Ruby Y Combinator

y.rb

def Y f = nil, &b
  f ||= b; -> g { g[g] }.call -> g { f[-> *a { g[g][*a] }] }
end

# Hash Autovivification
Y { |d| -> { Hash.new do |h, k| h[k] = d[] end }[]

# Factorial
Y -> fac { -> n { n <= 2 ? n : n * fac[n - 1] } }


# Alternate Using Monkey-Patched Proc
Proc.class_eval do
  def y
    this = self
    -> g { g[g] }[-> g { -> *a { g[g].instance_exec *a, &this } }]
  end
end

# Factorial
-> n { n <= 2 ? n : n * self[n - 1] }.y[5]
# => 120

And for hash autovivification:

-> { Hash.new do |h, k| h[k] = self[] end }.y[]

fix1 = ->f{->g{g[g]}[->g{f[->*a{g[g][*a]}]}]}
fix2 = ->f{->g{g[g]}[->g{->*a{g[g].instance_exec(*a,&f)}}]}