[ruby-core:102216] [Ruby master Feature#17576] Partial Functions (procs, lambdas)

Issue #17576 has been reported by temabolshakov (Tema Bolshakov).

----------------------------------------
Feature #17576: Partial Functions (procs, lambdas)
https://bugs.ruby-lang.org/issues/17576

* Author: temabolshakov (Tema Bolshakov)
* Status: Open
* Priority: Normal
----------------------------------------
We already have pattern matching and functions. Let's combine them and introduce a "partial procs" as first-class citizens. 

What are partial procs? This is a function that works on a subset of arguments. The partial proc's main advantage is that a caller may decide in advance if this proc can accept an argument or not and do something different rather than calling it.


That's how it may look like:


```ruby
partial_proc = proc do |arg|
in x if x.odd?
  "#{x} is odd"
end
```

One can check if a proc is defined on the argument 

```ruby 
partial_proc.defined?(42) #=> false
partial_proc.defined?(41) #=> true
```

You can call such a partial proc and it raises an error when it's not defined on this argument:


```ruby
partial_proc.call(42) #=> raises NoMatchingPatternError (42)
partial_proc.call(41) #=> 41 is odd
```

And finally, we can call or fallback to a default value:


```ruby
partial_proc.call_or_else(42) { "fallback value" } #=> 'fallback value'
partial_proc.call_or_else(41) { "fallback value" } #=> 41 is odd
```



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>