[ruby-core:94805] [Ruby master Feature#16147] List Comprehensions in Ruby

[sammomichael@gmail.com]

Issue #16147 has been updated by sammomichael (Samuel Michael).


There is nothing wrong with the current ruby syntax of using enumerables such as map/select/filter_map to generate the same type of list. But as you said ruby is all about many ways of doing things. And as many languages offer some version of this feature (Julia, Python, CoffeeScript, etc.) it could be beneficial for ruby to incorporate its own variation. There is something natural and refreshing about using pure mathematical set builder notation with keywords for...in to generate an array. `[*1..10].filter_map{@1**2 if @1.even?}` vs `[x**2 for x in 1..10 if x.even?]`  The second version has less symbols, less keystrokes and is more human readable for beginners imo. Also I didn't mean to suggest that we retire the normal for...in loop (which I use myself) to break older code or that nobody ever uses it, but I am suggesting that we extend it if possible, just as was done with making "in" the keyword for the new pattern matching system.

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Feature #16147: List Comprehensions in Ruby
https://bugs.ruby-lang.org/issues/16147#change-81427

* Author: sammomichael (Samuel Michael)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
## List comprehensions are present in many languages and programmers are quite fond of their simplicity and power. Add to that the fact that Ruby has a for...in loop that is rarely used but could possibly be repurposed. 

### Currently we can already do a hack like this to make Ruby support list comprehension syntax:

``` ruby
S = [for x in 0...9 do $* << x*2 if x.even? end, $*][1]
# [0, 4, 8, 12, 16]
```

Still, it would be far nicer if the for...in loop would return the desired array automatically, this is one way to approach that taking advantage of lambda bracket invocation syntax:

``` ruby
c = -> x do $*.clear             
  if x['if'] && x[0] != 'f' .  
    y = x[0...x.index('for')]    
    x = x[x.index('for')..-1]
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << #{y}")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  elsif x['if'] && x[0] == 'f'
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << x")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  elsif !x['if'] && x[0] != 'f'
    y = x[0...x.index('for')]
    x = x[x.index('for')..-1]
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << #{y}")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  else
    eval(x.split[3]).to_a
  end
end 

```

so basically we are converting a string to proper ruby syntax for loop then we can use python syntax in a string to do:

``` ruby

c['for x in 1..10']
c['for x in 1..10 if x.even?']
c['x**2 for x in 1..10 if x.even?']
c['x**2 for x in 1..10']

# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# [2, 4, 6, 8, 10]
# [4, 16, 36, 64, 100]
# [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

```




-- 
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