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====== Lua Tips ==
===== Links ==

[[lua>|Programming in Lua (5.0) ]] [[http://lua-users.org/wiki/TutorialDirectory|Tutorial]] --- [[lua>manual/5.2/#contents|Reference Manual]] --- {{http://lua-users.org/files/wiki_insecure/users/thomasl/luarefv51.pdf|Short Reference}} --- IBM intro: [[http://www.ibm.com/developerworks/linux/library/l-lua.html|Embeddable scripting with Lua]] --- Linux Journal intro: [[http://www.linuxjournal.com/article/9605|A Look at Lua]]

===== Install ==

Use www.slackbuilds.org

External addons / Packages for Lua:
  * [[http://luarocks.org/repositories/rocks/|Automated "Rocks"]]
  * [[http://lua-users.org/wiki/LibrariesAndBindings|Manual]]

===== Implementations ==
==== Standalone lua ==
=== Shebang ==

''#!/usr/bin/lua''

=== Command Line Arguments ==

Use global table ''arg''

=== StdIo ==

<code lua>
local inp= io.stdin:read() -- Read a line from stdion
print()                    -- Print a line to stdout
io.stdout:write()          -- Print a string to stdout
io.stderr:write()          -- Print a string to stderr
</code>

==== Lua for Web Browsers ==

[[http://kripken.github.io/lua.vm.js/script_example.html|lua.vm.js]] thanks to asm.js

==== Lua for Java/Android ==

[[https://sourceforge.net/projects/luaj/|Luaj]], seen at [[https://github.com/M66B/XPrivacyLua/blob/master/README.md|XPrivacyLua]]

===== C-style ?-operator equivalent ==

This C code
<code c>
d= a ? b : c
</code>
is in lua:
<code lua>
d= a and b or c
</code>

Note that this works only for //single// assignment. Use auxiliary tables for mulitiple assignment:
<code lua>
d1, d2 = table.unpack(a and {b1, b2} or {c1, c2})
</code>

<note warning>Take care if b can become ''nil'' or ''false''. In this case the equivalent doesn't work. Instead use:</note>
<code lua>
if a then d=b else d=c end
</code>

===== Default values for missing arguments ==

<code lua>
function foo(a)
    a= a or "defaultvalue"
end
</code>

Take care, if ''false'' is a valid value, instead use:

<code lua>
function foo(a)
    if a==nil then a="defaultvalue" end
end
</code>


===== Difference between nil and no value ==
<code lua>
function return_nil()
    return nil
end

function return_nothing()
end

function count_args(...)
    print(table.pack(...).n)
end

count_args(nil)                   --> 1
count_args()                      --> 0
--=> Passed nils increment argument count

count_args(return_nil())          --> 1
count_args(return_nothing())      --> 0
--=> Returned nils increment return value count

print(tonumber(nil))              --> nil
print(tonumber(return_nil()))     --> nil
--=> tonumber() accepts nil as argument

print(tonumber())                 --> bad argument #1 to 'tonumber' (value expected)
print(tonumber(return_nothing())) --> bad argument #1 to 'tonumber' (value expected)
--=> tonumber() does not accept nothing as argument
</code>

<note warning>An example for a function returning nothing is the result of string.gmatch()</note>

===== Lexical scoping ==

<code lua>
local strict= require"fs.strict"
local g,h,i

local function f()
    g() -- ok
    h() -- ok
    --i() -- err, because i refers to old upvalue which is still nill
end

g= function() end      -- existing upvalue g is assigned a function
function h() end       -- same as above (mind the missing "local"!)
local function i() end -- a new upvalue is created which hides old i(assumption)

f()
i() -- ok
</code>

===== Numeric for Loop ==

<code lua>
for i = 0,24,4 do -- start, end (including), step
    print(i)
end
</code>

===== Tables ==
==== Length ==

Use ''#'' operator

==== Iterate over ==

<code lua>
-- Iterate over array part:
for i,v in ipairs(now) do print(i,v) end

-- Iterate over complete table:
for k,v in pairs(now) do print(k,v) end
</code>

<note important>Oder is unspecified when iterating over a map with ''pairs()''</note>

===== Copying Variables ==

<code lua>
#!/usr/bin/lua

-- Numbers:
a=3 -- a contains 3
b=a -- b contains 3
a=4 -- a contains 4, b still contains 3
print(a,b) -- 4  3

