brainfuck, recursive interpreter implementation, cuz why not.

bf-fn.lua

function abs(x)
  if x < 0 then
    return -1
  else
    return 1
  end
end

oper = {}

oper['+'] = function(prog, instr, data, ptr)
  data[ptr] = (data[ptr] or 0) + 1
  return step(prog, instr+1, data, ptr)
end

oper['-'] = function(prog, instr, data, ptr)
  data[ptr] = (data[ptr] or 0) - 1
  return step(prog, instr+1, data, ptr)
end

oper['>'] = function(prog, instr, data, ptr)
  return step(prog, instr+1, data, ptr+1)
end

oper['<'] = function(prog, instr, data, ptr)
  return step(prog, instr+1, data, ptr-1)
end

oper['.'] = function(prog, instr, data, ptr)
  io.write(string.char(data[ptr] or 0))
  return step(prog, instr+1, data, ptr)
end

oper[','] = function(prog, instr, data, ptr)
  data[ptr] = string.byte(io.read(1))
  return step(prog, instr+1, data, ptr)
end

oper['['] = function(prog, instr, data, ptr)
  if (data[ptr] or 0) == 0 then
    return seek(prog, instr+1, data, ptr, 1)
  else
    return step(prog, instr+1, data, ptr)
  end
end

oper[']'] = function(prog, instr, data, ptr)
  if (data[ptr] or 0) ~= 0 then
    return seek(prog, instr-1, data, ptr, -1)
  else
    return step(prog, instr+1, data, ptr)
  end
end

function seek(prog, instr, data, ptr, loop)
  if loop == 0 then
    return step(prog, instr, data, ptr)
  elseif prog[instr] == '[' then
    return seek(prog, instr+abs(loop), data, ptr, loop+1)
  elseif prog[instr] == ']' then
    return seek(prog, instr+abs(loop), data, ptr, loop-1)
  else
    return seek(prog, instr+abs(loop), data, ptr, loop)
  end
end

function step(prog, instr, data, ptr)
  if instr > #prog then
    return data, ptr
  else
    local fn = oper[prog[instr]]
    if fn then
      return fn(prog, instr, data, ptr)
    else
      return step(prog, instr+1, data, ptr)
    end
  end
end

function run(filename, ...)
  if filename == nil then return end
  local str = io.open(filename):read('*a')
  local prg = {}
  str:gsub(".", function(c) table.insert(prg, c) end)
  step(prg, 1, {}, 1)
  return run(...)
end

run(...)