multiple value return in lua

Lua functions can return multiple values, and the language will natively assign them to the variables on the other side of the equal sign.

local a, b = (function() return 1,2 end)()
print(a,b) => 1     2

That's fine, but when things get a bit more tricky is when the values are not returned in tail call position.

local a, b = (function()
               local res = (function() return 1 , 2 end)()
               return res
             end)()
print(a, b) =>  1    nil

The catch is that lua assigns the 'rest' of the values only to the last element of tables, or argument lists. If we want to wrap a function into another while not being in tail position, we have to use a little trick. This trick is unpack.

local a, b = (function()
               local res = {(function() return 1 , 2 end)()}
               return unpack(res)
             end)()
print(a, b) =>  1    2

Presenting Eva

I've hacked a tiny little lisp interpreter in lua, and I called Eva, for obvious reasons.

It's not pretty, it's not complete at all. Hell, it doesn't even have strings, you can't call native lua functions, and there are no conses (although you could get them using functions for that).

The whole point of it was to have some fun implementing a tiny lisp-like thingie, and to make it in lua, which is a language that I like quite a lot (although some of its table missfeatures make me cry sometimes)

Without further ado, Eva .

erc makes you faster at jokes!

Hi, today's post is a simple sinppet I wrote to ease the pain of looking for recurrent references on the internet.

At work we use IRC heavily (it's our main channel for comunication). As you know,every platform or channel of comunication has its own processes and joke (slightly related (but interesting nonetheless): The medium is the Massage ) . So IRC is mostly about sending links to gifs and to funny images, and funny images with captions in them.

It's so simple that it doesn't even need explanation. You just have to know that all defuns named "erc-cmd-SOMETHING" will be automatically callable from erc as a /something.

(defun erc-cmd-MEME (wat)
  "Fetch links from the alist of knowledge"
  (let ((h '(("trap" "https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiM1KvHzrpRDpci5HCGe7mjpaRQ3uKRgNjbyTpUG6B8nG_qXnxGT2ccuskYnK_FlvsOLxX8jYIJIHsWYYAlMCqP77UaQRSrBkoILLHy3noG7cHhGcYpUrLA3iMGkczBE8FS-7G5sDuu9X_1/s1600/original.jpg"))))
    (erc-send-message (cadr (assoc wat h)))))

Combined with erc-image.el, it makes a really nice IRC experience :)

git reset. again

This is shamelessly copied from the web archive.

Assume you start from a "everything is committed and golden" state. Let's pretend you came to this starting point via a git checkout: and then you edit a bunch of stuff, compile/test, and then git add and git commit:

$ git checkout mywork
...edit edit edit...
$ git add ...
$ git commit

If at this point you do a git reset, here's how the type of reset ("soft", "hard", or the default, which is "mixed") affects things:

$ git checkout mywork
            # --hard resets to this point
...edit edit edit...
            # --mixed (default) resets to this point
$ git add ...
            # --soft resets to this point
$ git commit

Also note, as gitster says, that git commit --amend makes the git reset --soft mostly redundant.

sed vs unix

 UNIX         |  SED
 -------------+----------------------------------------------------------------
 cat          |  sed ':'
 cat -s       |  sed '1s/^$//p;/./,/^$/!d'
 tac          |  sed '1!G;h;$!d'
 grep         |  sed '/patt/!d'
 grep -v      |  sed '/patt/d'
 head         |  sed '10q'
 head -1      |  sed 'q'
 tail         |  sed -e ':a' -e '$q;N;11,$D;ba'
 tail -1      |  sed '$!d'
 tail -f      |  sed -u '/./!d'
 cut -c 10    |  sed 's/\(.\)\{10\}.*/\1/'
 cut -d: -f4  |  sed 's/\(\([^:]*\):\)\{4\}.*/\2/'
 tr A-Z a-z   |  sed 'y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/'
 tr a-z A-Z   |  sed 'y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/'
 tr -s ' '    |  sed 's/ \+/ /g'
 tr -d '\012' |  sed 'H;$!d;g;s/\n//g'
 wc -l        |  sed -n '$='
 uniq         |  sed 'N;/^\(.*\)\n\1$/!P;D'
 rev          |  sed '/\n/!G;s/\(.\)\(.*\n\)/&\2\1/;//D;s/.//'
 basename     |  sed 's,.*/,,'
 dirname      |  sed 's,[^/]*$,,'
 xargs        |  sed -e ':a' -e '$!N;s/\n/ /;ta'
 paste -sd:   |  sed -e ':a' -e '$!N;s/\n/:/;ta'
 cat -n       |  sed '=' | sed '$!N;s/\n/ /'
 grep -n      |  sed -n '/patt/{=;p;}' | sed '$!N;s/\n/:/'
 cp orig new  |  sed 'w new' orig
 hostname -s  |  hostname | sed 's/\..*//'

shamelessly copypasted from here

More Lua iterators

After the permutations post, I thought that we needed something like a 'take' function, that would allow us to use infinite streams safely.

local function take(n, it , param, state)
  local count = 0
  return function()
    count = count+1
    if count <= n then
      return it()
    end
    return nil
  end
end


local function take_while(fun, it , param, state)
  return function()
    if fun() then
      return it()
    end
    return nil
  end
end

Simple but nice.

Permutations in Lua. An iterator example

Here's some code I just wrote when messing with an algorithm to generate permutations. The code is just perfect to be used in the form of an iterator, and although I was reading the example in Perl (I read it in Higher Order Perl, an amazing book no matter what's your programming language of choice), I thought Lua would be a good candidate for that.

Along the way, I wrote a few utility functions you can see in there. Mostly tests on function composition and mapping over iterators.

Here's the code. If you need to generate permutations, I found this algorithm (which I don't know the name, but let's call it 'odometer counting') very easy to implement and understand. At least easier than Randal's way of doing it.

local inspect = require'inspect'

function map(f, t)
  local r = {}
  if type(t) == 'table' then
    for _, x in ipairs(t) do
      r[#r+1] = f(x)
    end
  else
    for x in t do
      r[#r+1] = f(x)
    end
  end
  return r
end

function count()
  local c = 0
  return function()
    c = c + 1
    return c
  end
end

-- for x in count() do
--   print(x)
--   if x > 10 then break end
-- end

function permutations(...)
  local function inc(t, pos)
    if t[pos][3] == t[pos][2] then
      if pos == 1 then return nil end
      t[pos][3] = 1
      return inc(t, pos-1)
    else
      t[pos][3] = t[pos][3] + 1
      return true
    end

  end

  local sets = {...}
  local state = map(function(x)
                      return {x, #x , 1}
                    end , sets)
  state[#state][3] = 0

  local curr = #state

  return function()
    while true do
      if inc(state, curr) then
        return map(function(s)
                     return s[1][s[3]] end,
                   state)
      else
        return nil
      end
    end
  end
end

function compose(f,g)
  return function (...)
    return f(g(unpack(arg)))
  end
end

pinspect = compose(print, inspect)

map(pinspect, permutations({1,2,3}, {5,6,7}))

local c = 0
for i in permutations({1,2,3,5,6} , {3,4,5,6,7,6}) do
  c = c+1
  if c == 10 then break end
  pinspect(i)
end