Can I reference a lambda from within itself using Ruby?

Question

I want to be able to call an anonymous lambda from within itself using Ruby. Consider the following recursive block (returns a factorial). I know I can assign it to a variab

Answer 1

In addition to KL-7's comment, here's a Y combinator solution:

lambda { |f|
  lambda { |x| x.call(x) }.call(
  lambda { |x| f.call( lambda { |v| x.call(x).call(v) } ) } )
}.call(
  lambda { |f|
    lambda { |n| n == 0 ? 1 : n * f.call(n - 1) }
  }
).call(5) #=> 120

You would normally split these:

y = lambda { |f|
  lambda { |x| x.call(x) }.call(
  lambda { |x| f.call( lambda { |v| x.call(x).call(v) } ) } )
}

fac = y.call(
  lambda { |f| lambda { |n| n == 0 ? 1 : n * f.call(n - 1) } }
)

fac.call(5) #=> 120

Note that although fac is being assigned, it is not used within the lambda.

I'd use Ruby's -> syntax and .() instead of .call():

y = ->(f) {
  ->(x) { x.(x) }.(
  ->(x) { f.(->(v) { x.(x).(v) }) } )
}

fac = y.(->(f) {
  ->(n) { n == 0 ? 1 : n * f.(n - 1) }
})

fac.(5) #=> 120

The y invocation can be simplified a bit by using curry:

y = ->(f) {
  ->(x) { x.(x) }.(
  ->(x) { f.curry.(->(v) { x.(x).(v) }) } )
}

fac = y.(
  ->(f, n) { n == 0 ? 1 : n * f.(n - 1) }
)

fac.(5) #=> 120