum-types, though it's still very different. And again, underlining that because of the properties of sum-types, thanks to their specific formalization, they're capable of things class hierarchies aren't. Namely, enforcing valid program-states at a type-level. Somebody more familiar with object-oriented formalizations may be a better person to ask than me on why that is the case. It's a pretty complicated space to talk about, because these type systems deviate on a very basic and fundamental level. Shit just doesn't translate well, and it's easy to find false friends. Like how the Japanese word for "name" sounds like the English word, despite not being a loan word.

ackfoobar 1y ago
You wrote a lot of words to say very little. Anyway, to translate your example: sealed interface Shape permits Point, Bound {} final class Point implements Shape {} sealed interface Bound extends Shape permits Circle, Rectangle {} record Circle(double radius) implements Bound {} record Rectangle(double width, double height) implements Bound {} A `Rectangle` is both a `Bound` (weird name choice but whatever), and a `Shape`. Thanks to subtyping, no contortion needed. No need to use 7 more lines to create a separate, unrelated type. > the Japanese word for "name" sounds like the English word, despite not being a loan word. Great analogy, except for the fact that someone from the Java team explicitly said they're drawing inspirations from ML. https://news.ycombinator.com/item?id=24203363 https://news.ycombinator.com/item?id=24203363
- sunnydiskincali 1y ago
  > You wrote a lot of words to say very little. Substantiate this. > weird name choice but whatever I don't think this kind of snarky potshot is in line with the commentary guidelines. Perhaps you could benefit from a refresher? https://news.ycombinator.com/newsguidelines.html#comments https://news.ycombinator.com/newsguidelines.html#comments > Thanks to subtyping, no contortion needed I see the same degree of contortion, actually. Far more noisy, at that. > No need to use 7 more lines to create a separate, unrelated type. You're still creating a type, because you understand that a sum-type with a different set of cases is fundamentally a different type. Just like a class with a different set of inheritance is a different type. And while it's very cute to compress it all into a single line, it's really not compelling in the context of readability and "write once, use many". Which is the point you were making, although it was on an entirely different part of the grammar. > Great analogy, except for the fact that someone from the Java team explicitly said they're drawing inspirations from ML. ML didn't invent ADTs, and I think you know it's more than disingenuous to imply the quotation means that the type-system in Java which hasn't undergone any fundamental changes in the history of the language (nor could it without drastically changing the grammar of the language and breaking the close relationship to the JVM) was lifted from ML.
  - 1y ago