Recursive Proc in Crystal

Question

Is recursive proc posible in Crystal?

Something like lambda in Ruby

I'm trying to do a y-combinator in Crystal,something like Ruby one:

puts -> {
  fact_improver = ->(partial) {
    -> (n) { n.zero? ? 1 : n * partial.(n-1) }
  }
  y = ->(f) {
    ->(x) { f.(->(v) { x.(x).(v) }) }.(
    ->(x) { f.(->(v) { x.(x).(v) }) }
    )
  }
  fact = y.(fact_improver)
  fact = fact_improver.(fact)
  fact.(100)
}.()

The above code was taken from Y Not- Adventures in Functional Programming

Disorderly

Brian Cardiff says you should look at github.com/crystal-lang/crystal/issues/… 👍 – Pareto

Answer 1

As far as I know Crystal does not have recursive procs. But to create Y combinator you don't need a recursive proc. Actually, according to the definition:

In functional programming, the Y combinator can be used to formally define recursive functions in a programming language that doesn't support recursion.

Here is an example of Y combinator written in Crystal using recursive types:

alias T = Int32
alias Func = T -> T
alias FuncFunc = Func -> Func
alias RecursiveFunction = RecursiveFunction -> Func

fact_improver = ->(partial : Func) {
  ->(n : T) { n.zero? ? 1 : n * partial.call(n - 1) }
}

y = ->(f : FuncFunc) {
  g = ->(r : RecursiveFunction) { f.call(->(x : T) { r.call(r).call(x) }) }
  g.call(g)
}

fact = y.call(fact_improver)
fact = fact_improver.call(fact)
fact.call(5) # => 120

UPDATE: it is possible to create recursive proc in Crystal with uninitialized keyword:

g = uninitialized Int32 -> Int32
g = ->(n : Int32) { n.zero? ? 1 : n * g.call(n - 1) }
g.call(5) # => 120

Thanks to @mgarciaisaia for the comment.

Dody