-- Booleans:
a=true; b=a; a=false
print(a,b) -- false  true

-- Strings:
a="World"  -- a points to the string "World"
b=a        -- b points to the same string
a="Hello"  -- a points to "World", b still points to "Hello"
print(a,b) --> Hello  World

-- Functions:
a= function() print("World") end
b=a
-- a and b point to the same function:
print(a,b) --> function: 0x80500e8  function: 0x80500e8
a= function() print("Hello") end
-- a points to a new function and b still points to the old function:
print(a,b) --> function: 0x8050a48  function: 0x80500e8
a() -- Hello
b() -- World
</code>

<note tip>A copy of a number, boolean, string or function can't modify the content of its origin or vice versa, they are independent of each other</note>

<code lua>
#!/usr/bin/lua

-- Tables:
a={3}
b=a;
-- a and b point to the same table:
print(a,b) -- table: 0x80509e0	table: 0x80509e0
print(a[1],b[1]) -- 3  3
a[1]=4;
print(a[1],b[1]) -- 4  4
b[1]=5
print(a[1],b[1]) -- 5  5

a={6}
-- a points to a new table and b still points to the old table:
print(a,b) -- table: 0x8050a30	table: 0x80509e0
print(a[1],b[1]) -- 6  5
b[1]=7
print(a[1],b[1]) -- 6  7
</code>

<note tip>A copy of a table can modify the value of the table entries of its origin and vice versa, because both table variables point to the same table object</note>

===== Error Handling ==
==== pcall() differs from Lua Convention ==
''pcall()'':
  success: true, value(s)
  failure: false, error

Lua [[http://lua-users.org/wiki/FinalizedExceptions|convention]]:
  success: value(s)
  failure: nil, error

<note tip>Error handling code after ''pcall()'' differs from error code after calling a function which complies to the convention!</note>

==== Raise error or return error? ==
=== Converting between raise and return ==

''pcall()'' converts an error raising function into an error returning function.

''assert()'' or similar converts an error returning function into an error raising function.

=== Pro Error Raising ==

  - A function which //returns// the error forces the user of that function to //always// check the return code (or convert to raise) , regadless if he wants to handle the error or just pass it upward.
  - A function which //raises// the error can not get forgotten to check for errors.
  - If the user wants to pass the error upward, checking and converting can be avaoided completely if both the called function and the caller use error raising. In any other case he has to check and/or convert:

^ caller⇓ callee⇒ ^ return ^ raise ^
^ return | check | pcall() + check |
^ raise | assert() | nothing |

=== Pro Error Returning ==

  - If the user wants to handle the error, he simply checks the return value of an error returning function. If the function raises the error, the user must convert it to a error returning function by help of pcall() before.
  - FIXME A typical ''try {...}''-block is done with a function in Lua. That means you end up having one function more for each try/catch usage

=== Rules: ==

<note tip>

  - //Return// error if it can be expected, that the user needs to handle the error directly after calling the func in question.
  - //Raise// the error if it can be expected, that the user does not care for errors directly after calling the func in question and simple passes it upward.
  - <del>If in doubt, raise the error</del> FIXME What to do when in doubt?
</note>

===== Classes and Objects ==

See also projects/accounting/sbaccimport which has since rev 482:c85c255f1870 classes with inheritance where the constructor new() is a class method and can therefore be reused in child classes similar to PIL capter 21 
==== Classes as Object templates ==
<code lua>
-- rectangle prototype:
rectangle_proto= {species="rectangle"}
function rectangle_proto:area() return self.width * self.height end

-- rectangle constructor:
function new_rectangle(width, height)
    local rectangle= {width= width, height= height}
    setmetatable(rectangle, {__index=rectangle_proto})
    return rectangle
end

-- rectangle usage:
rectangle= new_rectangle(3,4)
print(rectangle:area())  --> 12
print(rectangle.species) --> rectangle


-- circle prototype:
circle_proto= {species="circle"}
function circle_proto:area() return self.radius^2 * math.pi end

-- circle constructor:
function new_circle(radius)
    local circle= {radius= radius}
    setmetatable(circle, {__index=circle_proto})
    return circle
end

circle= new_circle(1)
print(circle:area())  --> 3.14
print(circle.species) --> circle
</code>

