Ruby Koan 151 raising exceptions

I'm going through the ruby koans, I'm on 151 and I just hit a brick wall.

Here is the koan:

# You need to write the triangle method in the file 'triangle.rb'
require 'triangle.rb'

class AboutTriangleProject2 < EdgeCase::Koan
  # The first assignment did not talk about how to handle errors.
  # Let's handle that part now.
  def test_illegal_triangles_throw_exceptions
    assert_raise(TriangleError) do triangle(0, 0, 0) end
    assert_raise(TriangleError) do triangle(3, 4, -5) end
    assert_raise(TriangleError) do triangle(1, 1, 3) end
    assert_raise(TriangleError) do triangle(2, 4, 2) end
 end
end

Then in triangle.rb we have:

def triangle(a, b, c)
  # WRITE THIS CODE
  if a==b && a==c
    return :equilateral
  end
  if (a==b && a!=c) || (a==c && a!=b) || (b==c && b!=a)
    return :isosceles
  end
  if a!=b && a!=c && b!=c
    return :scalene
  end
  if a==0 && b==0 && c==0
    raise new.TriangleError
  end



end

# Error class used in part 2.  No need to change this code.
class TriangleError < StandardError

end

I am beyond confused - any help at all would be much appreciated!

EDIT: To complete this koan, I need to put something in the TriangleError class - but I have no idea what

UPDATE: Here is what the koan karma thing is saying:

<TriangleError> exception expected but none was thrown.

1. A triangle should not have any sides of length 0. If it does, it's either a line segment or a point, depending on how many sides are 0.
2. Negative length doesn't make sense.
3. Any two sides of a triangle should add up to more than the third side.
4. See 3, and focus on the "more".

You shouldn't need to change the TriangleError code, AFAICS. Looks like your syntax is just a little wacky. Try changing

raise new.TriangleError

to

raise TriangleError, "why the exception happened"

Also, you should be testing the values (and throwing exceptions) before you do anything with them. Move the exception stuff to the beginning of the function.

You forgot the case when a,b, or c are negative:

def triangle(a, b, c)
  raise TriangleError if [a,b,c].min <= 0
  x, y, z = [a,b,c].sort
  raise TriangleError if x + y <= z
  [:equilateral,:isosceles,:scalene].fetch([a,b,c].uniq.size - 1)
end

Ended up doing this:

def triangle(a, b, c)
  a, b, c = [a, b, c].sort
  raise TriangleError if a <= 0 || a + b <= c
  [nil, :equilateral, :isosceles, :scalene][[a, b, c].uniq.size]
end

Thanks to commenters here :)

def triangle(a, b, c)
  [a, b, c].permutation do |sides|
    raise TriangleError unless sides[0] + sides[1] > sides[2]
  end
  case [a,b,c].uniq.size
    when 3; :scalene
    when 2; :isosceles
    when 1; :equilateral
  end
end