Scala版の焼き直しです。
Elixirっぽさがでているかどうかは微妙。

問題2

Problem2.ex

defmodule Problem2 do
	def mutually_list(a, b) do
		Enum.zip(a,b) |> Enum.flat_map(fn {x,y} -> [x,y] end)
	end
end

問題3

Problem3.ex

defmodule Problem3 do
	def fib do
		Stream.unfold({0, 1}, fn {a,b} -> {a , {b, a + b}} end)
		|> Enum.take(100)
	end
end

問題4

Problem4.ex

defmodule Problem4 do
    def permutations([]), do: [[]]
    def permutations(list) do
        for h <- list, t <- permutations(list -- [h]), do: [h | t]
    end

    def max(list) do
        permutations(list) 
        |> Enum.map(fn p -> Enum.map(p, fn i -> Integer.to_string(i) end) |> List.to_string end) 
        |> Enum.max
    end
end

問題5

Problem5.ex

defmodule Problem5 do
	def calc(str, acc) do
		case {Regex.run(~r/(^[1-9]+)(.*)/, str), Regex.run(~r/([+-])([1-9]+)(.*)/, str)} do
			{[_, n, last], _} -> calc(last, acc + String.to_integer(n))
			{_, [_, "+", n, last]} -> calc(last, acc + String.to_integer(n))
			{_, [_, "-", n, last]} -> calc(last, acc - String.to_integer(n))
			_ -> acc
		end
	end
		
	def comb do
		Enum.to_list(2..9)
		|> List.foldl(["1"], fn (x, acc) -> Enum.flat_map(acc, fn n -> ["#{n}#{x}", "#{n}+#{x}", "#{n}-#{x}"] end) end)
	end
	
	def find do
		Enum.filter(comb, fn str -> calc(str, 0) == 100 end)
	end
end