==== Classic Inheritance ==

Classic single inheritance chain:

<code lua>
-- Inheritance: shape prototype:
shape_proto= {family="shape"}
function shape_proto:get_species() return self.species end
function shape_proto:get_family() return self.family end

setmetatable(rectangle_proto, {__index=shape_proto})
setmetatable(circle_proto, {__index=shape_proto})

print(circle:get_family().."/"..circle:get_species()) --> shape/circle
print(rectangle:get_family().."/"..rectangle:get_species()) --> shape/rectangle
</code>


==== Implementing Interfaces ==

FIXME

  * http://code.google.com/p/go-wiki/wiki/GoForCPPProgrammers#Interfaces
  * Option 1: No metatables. Just implant all methods of the interface directly into the table which represents the object. Simple
  * Option 2: One metatable for every type. No metatable chain. Each type hast a dedicated factory and metatable. All implemented interface methods are copied into the metatable of the type. Saves space.

==== Special Case: tostring() ==

Lua ''print()'' does not look for a ''tostring()'' method directly in the table, instead it looks for a method called ''%%__tostring()%%'' in its metatable. Thus, to enable both ''print(myobject:tostring())'' (explicit call) and ''print(myobject) '' (implicit call), provide the tostring() method in the prototype table of the object and set ''%%__tostring%%'' attribute of the metatable to that method:

<code lua>
-- rectangle prototype:
rectangle_proto= {}
function rectangle_proto:tostring() return "["..self.width.."|"..self.height.."]" end

-- rectangle constructor:
function new_rectangle(width, height)
    rectangle= {width= width, height= height}
    setmetatable(rectangle, {
        __index=rectangle_proto,
        __tostring=rectangle_proto.tostring
    })
    return rectangle
end

-- rectangle usage:
rectangle= new_rectangle(3,4)
print(rectangle:tostring()) --> [3|4]
print(rectangle)            --> [3|4]
</code>

This works also if ''tostring()'' is defined in the base clase:

<code lua>
-- shape as base class:
shape_proto= {}
function shape_proto:tostring() return "["..self.width.."|"..self.height.."]" end

-- rectangle prototype inherits from shape:
rectangle_proto= {}
setmetatable(rectangle_proto, {
    __index=shape_proto
})

-- rectangle constructor:
function new_rectangle(width, height)
    rectangle= {width= width, height= height}
    setmetatable(rectangle, {
        __index=rectangle_proto,
        __tostring=rectangle_proto.tostring
    })
    return rectangle
end

-- rectangle usage:
rectangle= new_rectangle(3,4)
print(rectangle:tostring()) --> [3|4]
print(rectangle)            --> [3|4]
</code>

==== Privacy ==
=== Simple ==

<code lua point.lua>
local M= {}
local private= {} -- holds private data of all objecs in subtables

-- Class Point: --

local Point_proto= {} -- Class prototype
function Point_proto:set(x,y)
    local priv= private[self]
    priv.x= x
    priv.y= y
end
function Point_proto:get()
    local priv= private[self]
    return priv.x, priv.y
end
function Point_proto:tostring()
    local priv= private[self]
    return "["..priv.x.."|"..priv.y.."]"
end
Point_proto.__index   = Point_proto
Point_proto.__newindex= function() assert(false,"object is write-protected") end

setmetatable(Point_proto, { -- ensure read protection for Point and all childs
    __index= function() assert(false, "object is read-protected") end
})

function M.new_Point(x,y) -- constructor
    local instance= {}
    setmetatable(instance, Point_proto)
    private[instance]= {} -- create private part of object
    instance:set(x,y)
    return instance
end

return M
</code>

<code lua main.lua>
#!/usr/bin/lua
local point= require "point"

local print_r= function(table)
    for key,value in pairs(table) do
        io.write("[", key, "] = ", tostring(value), "\n")
    end
end

local p= point.new_Point(3,4)
--p.x= 8     --> Write access throws an error
--print(p.y) --> Read access throws an error
print(p:tostring()) --> [3|4]
print_r(p)          --> prints nothing
</code>

=== With inhteritance ==

<code lua point.lua>
local M= {}
local private= {} -- holds private data of all objecs in subtables

-- Class Point: --
-- ...

