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Feature #15865

Updated by mame (Yusuke Endoh) over 1 year 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

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