Question

Here’s Mathematica’s Nest function Definition. What’s the eqv. in Ruby?

The idea is this:

nest(f, x, 3) #=> f(f(f(x)))

Answer 1

You could define your own using inject:

def nest(f, x, n)
  n.times.inject(x) { |m| f.call(m) }
end

Then you could call it like this:

>> def f(x) 2*x end
>> nest(method(:f), 1, 3)
=> 8

If you want a function back (i.e. leave x unspecified) then you could return a lambda:

def nestx(f, n)
  ->(x) { n.times.inject(x) { |m| f.call(m) } }
end

and use it like this:

>> nestx(method(:f), 3).call(1)
=> 8

Or you could rearrange the nest arguments and use Proc#curry:

def nest(f, n, x)
  n.times.inject(x) { |m| f.call(m) }
end

>> method(:nest).to_proc.curry.call(method(:f), 3).call(1)
=> 8

You can also use [] in place of call if want something that looks more like a function call:

def nest(f, n, x)
  n.times.inject(x) { |m| f[m] }
end

>> method(:nest).to_proc.curry[method(:f), 3][1]
=> 8