From: shevegen@... Date: 2019-05-21T08:21:39+00:00 Subject: [ruby-core:92739] [Ruby trunk Feature#15865] ` in ` expression Issue #15865 has been updated by shevegen (Robert A. Heiler). I don't have a big pro/contra opinion per se on the whole topic of pattern matching, but I would wait a bit before finalizing more and more additions on top of it. Now pattern matching is probably there to say, I understand that, but it may still be better to not add lots of further building blocks to it. Another smaller reason is that at the least I find pattern matching to be somewhat more complex to understand; I understand that the suggestion here is simpler than long case/in statements, but I am still not sure if it would be a good idea to add too much to ideas/changes in a short period of time, without seeing how it may be used in "real" production code. ---------------------------------------- Feature #15865: ` in ` expression https://bugs.ruby-lang.org/issues/15865#change-78101 * Author: mame (Yusuke Endoh) * Status: Open * Priority: Normal * Assignee: matz (Yukihiro Matsumoto) * Target version: ---------------------------------------- How about adding a syntax for one-line pattern matching: ` in ` ? ``` [1, 2, 3] in x, y, z #=> true (with assigning 1 to x, 2 to y, and 3 to z) [1, 2, 3] in 1, 2, 3 #=> false ``` More realistic example: ``` json = { name: "ko1", age: 39, address: { postal: 123, city: "Taito-ku" } } if json in { name:, age: (20..), address: { city: "Taito-ku" } } p name #=> "ko1" else raise "wrong format" end ``` It is simpler and more composable than "case...in" when only one "in" clause is needed. I think that in Ruby a pattern matching would be often used for "format-checking", to check a structure of data, and this use case would usually require only one clause. This is the main rationale for the syntax I propose. Additional two small rationales: * It may be used as a kind of "right assignment": `1 + 1 in x` behaves like `x = 1 + 1`. It returns true instead of 2, though. * There are some arguments about the syntax "case...in". But if we have ` in `, "case...in" can be considered as a syntactic sugar that is useful for multiple-clause cases, and looks more natural to me. There are two points I should note: * ` in ` is an expression like ` and `, so we cannot write it as an argument: `foo(1 in 1)` causes SyntaxError. You need to write `foo((1 in 1))` as like `foo((1 and 1))`. I think it is impossible to implement. * Incomplete pattern matching also rewrites variables: `[1, 2, 3] in x, 42, z` will write 1 to the variable "x". This behavior is the same as the current "case...in". Nobu wrote a patch: https://github.com/nobu/ruby/pull/new/feature/expr-in-pattern -- https://bugs.ruby-lang.org/ Unsubscribe: