[ruby-core:95047] [Ruby master Feature#16147] List Comprehensions in Ruby

[sammomichael@gmail.com]

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


nobu (Nobuyoshi Nakada) wrote:
> sammomichael (Samuel Michael) wrote:
> > ```ruby
> > [for x in 1..10 do x**2 if x.even? end] #=> 1..10  (normal Ruby)
> > ```
> 
> It is an `Array` from 0 to 10 now. Correct thanks I should have wrote [1..10]
> 
> > below we propose a syntax in which we splat the for loop to return the stored result not the caller
> > 
> > ```ruby
> > [*for x in 1..10 do x**2 if x.even? end] #=> [4, 16, 36, 64, 100]
> > ```
> 
> Obviously the latter result conflicts with the former.
> Which result do you expect from the following code?
 Hi, I was suggesting a splat operator as one way to create a special syntax, under the hood instead of calling each method it would treat this as a filter map, map, or filter operation, and return the result in place. My intention is to create a flexible alternative syntax to enumerable syntax or using external iteration to do the same thing. Of course it doesn't have to be a splat, just some way to indicate you are invoking the shorthand for Ruby list comprehension, not using Ruby traditional for (.each) loop. Splat could be interpreted here as spreading the values from the block into a new array.

> 
> ```ruby
> a = for x in 1..10 do x**2 if x.even? end
> [*a] #=> [4, 16, 36, 64, 100] a = 1..10 => [1,2,3,4,5,6,7,8,9,10]  for loop results in 1..10 and spreads out to an array
the new syntax will not work on ranges only can be declared within [] square brackets at the time of invocation

> [*(for x in 1..10 do x**2 if x.even? end)] #=> [1,2,3,4,5,6,7,8,9,10] here the expression in parenthesis would resolve to range 1..10 and be splatted to an array
I'm not sure if there is a reason to do this though, but I am open to other opinions if there is something better that should happen
> ```



----------------------------------------
Feature #16147: List Comprehensions in Ruby
https://bugs.ruby-lang.org/issues/16147#change-81678

* 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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