[Hirrolot]

This is more of syntax sugar than power and generality, since nested pattern matching can be mechanically translated into "top-level" matching (e.g., see [1] and [2]).

[1] L. Augustsson. Compiling Pattern Matching. In Functional Programming Languages and Computer Architecture, pages 368– 381, 1985.

[2] P. Wadler. Efficient Compilation of Pattern Matching. In S.L. Peyton Jones, editor, The Implementation of Functional Programming Languages, pages 78–103. Prentice Hall, 1987.

[bunderbunder]

This is where I like to cite Dijstra's "Go To Statement Considered Harmful".

What a lot of people miss about that paper is that he wasn't just talking about goto statements. He was also making a more general observation about how more powerful and general programming language features are not necessarily desirable, because they tend to adversely impact developer productivity.

The reason I, as a user, prefer structured control flow statements over goto is not that I believe they are powerful. It's precisely because they are less powerful. The resulting constraints on how the program can be structured make it easier for me to read and reason about existing code. That makes maintaining code easier. It also makes optimization and static analysis easier. And it makes writing tests easier, too.

I have similar feelings about ADTs. The reason I prefer them to other ways of doing composite data types is not that I think they're more powerful. It's that they create constraints that tend to reduce the semantic complexity of the domain models people create in programming languages that use them. And that makes my job easier.

The corollary to that, though, is that I'm not actually all that hype about adding ADTs to existing languages. For reasons that are similar to how the mere availability of structured, reentrant function calls is small consolation in a codebase that's already riddled with goto statements. The real win doesn't come from using ADTs, it comes from not having to worry about all those other confusing overpowered things that aren't ADTs.

[ajross]

That's exactly where I am. Pattern matched sum types "feel great" to code in to an expert because they are a concise and reasonably tight way to express the otherwise boring "enumerate over possibilities" code.

But they're hard to read for anyone who isn't an expert on not just the language but the type in question (c.f. Rust's Option() idioms all looks like line noise to newbies, etc...). And that's a bad trade.

In essence, this stuff is just Perl all over again. It's a language feature that prioritizes concision over comprehension. And I say that as someone who really likes coding in perl. But "people" don't like perl, and the community moved on, and the reasons are... the same reason that uptake in ADTs is lagging where the experts want it to be.

[chongli]

This argument is the most common fallacy I see in programming language discussions. I might as well give it a name right here: "Turing equivalence fallacy" or perhaps "syntax sugar fallacy."

All Turing Complete programming languages are Turing equivalent to one another. Programs written in one language can be mechanically transformed into those written in another. This is irrelevant to the discussion of programming languages. The whole point of creating different programming languages is to explore different ways to express the same program!

[Hirrolot]

In programming language design, we tend to distinguish between global and local analysis. While type checking and elaboration is an example of global analysis, desugaring is inherently local to some piece of code. Therefore, "power" or "expressiveness" usually mean that something cannot be syntactically "expanded"; e.g., while type classes elaborate into explicit dictionaries, they still require information from the type checker, and therefore considered a "real" feature of a programming language. On the other hand, nested pattern matching can be formulated as local syntax transformation, and therefore it doesn't bring anything fundamentally new to the type system or dynamic semantics.

There's also a great talk on the matter [1], if somebody is interested in formalities.

[1] https://www.youtube.com/watch?v=43XaZEn2aLc

[OskarS]

Of course the syntax sugar is a good thing if it makes it easier to write the code, but if the question is about "expressive power of the type system", it's not really relevant: Zig's type system can properly express a sum type. In addition: pattern matching is orthogonal to ADT, you can have pattern matching in both languages with and without algebraic types. Neither one implies the other.

[gpderetta]

> Turing equivalence fallacy

Better known as the Turing Tar Pit.

[xdavidliu]

syntax sugar IS power. also if the mechanical transformation is nontrivial, then so is the power of the sugar

[thesz]

I once explored the Epigram dependently typed programming language. It used to preclude many types of free form pattern matches, due to heavy dependence on structured editor (you speciy a type, it generates pattern matching, you cannot change the structure), so it was almost completely unuseable for many, many tasks.

So, while you are formally right, the need of shortcuts in pattern matching is undeniable to me.