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Haskell-like list comprehensions in Ruby
$stack, $draws = [], {}
def method_missing *args
return if args[0][/^to_/]
$stack << { |a| a or $stack.pop }
$draws[$stack.pop(2)[0][0]] = args[1] if args[0] == :<
class Array
def +@
keys = $draws.keys & $stack
draws = $draws.values_at *keys
comp = draws.shift.product(*draws).map do |draw|
$ { |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
$stack, $draws = [], {}
Symbol === last ? : comp
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
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]]

tomekw commented Aug 15, 2012


be commented Aug 15, 2012


rahult commented Aug 16, 2012


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 ?

elight commented Aug 16, 2012

Holy shit.....

elight commented Aug 16, 2012

Gem it. Want. Want now.

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


andkerosine commented Aug 16, 2012

@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 amazing!

gcao commented Aug 17, 2012

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

styx commented Aug 18, 2012


Wow, totally ridonkulous. Nicely done!

rdp commented Aug 20, 2012

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...

xatier commented May 12, 2013

Really cool!


Bonus points for the name!

Amazing! Some comments would be useful, though.

so cool!

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

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

Ruby 2.1!

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.

igbanam commented May 12, 2013

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

darth10 commented May 13, 2013

Awesome job! ❤️ Gem it please?

wteuber commented May 15, 2013

"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|{|b| a*b}}}.call

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

hauleth commented May 18, 2013

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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