diff options
Diffstat (limited to 'third_party/immer/doc/design.rst')
| -rw-r--r-- | third_party/immer/doc/design.rst | 268 |
1 files changed, 0 insertions, 268 deletions
diff --git a/third_party/immer/doc/design.rst b/third_party/immer/doc/design.rst deleted file mode 100644 index 27d29e9e61..0000000000 --- a/third_party/immer/doc/design.rst +++ /dev/null @@ -1,268 +0,0 @@ -.. _design: - -Design -====== - -This is a library of **immutable containers**. - -These containers have all their methods marked ``const``. Instead of -mutating them *in place*, they provide manipulation functions that -*return a new transformed value*, leaving the original value -unaltered. In the context of data-structures, this property of -preserving old values is called **persistence**. - -Most of these containers use data-structures in which these operations -can be done *efficiently*. In particular, not all data is copied when -a new value is produced. Instead, the new values may share, -internally, common data with other objects. We sometimes refer to -this property as **structural sharing**. This behaviour is -transparent to the user. - -Assigment ---------- - -We are sorry, we lied. These containers provide *one mutating -operation*: **assignment** --- i.e. ``operator=``. - -There is a good reason: without ``operator=`` everything becomes -complicated in C++. For example, one may not contain non-assignable -types in many standard containers, assignment would also be disabled -from your custom types holding immutable containers, and so on and so -forth. - -C++ is a multi-paradigm language with an imperative core. Thus, it is -built on the foundation that *variables* can be mutated --- -i.e. assigned to. We don't want to ride against this tide. What we -want to prevent is in-place *object* manipulation. Because of C++ -semantics, *variable* assignment is defined in terms of *object -mutation*, so we have to provide *this very particular mutating -operation*, but nothing else. Of course, you are free to mark your -variables ``const`` to completely forbid assignment. - -.. warning:: - - **Assignment is not thread safe**. When a *mutable* variable is - shared across multiple threads, protect access using some other - mechanism. - - For obvious reasons, all other methods, which are ``const``, are - thread-safe. It is safe to share *immutable* state across multiple - threads. - -To ``const`` or not to ``const`` --------------------------------- - -Many C++ programmers, influenced by functional programming, are trying -to escape the evils of mutability by using ``const`` whenever -possible. We also do it ourselves in many of our examples to -reinforce the property of immutability. - -While in general this is a good practice backed up with very good -intentions, it has one caveat: *it disables moveability*. It does so -even when ``std::move()`` is used. This makes sense, since moving from -an object may mutate it, and ``const``, my friends, prevents *all* -mutations. For example: - -.. literalinclude:: ../example/vector/move.cpp - :language: c++ - :start-after: move-bad/start - :end-before: move-bad/end - -One may think that the variable ``v`` is moved into the -``push_back()`` call. This is not the case, because the variable -``v`` is marked ``const``. Of course, one may enable the move by -removing it, as in: - -.. literalinclude:: ../example/vector/move.cpp - :language: c++ - :start-after: move-good/start - :end-before: move-good/end - -So, is it bad style then to use ``const`` as much as possible? I -wouldn't say so and it is advisable when ``std::move()`` is not used. -An alternative style is to not use ``const`` but adopt an `AAA-style -<aaa>`_ (*Almost Always use Auto*). This way, it is easy to look for -mutations by looking for lines that contain ``=`` but no ``auto``. -Remember that when using our immutable containers ``operator=`` is the -only way to mutate a variable. - -.. _aaa: https://herbsutter.com/2013/08/12/gotw-94-solution-aaa-style-almost-always-auto/ - -.. admonition:: Why does ``const`` prevent move semantics? - - For those adventurous into the grainy details C++, here is why. - ``std::move()`` does not move anything, it is just *a cast* from - normal *l-value* references (``T&``) to *r-value* reference - (``T&&``). This is, you pass it a variable, and it returns a - reference to its object disguised as an intermediate result. In - exchange, you promise not to do anything with this variable later - [#f1]_. It is the role of the thing that *receives the moved-from - value* (in the previous example, ``push_back``) to actually do - anything interesting with it --- for example, steal its contents - ๐. - - So if you pass a ``T&`` to ``std::move()`` you get a ``T&&`` and, - unsurprisingly, if you pass a ``const T&`` you get a ``const T&&``. - But the receivers of the moved-from value (like constructors or our - ``push_back()``) maybe be moved-into because they provide an - overload that expects ``T&&`` --- without the ``const``! Since a - ``const T&&`` can not be converted into a ``T&&``, the compiler - looks up for you another viable overload, and most often finds a - copy constructor or something alike that expects a ``const T&`` or - just ``T``, to which a ``const T&&`` can be converted. The code - compiles and works correctly, but it is less efficient than we - expected. Our call to ``std::move()`` was fruitless. - - .. [#f1] For the sake of completeness: it is actually allowed to do stuff - with the variable *after another value is assigned to it*. - -.. _move-semantics: - -Leveraging move semantics -------------------------- - -When using :ref:`reference counting<rc>` (which is the default) -mutating operations can often be faster when operating on *r-value -references* (temporaries and moved-from values). Note that this -removes *persistence*, since one can not access the moved-from value -anymore! However, this may be a good idea when doing a chain of -operations where the intermediate values are not important to us. - -For example, let's say we want to write a function that inserts all -integers in the range :math:`[first, last)` into an immutable vector. -From the point of view of the caller of the function, this function is -a *transaction*. Whatever intermediate vectors are generated inside -of it can be discarded since the caller can only see the initial -vector (the one passed in as argument) and the vector with *all* the -elements. We may write such function like this: - -.. literalinclude:: ../example/vector/iota-move.cpp - :language: c++ - :start-after: myiota/start - :end-before: myiota/end - -The intermediate values are *moved* into the next ``push_back()`` -call. They are going to be discarded anyways, this little -``std::move`` just makes the whole thing faster, letting ``push_back`` -mutate part of the internal data structure in place when possible. - -If you don't like this syntax, :doc:`transients<transients>` may be -used to obtain similar performance benefits. - -.. admonition:: Assigment guarantees - - From the language point of view, the only requirement on moved from - values is that they should still be destructible. We provide the - following two additional guarantees: - - - **It is valid to assign to a moved-from variable**. The variable - gets the assigned value and becomes usable again. This is the - behaviour of standard types. - - - **It is valid to assign a moved-from variable to itself**. For - most standard types this is *undefined behaviour*. However, for our - immutable containers types, expressions of the form ``v = - std::move(v)`` are well-defined. - -Recursive types ---------------- - -Most containers will fail to be instantiated with a type of unknown -size, this is, an *incomplete type*. This prevents using them for -building recursive types. The following code fails to compile: - -.. code-block:: c++ - - struct my_type - { - int data; - immer::vector<my_type> children; - }; - -However, we can easily workaround this by using an ``immer::box`` to wrap -the elements in the vector, as in: - -.. code-block:: c++ - - struct my_type - { - int data; - immer::vector<immer::box<my_type>> children; - }; - -.. admonition:: Standard containers and incomplete types - - While the first example might seem to compile when using some - implementations of ``std::vector`` instead of ``immer::vector``, such - use is actually forbidden by the standard: - - **17.6.4.8** *Other functions (...)* 2. the effects are undefined in - the following cases: (...) In particular---if an incomplete type (3.9) - is used as a template argument when instantiating a template - component, unless specifically allowed for that component. - -.. _batch-update: -Efficient batch manipulations ------------------------------ - -Sometimes you may write a function that needs to do multiple changes -to a container. Like most code you write with this library, this -function is *pure*: it takes one container value in, and produces a -new container value out, no side-effects. - -Let's say we want to write a function that inserts all integers in the -range :math:`[first, last)` into an immutable vector: - -.. literalinclude:: ../example/vector/iota-slow.cpp - :language: c++ - :start-after: include:myiota/start - :end-before: include:myiota/end - -This function works as expected, but it is slower than necessary. -On every loop iteration, a new value is produced, just to be -forgotten in the next iteration. - -Instead, we can grab a mutable view on the value, a :ref:`transient`. -Then, we manipulate it *in-place*. When we are done with it, we -extract back an immutable value from it. The code now looks like -this: - -.. _iota-transient: - -.. literalinclude:: ../example/vector/iota-transient.cpp - :language: c++ - :start-after: include:myiota/start - :end-before: include:myiota/end - -Both conversions are :math:`O(1)`. Note that calling ``transient()`` -does not break the immutability of the variable it is called on. The -new mutable object will adopt its contents, but when a mutation is -performed, it will copy the data necessary using *copy on write*. -Subsequent manipulations may hit parts that have already been copied, -and these changes are done in-place. Because of this, it does not -make sense to use transients to do only one change. - -.. tip:: - - Note that :ref:`move semantics<move-semantics>` can be used instead to - support a similar use-case. However, transients optimise updates - even when reference counting is disabled. - -.. _std-compat: -Standard library compatibility ------------------------------- - -While the immutable containers provide an interface that follows a -functional style, this is incompatible with what the standard library -algorithms sometimes expect. :ref:`transients` try to provide an -interface as similar as possible to similar standard library -containers. Thus, can also be used to interoperate with standard -library components. - -For example the :ref:`myiota() function above<iota-transient>` may as -well be written using standard library tools: - -.. literalinclude:: ../example/vector/iota-transient-std.cpp - :language: c++ - :start-after: include:myiota/start - :end-before: include:myiota/end |