-- CLASS MovablePoint: --

local MovablePoint_proto= {} -- Class prototype
function MovablePoint_proto:move(dx, dy)
    local priv= private[self]
    priv.x= priv.x+dx
    priv.y= priv.y+dy
end
MovablePoint_proto.__index   = MovablePoint_proto
MovablePoint_proto.__newindex= Point_proto.__newindex

setmetatable(MovablePoint_proto, Point_proto) -- Inherit from Point

function M.new_MovablePoint(x,y) -- constructor
    local instance= M.new_Point(x,y) -- call parent constructor
    setmetatable(instance, MovablePoint_proto)
    return instance
end

return M
</code>

<code lua main.lua>
#!/usr/bin/lua
local point= require "point"

-- ...

local mp= point.new_MovablePoint(5,6)
mp.x= 9     --> Write access throws an error
print(mp.y) --> Read access throws an error
print(mp:tostring()) --> [5|6]
mp:move(2,1)
print(mp:tostring()) --> [7|7]
print_r(mp)          --> prints nothing
</code>

===== File access ==

Each of the following constructs is equivalent, it reads a file line by line:

<code lua>
local f= io.open"file.txt"
repeat
    local l= f:read("l")
    if l then print(l) end
until not l
f:close()
</code>

<code lua>
local f= io.open"file.txt"
for l in f:lines("l") do
    print(l)
end
f:close()
</code>

<code lua>
for l in io.lines("file.txt", "l") do
    print(l)
end
</code>

  - ''"l"'' as arg for ''read()'' is the default, it can be omitted
  - ''file:lines()'' and ''io.lines()'' accept the same format args as ''read()''. (Tested. Nowhere found in docu). Thus you could read a file eg chunk by chunk as well.
  - io.lines() without arguments returns an iterater wich returns stdin line by line

==== Load CSV-like files ==

<code lua>
#!/usr/bin/lua
local separator=":"
for line in io.lines() do
    print(line)
    for cell in line:gmatch("[^"..separator.."]+") do
        print("", cell)
    end
end
</code>
===== Includes and Modules ==
==== Include ==

To run some code sitting in a different file, use ''dofile()''

To load global variables of an external (config) file into a table locally, you have to pass the table as environment:

<code lua config.lua>
a="Harry"
b="Hirsch"
</code>

<code lua main.lua>
#!/usr/bin/lua

local extenv={}
local chunk= loadfile("config.lua", "t" , extenv)
chunk()

for k,v in pairs(extenv) do
    io.write("[", k, "] = ", tostring(v), "\n")
end
</code>

<code text result>
[a] = Harry
[b] = Hirsch
</code>


==== Modules ==

See [[http://lua-users.org/wiki/ModuleDefinition|ModuleDefinition]] and [[http://lua-users.org/wiki/LuaStyleGuide|LuaStyleGuide]] -> Modules

''module()'' is [[lua>manual/5.2/manual.html#8.2|deprecated]]!

===== Time and Date ==
==== Unix Timestamp ==

<code lua>
now= os.time()
</code>

==== Time Map / Time String ==

<code lua>
-- Return a table containing hour, min, wday, day, ... of current time:
now = os.date("*t")

-- Return a table containing hour, min, wday, day, ... of a certain time:
atime = os.date("*t", timestamp)

-- Return a formated string of current time:
now = os.date("Today is %A") -- Today is Thursday
</code>

[[http://www.lua.org/pil/22.1.html|Source]]

===== Performance ==
==== Using Locals ==

<code lua global.lua>
do
    var= 0
    for i=1,102000000 do
        var= var+1
    end
end
-- real	0m4.516s
</code>

<code lua local.lua>
do
    local var= 0
    for i=1,102000000 do
        var= var+1
    end
end
-- real	0m1.003s
</code>

<code lua chunk.lua>
local var= 0
do
    for i=1,102000000 do
        var= var+1
    end
end
--real 0m0.970s
</code>

<code lua closure.lua>
do
    local var= 0
    local closure= function()
        for i=1,102000000 do
            var= var+1
        end
    end
    closure()
end
-- real	0m1.666s
</code>

