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diff --git a/Data/DefaultContent/Libraries/luafun/doc/under_the_hood.rst b/Data/DefaultContent/Libraries/luafun/doc/under_the_hood.rst deleted file mode 100644 index 802659e..0000000 --- a/Data/DefaultContent/Libraries/luafun/doc/under_the_hood.rst +++ /dev/null @@ -1,154 +0,0 @@ -Under the Hood -============== - -.. module:: fun - -The section shed some light on the internal library structure and working -principles. - -Iterators ---------- - -A basic primitive of the library after functions is an iterator. Most functions -takes an iterator and returns a new iteraror(s). Iterators all the way down! -[#iterators]_. - -The simplest iterator is (surprise!) :func:`pairs` and :func:`ipairs` -Lua functions. Have you ever tried to call, say, :func:`ipairs` function -without using it inside a ``for`` loop? Try to do that on any Lua -implementation: - -.. _iterator_triplet: -.. code-block:: bash - - > =ipairs({'a', 'b', 'c'}) - function: builtin#6 table: 0x40f80e38 0 - -The function returned three strange values which look useless without a ``for`` -loop. We call these values **iterator triplet**. -Let's see how each value is used for: - -``gen`` -- first value - A generating function that can produce a next value on each iteration. - Usually returns a new ``state`` and iteration values (multireturn). - -``param`` -- second value - A permanent (constant) parameter of a generating function is used to create - specific instance of the generating function. For example, a table itself - for ``ipairs`` case. - -``state`` -- third value - A some transient state of an iterator that is changed after each iteration. - For example, an array index for ``ipairs`` case. - -Try to call ``gen`` function manually: - - .. code-block:: lua - - > gen, param, state = ipairs({'a', 'b', 'c'}) - > =gen(param, state) - 1 a - -The ``gen`` function returned a new state ``1`` and the next iteration -value ``a``. The second call to ``gen`` with the new state will return the next -state and the next iteration value. When the iterator finishes to the end -the ``nil`` value is returned instead of the next state. - -**Please do not panic!** You do not have to use these values directly. -It is just a nice trick to get ``for .. in`` loop working in Lua. - -Iterations ----------- - -What happens when you type the following code into a Lua console:: - - for _it, x in ipairs({'a', 'b', 'c'}) do print(x) end - -According to Lua reference manual [#lua_for]_ the code above is equivalent to:: - - do - -- Initialize the iterator - local gen, param, state = ipairs({'a', 'b', 'c'}) - while true do - -- Next iteration - local state, var_1, ยทยทยท, var_n = gen(param, state) - if state == nil then break end - -- Assign values to our variables - _it = state - x = var_1 - -- Execute the code block - print(x) - end - end - -What does it mean for us? - -* An iterator can be used together with ``for .. in`` to generate a loop -* An iterator is fully defined using ``gen``, ``param`` and ``state`` iterator - triplet -* The ``nil`` state marks the end of an iteration -* An iterator can return an arbitrary number of values (multireturn) -* It is possible to make some wrapping functions to take an iterator and - - return a new modified iterator - -**The library provides a set of iterators** that can be used like ``pairs`` -and ``ipairs``. - -Iterator Types --------------- - -Pure functional iterators -````````````````````````` - -Iterators can be either pure functional or have some side effects and returns -different values for some initial conditions [#pure_function]_. An **iterator is -pure functional** if it meets the following criteria: - -- ``gen`` function always returns the same values for the same ``param`` and - ``state`` values (idempotence property) -- ``param`` and ``state`` values are not modified during ``gen`` call and - a new ``state`` object is returned instead (referential transparency - property). - -Pure functional iterators are very important for us. Pure functional iterator -can be safety cloned or reapplied without creating side effects. Many library -function use these properties. - -Finite iterators -```````````````` - -Iterators can be **finite** (sooner or later end up) or **infinite** -(never end). -Since there is no way to determine automatically if an iterator is finite or -not [#turing]_ the library function can not automatically resolve infinite -loops. It is your obligation to do not pass infinite iterator to reducing -functions. - -Tracing JIT ------------ - -Tracing just-in-time compilation is a technique used by virtual machines to -optimize the execution of a program at runtime. This is done by recording a -linear sequence of frequently executed operations, compiling them to native -machine code and executing them. - -First profiling information for loops is collected. After a hot loop has been -identified, a special tracing mode is entered which records all executed -operations of that loop. This sequence of operations is called a **trace**. -The trace is then optimized and compiled to machine code (trace). When this -loop is executed again the compiled trace is called instead of the program -counterpart [#tracing_jit]_. - -Why the tracing JIT is important for us? The LuaJIT tracing compiler can detect -tail-, up- and down-recursion [#luajit-recursion]_, unroll compositions of -functions and inline high-order functions [#luajit-optimizations]_. -All of these concepts make the foundation for functional programming. - -.. [#iterators] http://en.wikipedia.org/wiki/Turtles_all_the_way_down -.. [#lua_for] http://www.lua.org/manual/5.2/manual.html#3.3.5 -.. [#pure_function] http://en.wikipedia.org/wiki/Pure_function -.. [#turing] `Proved by Turing <http://en.wikipedia.org/wiki/Halting_problem>`_ -.. [#tracing_jit] http://en.wikipedia.org/wiki/Tracing_just-in-time_compilation -.. [#luajit-recursion] http://lambda-the-ultimate.org/node/3851#comment-57679 -.. [#luajit-optimizations] http://wiki.luajit.org/Optimizations |