The Boston Diaries

Friday, January 27, 2012

99 ways to program a hex, Part 19: Lua, recursion, closure as callback

Now, instead of passing along data just to be passed to the callback function, we can include such data as part of a closure to the function we pass to the do_dump() function.

#!/usr/bin/env lua
-- ***************************************************************
--
-- Copyright 2010 by Sean Conner.  All Rights Reserved.
--
-- This program is free software: you can redistribute it and/or modify
-- it under the terms of the GNU General Public License as published by
-- the Free Software Foundation, either version 3 of the License, or
-- (at your option) any later version.
--
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
--
-- You should have received a copy of the GNU General Public License
-- along with this program.  If not, see <http://www.gnu.org/licenses/>.
--
-- Comments, questions and criticisms can be sent to: sean@conman.org
--
-- ********************************************************************

-- Style: Lua 5.1, recursion, closure as callback

function do_dump(fpout,offset,callback)
  local line = callback(offset)
  if line == nil then return end
  fpout:write(
  	string.format("%08X: ",offset),
  	line:gsub(".",function(c) return string.format("%02X ",c:byte()) end),
  	string.rep(" ",3 * (16 - line:len())),
  	line:gsub("%c","."),
  	"\n"
  )
  return do_dump(fpout,offset + 16,callback)
end

-- **********************************************************************

if #arg == 0 then
  print("-----stdin-----")
  do_dump(io.stdout,0,cb,io.stdin)
else
  for i = 1 , #arg do
    local f = io.open(arg[1],"r")
    io.stdout:write("-----",arg[1],"-----","\n")
    do_dump(io.stdout,0,function(offset) return f:read(16) end)
    f:close()
  end
end

os.exit(0)

Here, our function (which is not named, as you don't really need to name functions in Lua) references our open file f, but in order to do so, Lua needs to include a reference to f to the function when said function is passed to do_dump(). It does so by creating what's called a “closure”—think of a closure as both a pointer (or reference) to a function, plus a pointer (or reference) to data that is outside the normal lexical scope of the function.

And why do I pass in the offset when my unnamed (“anonymous”) function doesn't use it? Because it might be useful in some contexts to know where to pull the data (say from a block of memory).

Friday, January 27, 2012

99 ways to program a hex, Part 19: Lua, recursion, closure as callback

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