compose.rb

if Gem::Version.new("2.6") <= Gem::Version.new(RUBY_VERSION)
  def compose(*funcs)
    funcs.map(&:to_proc).reduce(:<<)
  end
else
  def compose(*funcs)
    funcs.map(&:to_proc).reduce { |f, g| ->(*x) { f.call(g.call(*x)) } }
  end
end

def a(x)
  "a(#{x})"
end

def b(x)
  "b(#{x})"
end

def c(x)
  "c(#{x})"
end

results = %w[functional programming].map(&compose(method(:a), method(:b), method(:c)))
expected = ["a(b(c(functional)))", "a(b(c(programming)))"]

raise "unexpected results! 😵\nactual:   #{results}\nexpected: #{expected}" unless results == expected
puts "✅ #{results}"