[inopinatus]

or even¹

   h = g.curry(2)[f]
   h[1]

function composition is available:

   f = -> x { x * 2 }
   g = -> y { y + 4 }
   j = f >> g
   k = j << g

   j[1] #=> 6
   k[1] #=> 14

and, topically, the Y combinator:

    y = -> f {
        -> g { g[g] } [
        -> g { f[-> v { g[g][v] }] }
      ]
    }

hence

    fib = y[-> f {
      -> n { n < 2 ? Array(0..n) : f[n-1].then {  _1 << _1[-1] + _1[-2] } }
    }]

    fib[6] #=> [0, 1, 1, 2, 3, 5, 8]

although the more idiomatic Ruby might be

    fibo = Hash.new { |this, n| this[n] = n < 2 ? n : this[n-1] + this[n-2] }

    (0..6).map(&fibo) #=> [0, 1, 1, 2, 3, 5, 8]

in which the Hash self-converts to a closure via the & operator.

Anyone who says "Ruby doesn't have first-class functions" can fight me.

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⁽¹⁾ notwithstanding that I recall Matz once remarked the curry method is only included as an elaborate joke.