Issue #19764 has been updated by rubyFeedback (robert heiler). I do not have a particular strong opinion either way, largely because I am using ruby more from an OOP point of view, so pattern matching, strong (mandatory) types and so forth aren't quite the way how I use ruby. I did want to say something about style, though. The threadstarter made this statement: "in a more Ruby-like way that Rubyists are already learning and loving." I believe that style, beauty and elegance is subjective. For instance, to me personally - and I am sure others may agree or disagree - the above may not be extremely "ruby-like" or elegant. So I believe reasoning primarily from a, mostly subjective, point of view will not be correct at all times. People are different, so are their preferences; matz pointed this out in the past. I believe this is one reason why ruby is fairly flexible too, with the "more than one way to do something" philosophy (and to some extent, syntax-wise). In regards to syntax, I believe one thing that also should be discussed is whether a new def-keyword should be added in general; and, if so, specifically for pattern matching. zverok did not make a suggestion for this on the ruby issues tracker (I believe), so I would assume that for now he only discussed it on the blog. From this context I would reason that **an additional discussion should be whether new def-centric keywords should be added (or not)** . We have "def", which python also uses, and I think most will agree that this is short, succinct, and to the point; and we have define_method() which may be used for meta-programming like functionality (I use this in a few project with instance_eval or class_eval sometimes, for instance, batch-generation of HTML colour keywords, such as "def steelblue" where I did not want to write like 500x "def", so I just use define_method instead). So this should be part of a discussion, whether "defp" is necessary as a new keyword due to pattern matching needs. I am not sure this is the case, but either way my point is that it should be discussed as well, no matter the outcome. Also, Dan0042 gave some "def" examples, but I think these are different from "defp" examples. So that also seems a bit a separate discussion. Perhaps MRI itself should stay somewhat more conservative. We had unusual ideas in the past, e. g. evil.rb and shapechanging classes/objects. Perhaps we could have "sub-type" dialects of ruby - not necessarily in MRI, but as add-ons to the language itself directly (similar to evil.rb, such as a "functional" sub-project where all these ideas could be bundled; although people can say that this could be a separate gem and perhaps it may be bundled or not. The question then would be whether that should be made available for all ruby users to use or not. Not everyone uses pattern matching, for instance.) Last but not least, since zverok's blog mentioned it: it's not necessarily only matz that is not the best fan of (mandatory?) typing additions to ruby. I, for instance, always feel that the typing makes ruby uglier. This is also subjective of course, and as long as I can defend my code base against mandatory typing I don't quite care as much anyway. People can then use what they want - as long as I don't have to I have no real gripe with it, even if I am not the biggest fan. But it's not correct to assume only matz would not be the biggest fan - others may not be the biggest fans either. You had a similar situation with the "it versus implicit numbering of block arguments", by the way. People felt that "it" is more expressive than implicit numbers and even if I may not be completely convinced that this is the case, they do have a point too, at the least when one compares "it" to _1 _2 _3. (Kokubun also made another good argument that _1 may imply more than one number too, which is not always true, and I agree with that point of view.) PS: In regards to "defp", others may wonder why pattern matching gets its own keyword. Why not other functionality too? We could perhaps reason in favour of "defa", "defb" and so forth and I am not sure it may all be necessary or make sense to have a proliferation of special-purpose keywords. ---------------------------------------- Feature #19764: Introduce defp keyword for defining overloadable, pattern matched methods https://bugs.ruby-lang.org/issues/19764#change-103870 * Author: zeke (Zeke Gabrielse) * Status: Open * Priority: Normal ---------------------------------------- Pattern matching has become one of my favorite features of Ruby, if not my favorite. It changed the way I write and express my thoughts through clean, maintainable code. And I'd like to use it *more*. I propose a new keyword, `defp`, for defining a method which applies pattern matching to its arguments. ```ruby defp call(String => s unless s in /^[a-z]/) puts "string: #{s.inspect} (capitalized)" end defp call(String => s) puts "string: #{s.inspect}" end defp call(Hash(foo:, bar:) => h) puts "hash: #{h.inspect}" end defp call(**nil) puts "no keyword args" end call("Example") # => string: "Example" (capitalized) call("test") # => string: "test" call(foo: 1, bar: 2) # => hash: { :foo => 1, :bar => 2 } ``` Internally, this could be represented as the following `case..in` pseudocode: ```ruby def call(...) case ... in String => s unless s in /foo/ puts "string: #{s.inspect} (not foo)" in String => s puts "string: #{s.inspect}" in Hash(foo:, bar:) => h puts "hash: #{h.inspect}" in **nil puts "no keyword args" else raise NoMatchingMethod end end ``` As you could imagine, this could be used to refactor a lot of code, making the developer's intent much clearer. From [complex methods that use `case` statements](https://github.com/rails/rails/blob/593893c901f87b4ed205751f72df41519b4d2da3...) for taking varied arguments (I'm sure all our code bases contain such `case` statements), to defining smaller, simpler methods that handle particular argument patterns. In addition, not only can this improve code quality, but it brings in method overloads, and it also adds a way to define more typing to the language -- something that RBS has tried to do, to mixed reactions -- but in a more Ruby-like way that Rubyists are already learning *and loving*. Thoughts? Original idea by Victor Shepelev: https://zverok.space/blog/2023-05-05-ruby-types.html Further discussion: https://news.ycombinator.com/item?id=35834351 -- https://bugs.ruby-lang.org/

