Getting Started with NPL

The syntax of NPL is 100% compatible with lua. If you are unfamiliar with lua, the only book you need is

NPL/Lua Syntax in 15 minutes

The following NPL code is modified from http://learnxinyminutes.com/

-- Two dashes start a one-line comment.

--[[
     Adding two ['s and ]'s makes it a multi-line comment.
--]]

----------------------------------------------------
-- 1. Variables and flow control.
----------------------------------------------------

local num = 42;  -- All numbers are doubles.

s = 'string is immutable'  
-- comparing two strings is as fast as comparing two pointers
t = "double-quotes are also fine"; -- `;` is optional at line end
u = [[ Double brackets
       start and end
       multi-line strings.]]
-- 'nil' is like null_ptr. It undefines t. When data has no reference, 
-- it will be garbage collected some time later.
t = nil  

-- Blocks are denoted with keywords like do/end:
while num < 50 do
  num = num + 1  -- No ++ or += type operators.
end

-- If clauses:
if (num <= 40) then
  print('less than 40')
elseif ("string" == 40 or s) then
  print('lua has dynamic type,thus any two objects can be compared.');
elseif s ~= 'NPL' then
  print('less than 40')
else
  -- Variables are global by default.
  thisIsGlobal = 5  -- variable case is sensitive.

  -- How to make a variable local:
  local line = "this is accessible until next `end` or end of containing script file"

  -- String concatenation uses the .. operator:
  print("First string " .. line)
end

-- Undefined variables return nil.
-- This is not an error:
foo = anUnknownVariable  -- Now foo == nil.

aBoolValue = false

-- Only nil and false are falsy; 0 and '' are true!
if (not aBoolValue) then print('false') end

-- 'or' and 'and' are short-circuited. They are like ||, && in C.
-- This is similar to the a?b:c operator in C:
ans = (aBoolValue and 'yes') or 'no'  --> 'no'

local nSum = 0;
for i = 1, 100 do  -- The range includes both ends. i is a local variable.
  nSum = nSum + i
end

-- Use "100, 1, -1" as the range to count down:
-- In general, the range is begin, end[, step].
nSum = 0
for i = 100, 1, -1 do 
   nSum = nSum + i 
end

-- Another loop construct:
repeat
  nSum = nSum - 1
until nSum == 0


----------------------------------------------------
-- 2. Functions.
----------------------------------------------------

function fib(n)
  if n < 2 then return 1 end
  return fib(n - 2) + fib(n - 1)
end

-- Closures and anonymous functions are ok:
function adder(x)
  -- The returned function is created when adder is
  -- called, and remembers the value of x:
  return function (y) return x + y end
end
a1 = adder(9)
a2 = adder(36)
print(a1(16))  --> 25
print(a2(64))  --> 100

-- Returns, func calls, and assignments all work
-- with lists that may be mismatched in length.
-- Unmatched receivers are nil;
-- unmatched senders are discarded.

x, y, z = 1, 2, 3, 4
-- Now x = 1, y = 2, z = 3, and 4 is thrown away.

function bar(a, b, c)
  print(string.format("%s %s %s", a, b or "b", c))
  return 4, 8, 15, 16, 23, 42
end

x, y = bar("NPL")  --> prints "NPL b nil"
-- Now x = 4, y = 8, values 15..42 are discarded.

-- Functions are first-class, may be local/global.
-- These are the same:
function f(x) return x * x end
f = function (x) return x * x end

-- And so are these:
local function g(x) return math.sin(x) end
local g; g  = function (x) return math.sin(x) end

-- Calls with one parameter don't need parenthesis:
print 'hello'  -- Works fine.


----------------------------------------------------
-- 3. Tables.
----------------------------------------------------

-- Tables = Lua's only compound data structure;
--          they are associative arrays.
-- Similar to php arrays or js objects, they are
-- hash-lookup dicts that can also be used as lists.

-- Using tables as dictionaries / maps:

-- Dict literals have string keys by default:
t = {key1 = 'value1', key2 = false}

-- String keys can use js-like dot notation:
print(t.key1)  -- Prints 'value1'.
t.newKey = {}  -- Adds a new key/value pair.
t.key2 = nil   -- Removes key2 from the table.

-- Literal notation for any (non-nil) value as key:
u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
print(u[6.28])  -- prints "tau"

-- Key matching is basically by value for numbers
-- and strings, but by identity for tables.
a = u['@!#']  -- Now a = 'qbert'.
b = u[{}]     -- We might expect 1729, but it's nil:
-- b = nil since the lookup fails. It fails
-- because the key we used is not the same object
-- as the one used to store the original value. So
-- strings & numbers are more portable keys.

-- A one-table-param function call needs no parens:
function h(x) print(x.key1) end
h{key1 = 'Sonmi~451'}  -- Prints 'Sonmi~451'.

for key, val in pairs(u) do  -- Table iteration.
  print(key, val)
end

-- _G is a special table of all globals.
print(_G['_G'] == _G)  -- Prints 'true'.

-- Using tables as lists / arrays:

-- List literals implicitly set up int keys:
v = {'value1', 'value2', 1.21, 'gigawatts'}
for i = 1, #v do  -- #v is the size of v for lists.
  print(v[i])  -- Indices start at 1 !! SO CRAZY!
end
-- A 'list' is not a real type. v is just a table
-- with consecutive integer keys, treated as a list.

