G()('al')(github.com)

37 pointsby eatnumber1vor 12 Jahren46 comments

https://github.com/eatnumber1/goal

48 comments

I'm pretty sure this is cheating, but...

  #!/bin/sh
  grep -e "g\(()\)*('al')" $0 | sed 's/()/o/g' | sed "s/('al')/al/"
  exit
  g()('al')

On its own, g()('al') is valid but g()()('al') is a syntax error. Also, bash and sh don't have non-numeric return values, so regardless of the solution you're stuck with printing.

eatnumber1vor 12 Jahren

This is a great incomplete solution. Can you submit a pull-request for it?

csandreasenvor 12 Jahren

Just to clarify, I think it's a non-solution because the g()('al') is never actually executed - grep does all the work. You can add as many parenthesis as you want. The "exit" is there because you'll get a syntax error if you try to execute g()()('al'). g()('al') by itself does nothing (I'm not even sure why it's not flagged as a syntax error).

eatnumber1vor 12 Jahren

The three people who are judging this had a long discussion over this case, and basically decided that since it is in the file, it's fine. We frown on it a little because it's basically self-modifying code, but it's better than nothing for bash.

tolmaskyvor 12 Jahren

I actually prefer it vs. frowning on it for the record ;)

pathikritvor 12 Jahren

Quite trivial in idiomatic Scala:

  case class Goal(count: Int) {
    def apply() = Goal(count + 1)
    def apply(s: String) = s"g${"o" * count}$s"
  }

  object Goal {
    def g() = Goal(1)
  }

Non-idiomatic shorter answer:

  case class g(p: String = "go") {
    def apply() = g(p + "o")
    def apply(s: String = "o") = p + s
  }

tolmaskyvor 12 Jahren

This looks great! Can you submit a pull request? These things are coming in really fast and its way easier to take that way.

stevenheidelvor 12 Jahren

Also add

g(s: String) = "gal"

to handle the case of no 'o's.

juolvor 12 Jahren

    using System;
    using System.Text;

    namespace Goal
    {
        class Program
        {
            static void Main(string[] args)
            {
                Console.WriteLine(G("al"));
                Console.WriteLine(G()("al"));
                Console.WriteLine(G()()()()()("al"));
            }

            static StringBuilder accumulator = new StringBuilder();
            delegate dynamic GoalDelegate(string s = "o");

            static dynamic G(string s = "o")
            {
                switch (s)
                {
                    case "o":
                        accumulator.Append("o");
                        return (GoalDelegate)G;
                    case "al":
                        string result = "G" + accumulator.ToString() + "al";
                        accumulator.Clear();
                        return result;
                    default:
                        return null;
                }
            }
        }

    }

recursivevor 12 Jahren

Here's my C# solution that uses no global state, and features a cool (I think) variadic lambda.

    using System;
    using System.Linq;

    class Program {
        static void Main() {
            Console.WriteLine(g("al"));
            Console.WriteLine(g()("al"));
            Console.WriteLine(g()()("al"));
            Console.WriteLine(g()()()("al"));
            Console.WriteLine("Press any key to continue.");
            Console.ReadKey();
        }

        delegate dynamic Goal(params string[] args);

        static dynamic g(string suffix = null, string prefix = "g") {
            if (suffix == "al") return prefix + suffix;
            return (Goal)(a => g(a.FirstOrDefault(), prefix + "o"));
        }
    }

It was a fun exercise. I found that creating a github pull request was not such a fun exercise, and took about 10x longer than the actual task at hand. Perhaps I could learn to do it more smoothly with practice.

eatnumber1vor 12 Jahren

Cool! Can you submit a pull request?

[deleted]vor 12 Jahren

(1)

zeeboovor 12 Jahren

I made this Go one that might be cheating because instead of the ("al") call returning a string, you get a value that implements the String method. Works with fmt.Println :)

http://play.golang.org/p/zrp2sO6ZLU

eatnumber1vor 12 Jahren

This is awesome! Can you add a main method with an example of it working and submit a pull request?

personZvor 12 Jahren

This is close to identical to what I did, but the use of a global just felt too dirty. Still trying to figure out the use of a closure on it.

eatnumber1vor 12 Jahren

Definitely submit a pull-request if you figure out how to do it :)

bobbyi_settvvor 12 Jahren

The "complete" solutions for Python in the repo don't actually work because they don't reset the state. If you add a final line of:

  print g('al')

for solution1 or:

  print m('rton')

for solution2, you'll find they don't print gal/ mrton.

Here's what I came up with before looking at those solutions that seems to work:

  def g(arg=None):
      if arg == 'al':
          return 'gal'
      def inner(arg=None):
          if arg == 'al':
              return 'g' + (inner.counter * 'o') + 'al'
          inner.counter += 1
          return inner
      inner.counter = 1
      return inner

eatnumber1vor 12 Jahren

Resetting the state isn't one of the rules ;)

I'd make a solution which resets the state linked by the table though, so please submit one!

bobbyi_settvvor 12 Jahren

Rule 7 says:

   g('al') must return "gal".

