Brainfuck interpreter in Ruby

bf.rb

#!/usr/bin/env ruby
mem  = Array.new(30_000)
iptr = dptr = 0
prog = File.read ARGV.first
jmp  = lambda { |op, a, b|
  (iptr = iptr.send(op, 1); jmp.call(op, a, b) if prog[iptr] == a) until prog[iptr] == b
}
nop  = lambda {}
cmds = {
        '>' => lambda { dptr += 1 },
        '<' => lambda { dptr -= 1 },
        '+' => lambda { mem[dptr] = mem[dptr].to_i + 1 },
        '-' => lambda { mem[dptr] = mem[dptr].to_i - 1 },
        '.' => lambda { $stdout.printf mem[dptr].chr },
        ',' => lambda { mem[dptr] = $stdin.readbyte  },
        '[' => lambda { jmp.call(:+, '[', ']') if     mem[dptr].to_i.zero? },
        ']' => lambda { jmp.call(:-, ']', '[') unless mem[dptr].to_i.zero? }
       }
(cmds.fetch(prog[iptr], nop).call; iptr += 1) while prog[iptr]

hello_world.bf

 
+++++ +++++             initialize counter (cell #0) to 10
[                       use loop to set the next four cells to 70/100/30/10
    > +++++ ++              add  7 to cell #1
    > +++++ +++++           add 10 to cell #2 
    > +++                   add  3 to cell #3
    > +                     add  1 to cell #4
    <<<< -                  decrement counter (cell #0)
]                   
> ++ .                  print 'H'
> + .                   print 'e'
+++++ ++ .              print 'l'
.                       print 'l'
+++ .                   print 'o'
> ++ .                  print ' '
<< +++++ +++++ +++++ .  print 'W'
> .                     print 'o'
+++ .                   print 'r'
----- - .               print 'l'
----- --- .             print 'd'
> + .                   print '!'
> .                     print '\n'