<code lua map.lua>
do
    local tab= {var= 0}
    for i=1,102000000 do
        tab.var= tab.var+1
    end
end
-- real	0m4.248s
</code>

<code lua array.lua>
do
    local tab= {0}
    for i=1,102000000 do
        tab[1]= tab[1]+1
    end
end
-- real	0m4.767s
</code>

<note tip>

  - Local variables (compiled indexed access) are much faster than global variables (table access).
  - Chunk-local variables are as fast as function-local variables
  - Upvalues (non-local variables of closures) are only a little slower than local variables
  - Table access (array or map) is as slow as accessing global variables
</note>

Tested with Lua 5.2


==== Strings ==

<note warning>Concatenating a string piecemeal in a loop is inefficient. Consider using a table as buffer and then ''table.concat()''</note>

See also:
  * http://www.lua.org/pil/11.6.html
  * http://www.lua.org/gems/sample.pdf => About Strings

===== Coroutines ==
==== Basic Rules ==

A newly created coroutine is in state ''supended'':
<code lua>
co1= coroutine.create(function() end)
print(coroutine.status (co1)) --> suspended
</code>

A coroutine which resumes another coroutine goes from state ''running'' to ''normal'' while the resumed couroutine goes from state ''suspended''  to''running'':
<code lua>
co1= coroutine.create(function()
    print("in co1: state main:", coroutine.status(main))
    print("in co1: state co1", coroutine.status(co1))
end)

main= coroutine.running() -- get handle to main coroutine
print("in main: state main:", coroutine.status(main))
print("in main: state co1:", coroutine.status(co1))
coroutine.resume(co1)
</code>
<code>Result:
in main: state main:	running
in main: state co1:	suspended
in co1: state main:	normal
in co1: state co1	running
</code>


A ''running'' couroutine can yield() back to its resumer, which puts the resumer back to ''running'' and itself back to ''suspended'':
<code lua>
co1= coroutine.create(function()
    print("in co1: state main:", coroutine.status(main))
    print("in co1: state co1", coroutine.status(co1))
    coroutine.yield()
end)

main= coroutine.running() -- get handle to main coroutine
coroutine.resume(co1)
print("in main: state main:", coroutine.status(main))
print("in main: state co1:", coroutine.status(co1))
</code>
<code>Result:
in co1: state main:	normal
in co1: state co1	running
in main: state main:	running
in main: state co1:	suspended
</code>

Alternatively a ''running'' couroutine can resume another coroutine (which is in state ''suspended''):
<code lua>
co1= coroutine.create(function()
    coroutine.resume(co2)
end)

co2= coroutine.create(function()
    print("in co2: state main:", coroutine.status(main))
    print("in co2: state co1", coroutine.status(co1))
    print("in co2: state co2", coroutine.status(co2))
end)

main= coroutine.running() -- get handle to main coroutine
coroutine.resume(co1)
</code>
<code>Result:
in co2: state main:	normal
in co2: state co1	normal
in co2: state co2	running
</code>


But a ''running'' couroutine cannot resume a non-suspended coroutine. Only  coroutines in state ''suspended'' can be resumed:
<code lua>
co1= coroutine.create(function()
    local res, err= coroutine.resume(main)
    print(res, err)
end)

main= coroutine.running() -- get handle to main coroutine
coroutine.resume(co1)
</code>
<code>Result:
false	cannot resume non-suspended coroutine
</code>

==== Pipe-like Data exchange between Coroutines ==

<code lua>
#!/usr/bin/lua

co= coroutine.create(function()
    while true do
        io.stderr:write('co: waking up\n')
        local inp= io.stdin:read()
        io.stderr:write('co: read "'..inp..'", going to sleep\n')
        coroutine.yield(inp)
    end
end)

while true do
    io.stderr:write('main: going to sleep\n')
    local state, oup= coroutine.resume(co)
    io.stderr:write('main: waking up\n')
    print('> '..oup)
end
</code>

==== Misc ==

<note tip>Coroutines can be used to convert a recursive data source/sink into an iterative one. See "Programming in Lua, 2nd editon, p.79"</note>

===== Documentation Generation ==

[[http://stevedonovan.github.io/ldoc/topics/doc.md.html|Ldoc]] is preferred (over [[http://keplerproject.github.io/luadoc/manual.html#howto|LuaDoc]]).
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