Issue #19764 has been updated by baweaver (Brandon Weaver). Going to go through a few points here, sorry for the long reply. ### Taking - A gem implementation I had hacked this behavior together at one point with `Taking`: https://github.com/baweaver/taking ```ruby Point = Struct.new(:x, :y) def handle_responses(...) = case Taking.from(...) in Point[x, 10 => y] Point[x, y + 1] in 1, 2, 3 :numbers in 'a', 'b' :strings in :a, :b :symbols in x: 0, y: 0 :origin in x: 0, y: (10..) :north else false end # Array-like handle_responses(1,2,3) # => :numbers handle_responses('a', 'b') # => :strings handle_responses(:a, :b) # => :symbols handle_responses(:nope?) # => false # Hash-like handle_responses(x: 0, y: 0) # => :origin handle_responses(x: 0, y: 15) # => :north handle_responses(x: 10, y: 15) # => :false # Deconstructable Object handle_responses(Point[1, 10]) # => Point[1, 11] ``` ### Precedent with Rescue That said I could see a case for replicating the way `rescue` works currently, as it does establish a precedence: ```ruby def some_method(args) # body rescue # handling code end ``` I can see a case for doing that for pattern matching, amending the previous examples above: ```ruby def handle_responses in Point[x, 10 => y] Point[x, y + 1] in 1, 2, 3 :numbers in 'a', 'b' :strings in :a, :b :symbols in x: 0, y: 0 :origin in x: 0, y: (10..) :north else false end ``` ### Potential Issues and Pitfalls Now while I generally _like_ the idea there are a number of problems that this might present we must cover to be fair to the language. There are a _lot_ of questions here, and not all of them need to be answered to justify such a feature, but would need to be answered insofar as implementations are concerned for core folks, and these certainly are not comprehensive. #### Args vs Body What happens if someone uses the top level or uses the regular arguments syntax?: ```ruby def some_method(a, b, c) # body ??? in pattern end ``` Should pattern take precedence, or the top body? Should we even allow method arguments in the case there are patterns applied? Let's say we don't have arguments to the method in these cases and someone writes a method body outside of an `in pattern` right above the first one, what should we do then? #### Super How would we handle `super` with this? Argument forwarding and signatures could become very interesting here, especially if Sorbet or Steep get involved. #### Typing Speaking of, it would be real fun to make Sorbet and Steep play nicely with this. It'd pretty well crash the syntax as it exists today and require a decent amount of work to support. #### Rescue Would assume that this would remain top level and apply to every branch like so: ```ruby def some_method in pattern # ... rescue something # ... end ``` #### Absurdity What happens if we don't handle every case? What if there's no `else`? Should that raise an exception for an unhandled case? At least with static languages the absurdity clause (all inputs must be handled) it's easier to guarantee. In Ruby this would be a decent bit harder to do so perhaps just raising exceptions on unhandled is the easiest. #### Signatures What happens if you use a mix of positional and keyword arguments? Pattern matching also has find patterns (`*, arg, *`) that might not translate cleanly. How do we translate those into arguments? Perhaps in this case we don't and we pass it with `...` much like the above, but then that brings up more fun with the next area. #### Performance This would be hard to optimize, and probably to JIT. It's possible but essentially with the above you end up with forwarding all arguments instead of a very restricted set you know you'd need. ---- Anyways, not saying don't do it, but this would be quite a task to really sort out all the ways it could be (ab)used and all the edge cases. ---------------------------------------- Feature #19764: Introduce defp keyword for defining overloadable, pattern matched methods https://bugs.ruby-lang.org/issues/19764#change-103912 * Author: zeke (Zeke Gabrielse) * Status: Open * Priority: Normal ---------------------------------------- Pattern matching has become one of my favorite features of Ruby, if not my favorite. It changed the way I write and express my thoughts through clean, maintainable code. And I'd like to use it *more*. I propose a new keyword, `defp`, for defining a method which applies pattern matching to its arguments. ```ruby defp call(String => s unless s in /^[a-z]/) puts "string: #{s.inspect} (capitalized)" end defp call(String => s) puts "string: #{s.inspect}" end defp call(Hash(foo:, bar:) => h) puts "hash: #{h.inspect}" end defp call(**nil) puts "no keyword args" end call("Example") # => string: "Example" (capitalized) call("test") # => string: "test" call(foo: 1, bar: 2) # => hash: { :foo => 1, :bar => 2 } ``` Internally, this could be represented as the following `case..in` pseudocode: ```ruby def call(...) case ... in String => s unless s in /foo/ puts "string: #{s.inspect} (not foo)" in String => s puts "string: #{s.inspect}" in Hash(foo:, bar:) => h puts "hash: #{h.inspect}" in **nil puts "no keyword args" else raise NoMatchingMethod end end ``` As you could imagine, this could be used to refactor a lot of code, making the developer's intent much clearer. From [complex methods that use `case` statements](https://github.com/rails/rails/blob/593893c901f87b4ed205751f72df41519b4d2da3...) for taking varied arguments (I'm sure all our code bases contain such `case` statements), to defining smaller, simpler methods that handle particular argument patterns. In addition, not only can this improve code quality, but it brings in method overloads, and it also adds a way to define more typing to the language -- something that RBS has tried to do, to mixed reactions -- but in a more Ruby-like way that Rubyists are already learning *and loving*. Thoughts? Original idea by Victor Shepelev: https://zverok.space/blog/2023-05-05-ruby-types.html Further discussion: https://news.ycombinator.com/item?id=35834351 -- https://bugs.ruby-lang.org/