Haskell style list comprehensions in Ruby (opens in new tab)

(gist.github.com)

87 pointsldubinets13y ago28 comments

28 comments

23 comments · 8 top-level

andkerosine13y ago· 4 in thread

This has no business being at the top of HN.

Osmium13y ago

Why not? Seems pretty damn cool to me.

Edit: didn't realise you were the author :)

andkerosine13y ago

As its creator, I would certainly agree. Still, I'd feel bad if some poor soul considered it anything but an interesting demonstration of Ruby's capabilities.

1 more reply

vidarh13y ago

It's awesomely cool, but it also defines a global method_missing, which puts you in for a world of pain before even considering the overloaded operators.

It's one of those hacks that are cute and great to play with and awesome proof of concept for new features, but if I saw it in a production codebase, I'd want to strangle someone.

zerr13y ago

Not everything has to be related with entrepreneurship. It is a hacker news after all...

EDIT: just realized you're the author, so your comment must be out of modesty. Anyway, it deserves, and kudos to you ;)

VeejayRampay13y ago· 3 in thread

Cue to Haskell people lamenting the fact that this is an abomination and not by any means "Haskell-style list comprehensions".

Still very nice though.

weareconvo13y ago

Haskell person here... I personally find Haskell list comprehensions to be ridiculously ugly, and are an unnecessary bit of syntactic sugar over the List Monad.

dons13y ago

... you might want to read up on monad comprehensions :)

http://hackage.haskell.org/trac/ghc/wiki/MonadComprehensions

2 more replies

Osmium13y ago

I find this interesting. As someone who emphatically doesn't know Haskell, but has recently started learning, I think list comprehensions are wonderful. That said, I haven't got to the chapter on Monads yet, so... :)

What do you find ugly about them?

1 more reply

dopamean13y ago· 3 in thread

As someone who is still learning Ruby and would like to learn Haskell I have no idea what is going on here. Would someone be kind enough to explain? Thanks.

cschneid13y ago

Specifically, it is monkey patching the unary - and + operators on array, and has a clever global method missing.

The unary operators `def -@` gets called when you do something like: `-[1,2,3]`.

So in the example code at the bottom, it's doing `bar =+ [x | x <- [1..3]]` that's better parsed as `bar = +[x | x <- [1..3]]` (note the spacing difference).

Hacking unary operators has a long history in silliness in ruby. See also: https://github.com/jicksta/superators

phaer13y ago

It's just adding a syntactic construct to ruby which looks similar to haskells list comprehensions. Explanation for haskell: http://learnyouahaskell.com/starting-out#im-a-list-comprehen...

dopamean13y ago

Ah, I get it. Thanks dude.

tome13y ago· 2 in thread

It's neat that Ruby has programmable syntax like this, but Haskell's comprehensions are one of its worst syntactic features IMHO. They're inherently non-composable.

nimish13y ago

They're equivalent to do-notation in the List monad so as long as you don't really need the inline syntax you can just mechanically translate it to do nation and back.

And with -XMonadComprehensions you can have the compiler do that for a whole bunch more Monads.

surement13y ago

It's also possible to just use Applicative with <*> and <$>

gbog13y ago· 2 in thread

Side question, rant. I love the syntactic side of list or dict comprehension (in python) and often use them but as soon as I have to debug or expand the functionality, I have to slice them into for loop. And then I hate myself for being lazy/clever and more and more, when I start typing a = [, I hear an internal voice: wait, aren't you being wrongfully clever one again?

I am the only one?

obviouslygreen13y ago

I adore list comprehensions, but perhaps for this reason, I generally only use them for operations so simple as to be trivially correct or incorrect without any debugging.

Dict comprehensions are just a little muddier, even for simple things, but a single line of debug output after the comprehension should confirm whether or not you're getting the structure you expect, provided you are still doing only trivial work inside your comprehension.

hendzen13y ago

I like to use them for simple cases only (No nested comprehensions, etc). When in doubt, the Google Python style guide [0] has some useful guidance.

[0] - http://google-styleguide.googlecode.com/svn/trunk/pyguide.ht...

stiff13y ago· 1 in thread

Wow, that's some nontrivial Ruby, some explanation of this code would be nice, I program in Ruby for some 6 years now and I still had to do a lot of head-scratching to more or less figure this out. I had no idea Ruby allows overloading of prefix operators, for example.

saraid21613y ago

There is an excellent book called "Metaprogramming Ruby" I'd recommend.

wedesoft13y ago

Somewhat related: I've developed a library for operations involving multi-dimensional arrays [1, 2]. When possible it uses GCC for jit-compilation to achieve higher performance.

  require 'multiarray'
  include Hornetseye
  # Object
  lazy(4) { |i| i + 2 }
  # Sequence(INT):
  # [ 2, 3, 4, 5 ]
  lazy(3, 2) { |x, y| x }
  # MultiArray(INT,2):
  # [ [ 0, 1, 2 ],
  #   [ 0, 1, 2 ] ]
  lazy(3, 2) { |x, y| x + 1 }
  # MultiArray(INT,2):
  # [ [ 1, 2, 3 ],
  #   [ 1, 2, 3 ] ]
  lazy(3, 3) { |x, y| y + 4 }
  # MultiArray(INT,2):
  # [ [ 4, 4, 4 ],
  #   [ 5, 5, 5 ],
  #   [ 6, 6, 6 ] ]
  lazy(3, 3) { |x, y| (x + 1) * (y + 4) }
  # MultiArray(INT,2):
  # [ [ 4, 8, 12 ],
  #   [ 5, 10, 15 ],
  #   [ 6, 12, 18 ] ]
  lazy { |x,y| Sequence['n', 'p', 'r', 't'][x] + Sequence['a', 'i', 'u', 'e', 'o'][y] }
  # MultiArray(OBJECT,2):
  # [ [ "na", "pa", "ra", "ta" ],
  #   [ "ni", "pi", "ri", "ti" ],
  #   [ "nu", "pu", "ru", "tu" ],
  #   [ "ne", "pe", "re", "te" ],
  #   [ "no", "po", "ro", "to" ] ]
  s = lazy(33) { |i| 3 * (i+1) }
  # Sequence(INT):
  # [ 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, .... ]
  s.mask((s % 2).eq(0)).collect { |i| i ** 2 / 3 }
  # Sequence(INT):
  # [ 12, 48, 108, 192, 300, 432, 588, 768, 972, 1200, 1452, 1728, .... ]

[1] https://github.com/wedesoft/multiarray [2] http://www.wedesoft.de/hornetseye-api/

egonschiele13y ago

Oh wow, that is so wrong and so beautiful.

j / k navigate · click thread line to collapse

28 comments

23 comments · 8 top-level

andkerosine13y ago· 4 in thread

This has no business being at the top of HN.

Osmium13y ago

Why not? Seems pretty damn cool to me.

Edit: didn't realise you were the author :)

andkerosine13y ago

As its creator, I would certainly agree. Still, I'd feel bad if some poor soul considered it anything but an interesting demonstration of Ruby's capabilities.

1 more reply

vidarh13y ago

It's awesomely cool, but it also defines a global method_missing, which puts you in for a world of pain before even considering the overloaded operators.

It's one of those hacks that are cute and great to play with and awesome proof of concept for new features, but if I saw it in a production codebase, I'd want to strangle someone.

zerr13y ago

Not everything has to be related with entrepreneurship. It is a hacker news after all...

EDIT: just realized you're the author, so your comment must be out of modesty. Anyway, it deserves, and kudos to you ;)

VeejayRampay13y ago· 3 in thread

Cue to Haskell people lamenting the fact that this is an abomination and not by any means "Haskell-style list comprehensions".

Still very nice though.

weareconvo13y ago

Haskell person here... I personally find Haskell list comprehensions to be ridiculously ugly, and are an unnecessary bit of syntactic sugar over the List Monad.

dons13y ago

... you might want to read up on monad comprehensions :)

http://hackage.haskell.org/trac/ghc/wiki/MonadComprehensions

2 more replies

Osmium13y ago

What do you find ugly about them?

1 more reply

dopamean13y ago· 3 in thread

As someone who is still learning Ruby and would like to learn Haskell I have no idea what is going on here. Would someone be kind enough to explain? Thanks.

cschneid13y ago

Specifically, it is monkey patching the unary - and + operators on array, and has a clever global method missing.

The unary operators `def -@` gets called when you do something like: `-[1,2,3]`.

So in the example code at the bottom, it's doing `bar =+ [x | x <- [1..3]]` that's better parsed as `bar = +[x | x <- [1..3]]` (note the spacing difference).

Hacking unary operators has a long history in silliness in ruby. See also: https://github.com/jicksta/superators

phaer13y ago

It's just adding a syntactic construct to ruby which looks similar to haskells list comprehensions. Explanation for haskell: http://learnyouahaskell.com/starting-out#im-a-list-comprehen...

dopamean13y ago

Ah, I get it. Thanks dude.

tome13y ago· 2 in thread

It's neat that Ruby has programmable syntax like this, but Haskell's comprehensions are one of its worst syntactic features IMHO. They're inherently non-composable.

nimish13y ago

They're equivalent to do-notation in the List monad so as long as you don't really need the inline syntax you can just mechanically translate it to do nation and back.

And with -XMonadComprehensions you can have the compiler do that for a whole bunch more Monads.

surement13y ago

It's also possible to just use Applicative with <*> and <$>

gbog13y ago· 2 in thread

I am the only one?

obviouslygreen13y ago

I adore list comprehensions, but perhaps for this reason, I generally only use them for operations so simple as to be trivially correct or incorrect without any debugging.

hendzen13y ago

I like to use them for simple cases only (No nested comprehensions, etc). When in doubt, the Google Python style guide [0] has some useful guidance.

[0] - http://google-styleguide.googlecode.com/svn/trunk/pyguide.ht...

stiff13y ago· 1 in thread

saraid21613y ago

There is an excellent book called "Metaprogramming Ruby" I'd recommend.

wedesoft13y ago

Somewhat related: I've developed a library for operations involving multi-dimensional arrays [1, 2]. When possible it uses GCC for jit-compilation to achieve higher performance.

  require 'multiarray'
  include Hornetseye
  # Object
  lazy(4) { |i| i + 2 }
  # Sequence(INT):
  # [ 2, 3, 4, 5 ]
  lazy(3, 2) { |x, y| x }
  # MultiArray(INT,2):
  # [ [ 0, 1, 2 ],
  #   [ 0, 1, 2 ] ]
  lazy(3, 2) { |x, y| x + 1 }
  # MultiArray(INT,2):
  # [ [ 1, 2, 3 ],
  #   [ 1, 2, 3 ] ]
  lazy(3, 3) { |x, y| y + 4 }
  # MultiArray(INT,2):
  # [ [ 4, 4, 4 ],
  #   [ 5, 5, 5 ],
  #   [ 6, 6, 6 ] ]
  lazy(3, 3) { |x, y| (x + 1) * (y + 4) }
  # MultiArray(INT,2):
  # [ [ 4, 8, 12 ],
  #   [ 5, 10, 15 ],
  #   [ 6, 12, 18 ] ]
  lazy { |x,y| Sequence['n', 'p', 'r', 't'][x] + Sequence['a', 'i', 'u', 'e', 'o'][y] }
  # MultiArray(OBJECT,2):
  # [ [ "na", "pa", "ra", "ta" ],
  #   [ "ni", "pi", "ri", "ti" ],
  #   [ "nu", "pu", "ru", "tu" ],
  #   [ "ne", "pe", "re", "te" ],
  #   [ "no", "po", "ro", "to" ] ]
  s = lazy(33) { |i| 3 * (i+1) }
  # Sequence(INT):
  # [ 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, .... ]
  s.mask((s % 2).eq(0)).collect { |i| i ** 2 / 3 }
  # Sequence(INT):
  # [ 12, 48, 108, 192, 300, 432, 588, 768, 972, 1200, 1452, 1728, .... ]

[1] https://github.com/wedesoft/multiarray [2] http://www.wedesoft.de/hornetseye-api/

egonschiele13y ago

Oh wow, that is so wrong and so beautiful.

j / k navigate · click thread line to collapse