If you add

   assert g('al') == 'gal'

as the last line of solution 1, the assertion will not pass. I'm not saying there is a rule that mentions "resetting state". I'm saying that their failure to reset state is the bug leading to them not following the rules.

nickdepinetvor 12 Jahren

Thanks for pointing this out, it's now fixed.

https://github.com/eatnumber1/goal/pull/18

bobbyi_settvvor 12 Jahren

Nick,

That helps but doesn't completely solve the state problem. Try adding these two lines to the end of the program:

    g()()()()()
    print g('al')

I think 'gal' should be printed here. Rule 7 says that's what your function should return given 'al', and the fact that we've called it in some other way before doesn't change the fact that it is required to return 'gal' given 'al'.

The solution I posted above handles this scenario (and, as a bonus, is thus threadsafe in case multiple people were doing g()()()('al') simultaneously).

morganhankinsvor 12 Jahren

  # goal.rb
  module G
    def [](o=nil)
      o ? "g#{values[0].to_s}al" : ({ :n => values[0].to_s + "o" }.extend G)
    end
  end

  g = {}.extend G
  
  eval DATA.read.gsub("(", "[").gsub(")", "]")

__END__

  gal = g('al')
  p gal
  
  goal = g()('al')
  p goal

  goooooooal = g()()()()()()()('al')
  p goooooooal
  
  # ruby goal.rb
  # "gal"
  # "goal"
  # "goooooooal"

# https://gist.github.com/morganhankins/de13ee378907143a7789

GuiAvor 12 Jahren

The table at the bottom is interesting. I'd love to read articles about why it's impossible in language X. E.g. I'm familiar with C/Python and can implement a solution, and am surprised that it'd be impossible in Ruby - I don't know Ruby, and would love to know why.

More generally, what properties must a programming language satisfy for this problem to be solvable in it?

My current intuition is that if a language does not denote function calls with () enclosing arguments, AND if a language does not support metaprogramming (eg. LISP macros), then the problem is not solvable in it. Still thinking about it.

jchengvor 12 Jahren

Ruby and Java both lack first class functions--functions can't return values that can be directly called with () (unless this has changed with Java 8?).

eatnumber1vor 12 Jahren

For the impossible bullets, they link to informal arguments as to why it's impossible. The ruby one has code - it's a syntax error.

GuiAvor 12 Jahren

The bullets link to non-solutions, which are not enough to formally prove that it is impossible to solve this challenge in the language.

eatnumber1vor 12 Jahren

Agreed. These are informal. If you want to make a formal proof that it's impossible, I'll make a new category.

nathangriggvor 12 Jahren

Python:

    def g(a='', n=0):
        return 'g '+ 'o'*n + a if a else lambda a='': g(a, n+1)

kylecvor 12 Jahren

Ugh, so many solutions that mutate internal state. I went for a more purely functional approach and kept track of the count with a function parameter (JavaScript):

https://github.com/kylecronin/goal/commit/70c9277ec99f80dca2...

Veritaasvor 12 Jahren

The ruby non-solution seen here:

https://github.com/eatnumber1/goal/blob/master/non-solutions...

is actually doable if you parse the error.

capicuevor 12 Jahren

But it requires eval-ing a string literal instead of actually having g()("al") as an expression itself

jsmeatonvor 12 Jahren

Here's my attempt in python. Started off with the traditional approach but added a functional one too.

https://gist.github.com/jarshwah/0998143317f5ccc093b3

tantalorvor 12 Jahren

This is a syntax error in perl, so impossible?

  > perl -e "g()()"
  syntax error at -e line 1, near ")("
  Execution of -e aborted due to compilation errors.

eatnumber1vor 12 Jahren

Can you submit a pull request saying as much in the same format as the Ruby one?

dima55vor 12 Jahren

Nah, you can use a source filter. "man perlfilter"

jnbichevor 12 Jahren

Do languages that don't use () as a function call still have to use the same syntax? Or is () just a stand-in for a function call?

eatnumber1vor 12 Jahren

Languages that don't use () still have to use the same syntax. See the haskell solution.

krzysz00vor 12 Jahren

Not too difficult in Common Lisp (the parser is fully exposed to user code, so I just changed the meaning of "g").

eatnumber1vor 12 Jahren

Glad to hear it :)

Can you submit a solution then?

krzysz00vor 12 Jahren

I'm pull request #36.

eudoxvor 12 Jahren

This could be an interesting use case for Common Lisp reader macros.

eatnumber1vor 12 Jahren

Agreed! Please submit a solution!

[deleted]vor 12 Jahren

whoopdedovor 12 Jahren

Easy in Lua. You can even write G()'al'

eatnumber1vor 12 Jahren

Great! Can you submit a pull request?

whoopdedovor 12 Jahren

Straightforward solution:

    function g(o)
      if o then return "gal" end
      o = "o"
      local function go(al)
        if al then
          return "g"..o..al
        else
          o = o .. "o"
          return go
        end
      end
      return go
    end
    print(g()('al'))