## Feature #10064

### &:symbol in when clause

Status: | Feedback | ||
---|---|---|---|

Priority: | Normal | ||

Assignee: | Yukihiro Matsumoto |

**Description**

Now we can put Proc objects in a when clause as this code.

require 'prime' def test(n) print "#{n} is " case n when :zero?.to_proc puts 'zero' when :even?.to_proc puts 'an even number' when :prime?.to_proc puts 'a prime number' else puts 'a non-prime odd number' end end (1..10).each &method(:test)

I would like to write it as below:

require 'prime' def test(n) print "#{n} is " case n when &:zero? puts 'zero' when &:even? puts 'an even number' when &:prime? puts 'a prime number' else puts 'a non-prime odd number' end end (1..10).each &method(:test)

How do you think about this new syntax, Matz?

### History

#### #1 Updated by Koichi Sasada about 1 year ago

just idea.

```
class Symbol
def ~@
self.to_proc
end
end
case 0
when ~:zero?
p :zero
end
```

#### #2 Updated by Yukihiro Matsumoto about 1 year ago

**Status**changed from*Open*to*Feedback*

The original idea includes dispatch based on type (Symbol), which is not a good idea in general.

The one proposed by ko1 is slightly better, yet I still don't love it.

Matz.

#### #3 Updated by Tsuyoshi Sawada 11 months ago

What about allowing syntax like this:

```
->&Proc.new{|y| y.zero?} #=> Should be equivalent to ->{|x| x.zero?}
->&:zero? #=> Should be equivalent to ->{|x| x.zero?} (Symbol#to_proc applies implicitly)
```

This would be a parallel to

```
[*["foo"]] #=> ["foo"]
[*{foo: "bar"}] #=> [[:foo, "bar"]] (Symbol#to_a applies implicitly)
```

under the understanding that what `[]`

is to `*`

(converse operation) is what `->`

is to `&`

.

Then, we would be able to do:

```
case 0
when ->&:zero?
p :zero
end
```

#### #4 Updated by Marc-Andre Lafortune 11 months ago

Yukihiro Matsumoto wrote:

The original idea includes dispatch based on type (Symbol), which is not a good idea in general.

I'm not sure I understand. I think the original idea would work for other types too; the syntax would just be a shortcut for a call to `to_proc`

.

For example, I'd expect the following to work too:

x = Object.new def x.to_proc -> { true } end case 42 when &x puts "Always true" end