[ruby-core:98726] [Ruby master Feature#16946] Add an `intersperse` method

Issue #16946 has been updated by nobu (Nobuyoshi Nakada).


The example for `Array` looks simple, but it would be more complicated for `String`.
Is it OK by "char", not by "grapheme cluster"?

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Feature #16946: Add an `intersperse` method
https://bugs.ruby-lang.org/issues/16946#change-86082

* Author: sos4nt (Stefan Sch館ler)
* Status: Open
* Priority: Normal
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Haskell has an `intersperse` function which adds a separator between elements of a list.

It would be pretty useful to have such method(s) in Ruby, too.

Examples for `Array` and `String`:
```ruby
[1, 2, 3].intersperse(0)
#=> [1, 0, 2, 0, 3]

'Hello'.intersperse('-')
#=> "H-e-l-l-o"
```

I'm aware that I can achieve the above with built-in methods, but it's quite cumbersome: (requiring regular expressions / intermediate arrays)

```ruby
[1, 2, 3].flat_map { |i| [i, 0] }[0...-1]
#=> [1, 0, 2, 0, 3]

'Hello'.gsub(/(?<=.)./, '-\0')
#=> "H-e-l-l-o"

'Hello'.chars.join('-')
#=> "H-e-l-l-o"
```




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