Lists, and the concept of "head" and "tail", coupled with recursion are at the core of functional programming. This demonstrates how you can do it in Ruby, without ever writing a loop.

functional_ruby.rb

# foldl() is fundamental. With a fold, you can do everything else.
def foldl(list, acc, &fn)
  if list == [] # base case, return the accumulator
    acc
  else
    head, *tail = list
    foldl(tail, fn.call(acc, head), &fn) #recurse on the remainder
  end
end

# now we can write reverse()
def reverse(list)
  foldl(list, []) {|acc, v| [v, *acc]}
end

# and we can write foldr (reverse foldl)
def foldr(list, acc, &fn)
  foldl(reverse(list), acc, &fn)
end

# we can write map
def map(list, &fn)
  reverse(foldl(list, []) {|acc, v| [fn.call(v), *acc]})
end

# we can write foreach
def foreach(list, &fn)
  if list != []
    head, *tail = list
    fn.call(head)
    foreach(tail, &fn)
  end
end

# etc

foldl([1, 2, 3], 0) {|acc, n| acc + n} # => 6
reverse([1, 2, 3]) # => [3, 2, 1]

@matthayter yeah absolutely, but it was fun to observe the the splat really simplifies doing head|tail and cons operations in Ruby, without mutating data.

@d11wtq Totally, those splats are awesome, I can't believe how clean that code is. I doubt it would be any more concise if written in Haskell.

@matthayter without pattern matching you have the annoying conditionals, which is slightly less clean than the haskell/erlang equivalent:

foldl(_, Acc, []) -> Acc;
foldl(Fn, Acc, [Head|Tail]) ->
  foldl(Fn, Fn(Acc, Head), Tail).

RubyVM::InstructionSequence.compile_option = {
  tailcall_optimization: true,
  trace_instruction: false,
}

https://speakerdeck.com/jeg2/10-things-you-didnt-know-ruby-could-do?slide=46
(not really practical, only available in a sub-parser, I think.)

@pda wow, that's interesting. Presumably that will find its way into the default set of compile options then.