This is a brainfuck interpreter written using little more than lambdas, numbers, and a whole lot of symbols.

legibler_brainfuck.rb

SIZE = -> coll {
  coll == [] || coll == '' ? 0 : 1 + SIZE[coll[1..-1]]
}

MAP = -> coll, &fn {
  coll == [] ? [] : [fn[coll[0]]] + MAP[coll[1..-1], &fn]
}

KEYS   = -> hash { MAP[[*hash], &-> h { h[ 0] }] }
VALUES = -> hash { MAP[[*hash], &-> h { h[-1] }] }

INVERT = -> hash {
  ks, vs = KEYS[hash], VALUES[hash]
  sz = SIZE[ks]
  ret = {}
  i = 0

  (inverter = -> {
    ret[vs[i]] = ks[i]
    (i += 1) == sz ? ret : inverter[]
  })[]
}

MERGE = -> a, b {
  all = [*a] + [*b]
  sz  = SIZE[all]
  ret = {}
  i = 0

  (merger = -> {
    ret[all[i][0]] = all[i][1]
    (i += 1) == sz ? ret : merger[]
  })[]
}

brainfuck = -> prog {
  tape = []
  sz = SIZE[prog]
  pc = bp = i = 0
  jumps, stack = {}, []

  (jumper = -> {
    prog[i] == '[' ? (stack << i) : 0
    prog[i] == ']' ? (jumps[stack[-1]] = i; stack = stack[0..-2]) : 0
    (i += 1) == sz ? 0 : jumper[]
  })[]

  jumps = jumps == {} ? {} : MERGE[jumps, INVERT[jumps]]

  (brainfuck_p = -> {
    prog[pc] ==
    '+' ? (tape[bp] ||= 0; tape[bp] += 1) : prog[pc] ==
    '-' ? (tape[bp] ||= 0; tape[bp] -= 1) : prog[pc] ==
    '>' ? (bp += 1) : prog[pc] ==
    '<' ? (bp -= 1) : prog[pc] ==
    '[' ? (pc = (tape[bp] || 0) == 0 ? jumps[pc] : pc) : prog[pc] ==
    ']' ? (pc = (tape[bp] || 0) != 0 ? jumps[pc] : pc) : prog[pc] ==
    '.' ? ($> << ('' << tape[bp])) : 0

    (pc += 1) == sz ? prog : brainfuck_p[]
  })[]
}

brainfuck[File.read('hw.bf')]