ruby-core

Issue #20318 has been updated by mame (Yusuke Endoh).


I strongly feel "bad smell" for this proposal.

In principle, one method should do one thing.
But this syntax introduces a completly different branching depending on the arguments, which could encourage a weird method behavior.
In fact, the motivating example prints different messages depending on the arguments.  I believe such a method would never be recommended.

In particular, the method that accepts `foo(str)` and `foo(greeting: str)` but not `foo(str, greeting: str)` looks rather weird to me. I am concerned that such a method could be so easily defined.

Considering the Java overloading, each overloaded method definition would often delegate to the most common (receives the most arguments) definition. Given that, the motivating example should be:

```ruby
def foo(...)
  case ...
  in args: [name]
   puts "Hello #{ name }"
  in args: [first_name, last_name]
   foo(first_name + " " + last_name)
  in kwargs: {greeting:}
   foo(greeting)
  else
    raise ArgumentError
  end
end
````

I do not find this code easy to understand.

As far as I know, Ruby has the most complicated argument system in the world. I don't think we should make it even more complicated.

----------------------------------------
Feature #20318: Pattern matching `case ... in` support for triple-dot arguments
https://bugs.ruby-lang.org/issues/20318#change-107177

* Author: bradgessler (Brad Gessler)
* Status: Open
----------------------------------------

# Premise

Sometimes when I'm creating a method for an API, I'd like to do pattern matching against the arguments. Today I have to do something like this:

```ruby
def foo(*args, **kwargs, &block)
  case { args:, kwargs:, block: }
    in args: [name]
      puts name
    in args: [first_name, last_name]
      puts "Hi there #{first_name} #{last_name}"
    in kwargs: {greeting:}
      puts "Hello #{greeting}"
    else
      puts "No match: #{args}"
  end
end

foo "Hi"
foo "Brad", "Gessler"
foo greeting: "Brad"
```

Or an array like this:

```ruby
def bar(*args, **kwargs, &block)
  case [args, kwargs, block]
    in [name], {}, nil
      puts name
    in [first_name, last_name], {}, nil
      puts "Hi there #{first_name} #{last_name}"
    in [], {greeting:}, nil
      puts "Hello #{greeting}"
    else
      puts "No match: #{args}, #{kwargs}"
  end
end

bar "Howdy"
bar "Bradley", "Gessler"
bar greeting: "Bradley"
```

# Proposal

I'd like to propose the same thing, but for `...`, like this:

```ruby
def foo(...)
  case ...
    in args: [name]
      puts name
    in args: [first_name, last_name]
      puts "Hi there #{first_name} #{last_name}"
    in kwargs: {greeting:}
      puts "Hello #{greeting}"
    else
      puts "No match: #{args}"
  end
end

foo "Hi"
foo "Brad", "Gessler"
foo greeting: "Brad"
```

One thing I'm not sure sure about: the `args`, `kwargs`, and `block` names appear out of thin air, so ideally those could somehow be named or have a syntax that doesn't require those names.

The array would look like this:

```ruby
def bar(...)
  case ...
    in [name], {}, nil
      puts name
    in [first_name, last_name], {}, nil
      puts "Hi there #{first_name} #{last_name}"
    in [], {greeting:}, nil
      puts "Hello #{greeting}"
    else
      puts "No match: #{args}, #{kwargs}"
  end
end

bar "Howdy"
bar "Bradley", "Gessler"
bar greeting: "Bradley"
```



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