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Haskell-like list comprehensions in Ruby

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raskell.rb
Ruby
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$stack, $draws = [], {}
 
def method_missing *args
return if args[0][/^to_/]
$stack << args.map { |a| a or $stack.pop }
$draws[$stack.pop(2)[0][0]] = args[1] if args[0] == :<
end
 
class Array
def +@
$stack.flatten!
keys = $draws.keys & $stack
draws = $draws.values_at *keys
 
comp = draws.shift.product(*draws).map do |draw|
$stack.map { |s| draw[keys.index s] rescue s }.reduce do |val, cur|
op = Symbol === cur ? [:send, :method][val.method(cur).arity] : :call
val.send op, cur
end
end
 
$stack, $draws = [], {}
Symbol === last ? comp.select(&pop) : comp
end
 
def -@
case map(&:class).index Range
when 0 then first.to_a
when 1 then [first] + last.step(last.min.ord - first.ord).to_a
else self
end
end
end
 
foo =+ [x * y | x <- [1..3], y <- [4..6]]
# [4, 5, 6, 8, 10, 12, 12, 15, 18]
 
bar =+ [a + b | a <- ['n','p'..'t'], b <- %w[a i u e o]]
# ["na", "ni", "nu", "ne", "no", "pa", "pi", "pu", "pe", "po", "ra", "ri", "ru", "re", "ro", "ta", "ti", "tu", "te", "to"]
 
baz =+ [i ** 2 / 3 | i <- [3,6..100], :even?]
# [12, 48, 108, 192, 300, 432, 588, 768, 972, 1200, 1452, 1728, 2028, 2352, 2700, 3072]
 
quux =+ [s.size.divmod(2) | s <- %w[Please do not actually use this.]]
# [[3, 0], [1, 0], [1, 1], [4, 0], [1, 1], [2, 1]]

Seriously though, couldn't you make this non-global by only having it work within a block passed to a method named something like list_comprehend ?

Gem it. Want. Want now.

This is so, so "wrong", yet thrilling.

@pmarreck It could definitely be cleaned up and made passably useful by wrapping everything in, say, a Raskell do block, but Kernel#method_missing is the only way to capture the bare variables that help to fully emulate Haskell's syntax, so it'd still be pretty unpleasant. This was mostly just an interesting proof of concept.

@elight I just can't bring myself to do it, at least not while there are so many holes: the magical undefined variables become nil after they're used once, so you'd have to ensure you're always using a different set of names in each comprehension; there's no way to filter a specific draw, only the entire result set; I couldn't figure out how to cleanly allow for previous variables to constrain subsequent ranges, and as long as Infinity forces a range into floats, the real power of Haskell's list comprehensions isn't available.

@FranklinChen I'm looking forward to laziness and refinements in 2.0, when something like this might actually be considered sane.

This is crazy stuff! I'm still trying to understand how this works. If you could provide some inline comment, it'll be great.

Wow, totally ridonkulous. Nicely done!

now if it could avoid using method_missing somehow...

Wow, I'm amazed. I really wish Ruby supported this style.

Nice work. Let's see if this can be made fly with 2.0...

Bonus points for the name!

Amazing! Some comments would be useful, though.

So, what about using it in place of function argument?

Like:
[x * y | x <- [1..3], y <- [4..6]].each { |a| print a }

Looks really nice!

I played a bit with the idea to get that. Trying out ruby2 refinements. Too lazy for the "|" support but can do conditions.
https://gist.github.com/freakhill/5564328

+1 for "Raskell". Pun in "Rascal"?

Awesome job! :heart: Gem it please?

"Haskell-like" without lambdas?
Even if it doesn't look as much like Haskell, this snippet is much closer to the Haskell principles, without abusing Ruby.

foo = ->(x=(1..3), y=(4..6)){x.flat_map{|a| y.map{|b| a*b}}}.call

(Of course you usually wouldn't need to call any lambdas before actually executing the application)

Nice but for Ruby I prefer using Ruby syntax:

(1..3).zip(4..6).map { |a, b| a * b }

I'd humbly like to up-vote the previous comment.

Thats quite brilliant, the author does say "Please do not actually use this."
@hauleth zip is a bit different, list comprehensions are more like cross products:

(1..3).to_a.product((4..6).to_a).map{|a,b| a*b}

or you could do a double map_flat like @wteuber.

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