----------------------------------------------------
-- 3.1 Metatables and metamethods.
----------------------------------------------------

-- A table can have a metatable that gives the table
-- operator-overloadish behavior. Later we'll see
-- how metatables support js-prototypey behavior.

f1 = {a = 1, b = 2}  -- Represents the fraction a/b.
f2 = {a = 2, b = 3}

-- This would fail:
-- s = f1 + f2

metafraction = {}
function metafraction.__add(f1, f2)
  sum = {}
  sum.b = f1.b * f2.b
  sum.a = f1.a * f2.b + f2.a * f1.b
  return sum
end

setmetatable(f1, metafraction)
setmetatable(f2, metafraction)

s = f1 + f2  -- call __add(f1, f2) on f1's metatable

-- f1, f2 have no key for their metatable, unlike
-- prototypes in js, so you must retrieve it as in
-- getmetatable(f1). The metatable is a normal table
-- with keys that Lua knows about, like __add.

-- But the next line fails since s has no metatable:
-- t = s + s
-- Class-like patterns given below would fix this.

-- An __index on a metatable overloads dot lookups:
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal  -- works! thanks, metatable

-- Direct table lookups that fail will retry using
-- the metatable's __index value, and this recurses.

-- An __index value can also be a function(tbl, key)
-- for more customized lookups.

-- Values of __index,add, .. are called metamethods.
-- Full list. Here a is a table with the metamethod.

-- __add(a, b)                     for a + b
-- __sub(a, b)                     for a - b
-- __mul(a, b)                     for a * b
-- __div(a, b)                     for a / b
-- __mod(a, b)                     for a % b
-- __pow(a, b)                     for a ^ b
-- __unm(a)                        for -a
-- __concat(a, b)                  for a .. b
-- __len(a)                        for #a
-- __eq(a, b)                      for a == b
-- __lt(a, b)                      for a < b
-- __le(a, b)                      for a <= b
-- __index(a, b)  <fn or a table>  for a.b
-- __newindex(a, b, c)             for a.b = c
-- __call(a, ...)                  for a(...)

----------------------------------------------------
-- 3.2 Class-like tables and inheritance.
----------------------------------------------------

-- Classes aren't built in; there are different ways
-- to make them using tables and metatables.

-- Explanation for this example is below it.

Dog = {}                                   -- 1.

function Dog:new()                         -- 2.
  newObj = {sound = 'woof'}                -- 3.
  self.__index = self                      -- 4.
  return setmetatable(newObj, self)        -- 5.
end

function Dog:makeSound()                   -- 6.
  print('I say ' .. self.sound)
end

mrDog = Dog:new()                          -- 7.
mrDog:makeSound()  -- 'I say woof'         -- 8.

-- 1. Dog acts like a class; it's really a table.
-- 2. function tablename:fn(...) is the same as
--    function tablename.fn(self, ...)
--    The : just adds a first arg called self.
--    Read 7 & 8 below for how self gets its value.
-- 3. newObj will be an instance of class Dog.
-- 4. self = the class being instantiated. Often
--    self = Dog, but inheritance can change it.
--    newObj gets self's functions when we set both
--    newObj's metatable and self's __index to self.
-- 5. Reminder: setmetatable returns its first arg.
-- 6. The : works as in 2, but this time we expect
--    self to be an instance instead of a class.
-- 7. Same as Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.

----------------------------------------------------

-- Inheritance example:

LoudDog = Dog:new()                           -- 1.

function LoudDog:makeSound()
  s = self.sound .. ' '                       -- 2.
  print(s .. s .. s)
end

seymour = LoudDog:new()                       -- 3.
seymour:makeSound()  -- 'woof woof woof'      -- 4.

-- 1. LoudDog gets Dog's methods and variables.
-- 2. self has a 'sound' key from new(), see 3.
-- 3. Same as LoudDog.new(LoudDog), and converted to
--    Dog.new(LoudDog) as LoudDog has no 'new' key,
--    but does have __index = Dog on its metatable.
--    Result: seymour's metatable is LoudDog, and
--    LoudDog.__index = LoudDog. So seymour.key will
--    = seymour.key, LoudDog.key, Dog.key, whichever
--    table is the first with the given key.
-- 4. The 'makeSound' key is found in LoudDog; this
--    is the same as LoudDog.makeSound(seymour).

-- If needed, a subclass's new() is like the base's:
function LoudDog:new()
  newObj = {}
  -- set up newObj
  self.__index = self
  return setmetatable(newObj, self)
end

-- Copyright: modified from http://learnxinyminutes.com/
-- Have fun with NPL/Lua!

How To Practice

We have developed NPL Code Wiki to help you learn and practice NPL. To launch NPL Code Wiki, you need to

You can then enter any NPL code in the console. See below:

nplconsole

Advanced Language Features

Lua/NPL is easy to learn, but hard to master. Pay attention to following concepts.

  • Lua string is immutable, so comparing two strings is as fast as comparing two pointers.
  • Lua has lexical scoping, use it to accelerate your code by exposing frequently used objects as local variables in its upper lexical scope.
  • Always use local variables.
  • NPL recommends object oriented programming, one can implement all C++ class features with lua tables, like class inheritance and virtual functions, etc.