Feature #15865
Updated by mame (Yusuke Endoh) almost 5 years ago
How about adding a syntax for one-line pattern matching: `<expr> in <pattern>` ? ``` [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, 4 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 `<expr> in <pattern>`, "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: * `<expr> in <pattern>` is an expression like `<expr> and <expr>`, 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