[GuiA]

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.

[jcheng]

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?).

[eatnumber1]

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.

[GuiA]

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

[eatnumber1]

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