This resolves an intricate problem related to metaprogramming with
variadic templates and function signatures. Due to exceptional complexity,
a direct solution was blocked for several years, and required a better
organisation of the support code involved; several workarounds were
developed, gradually leading to a transition path, which could now
be completed in an focused clean-up effort over the last week.
Metaprogramming with sequences of types is organised into three layers:
- simple tasks can be solved with the standard facilities of the language,
using pattern match with variadic template specialisations
- the ''type-sequence'' construct `Types<T...>` takes the centre stage
for the explicit definition of collections of types; it can be re-bound
to other variadic templates and supports simple direct manipulation
- for more elaborate and advanced processing tasks, a ''Loki-style type list''
can be obtained from a type-sequence, allowing to perform recursive
list processing task with a technique similar to LISP.
A very performance relevant shortcoming of the existing implementation
of partial function closure is that the result is always wrapped into a
std::function, which typically causes a heap allocation when more than
a single pre-bound argument must be stored — which is annoying,
since the underlying Binder provides inline storage and thus
could be handled directly as a value object.
However, returning the Binder directly is also problematic, since
this object is outfitted with several overloaded function call operators,
which defeats most techniques to detect a function signature. Notably,
relevant down-stream metaprogramming code, like the tuple-closure used
in the `NodeBuilder` would break when being confronted directly with
a binder object.
An investigation shows that there is no direct remedy, short of
wrapping the binder into another functor. This can be accomplished
with a helper template, that generates a wrapper; however, this
wrapper builder must be supplied with explicit type information
regarding the function arguments (precisely because this type
signature can not be picked up from the Binder object itself)
A lot of repetitive, pre C++11 metaprogramming code can now be removed,
and several helper constructs, which were needed to handle generic
function application and passing a tuple of values to create a binder.
Note however, the highly complex and technical core of this header
still remains intact; which is to create a ''partial closure'' over
some arguments of a function, while keeping the remaining arguments
open as parameters for invocation.
TODO: Even in the remaining code there is a lot of redundancy
and helper construct which are no longer necessary
...can now be formulated in a single function,
based on the apply-to-λ technique invented by David Vandervoorde.
WARNING: the rewritten version of BindToArgument<...>::reduced()
does not compile in the out-of-bounds case, revealing a possibly
long standing defect in the typelist-metafunction Splice
This library header was developed at a time, where C++ had no built-in support
for so called "invokables"; `std::invoke` and `std::apply` were added much later;
So in that early version that was a significant technical hurdle to overcome.
seems like it might be possible to get rid of the TupleApplicator alltogether?
This is one of the most problematic headers, because it is highly complex
and comprises tightly interwoven definitions (in functional programming style),
which in turn are used deep within other features.
What concerns me is that this header is very much tangled
and pushes me (as the author) to my mental limits.
And on top of this comes that this code has to deal with intricate aspects
like perfect forwarding, and proper handling of binder instances and
function argument copying (which basically should be left to `std::bind`)
Fortunately, the changes ''for this specific topic'' are transparent:
Type sequences are not used on the API for function closure and composition,
but only as an internal tool to assemble argument tuples used for either
binding or invocation of the resulting (partially closed) function.
Attempting to reduce the remaining pre-C++11 workarounds before upgrade to C++20...
As a first step: rename the old type-sequence implementation into `TyOLD`
to make it clearly distinguishable; a new variadic implementation `TySeq`
was already introduced as partial workaround, and the next steps
will be to switch over essential parts of the type-sequence library.
* Lumiera source code always was copyrighted by individual contributors
* there is no entity "Lumiera.org" which holds any copyrights
* Lumiera source code is provided under the GPL Version 2+
== Explanations ==
Lumiera as a whole is distributed under Copyleft, GNU General Public License Version 2 or above.
For this to become legally effective, the ''File COPYING in the root directory is sufficient.''
The licensing header in each file is not strictly necessary, yet considered good practice;
attaching a licence notice increases the likeliness that this information is retained
in case someone extracts individual code files. However, it is not by the presence of some
text, that legally binding licensing terms become effective; rather the fact matters that a
given piece of code was provably copyrighted and published under a license. Even reformatting
the code, renaming some variables or deleting parts of the code will not alter this legal
situation, but rather creates a derivative work, which is likewise covered by the GPL!
The most relevant information in the file header is the notice regarding the
time of the first individual copyright claim. By virtue of this initial copyright,
the first author is entitled to choose the terms of licensing. All further
modifications are permitted and covered by the License. The specific wording
or format of the copyright header is not legally relevant, as long as the
intention to publish under the GPL remains clear. The extended wording was
based on a recommendation by the FSF. It can be shortened, because the full terms
of the license are provided alongside the distribution, in the file COPYING.
Investigation in test setup reveals that the intended solution
for dynamic configuration of the RandomDraw can not possibly work.
The reason is: the processing function binds back into the object instance.
This implies that RandomDraw must be *non-copyable*.
So we have to go full circle.
We need a way to pass the current instance to the configuration function.
And the most obvious and clear way would be to pass it as function argument.
Which however requires to *partially apply* this function.
So -- again -- we have to resort to one of the functor utilities
written several years ago; and while doing so, we must modernise
these tools further, to support perfect forwarding and binding
of reference arguments.
we made double use of our Tuple type, not only as a
generic record, but also as a metaprogramming helper.
This changeset replaces these helpers with other
metafunctions available for our typelists or type sequences
(with the exception of code directly related to Tuple itself,
since the intention is to delete this code alltogether shortly)
to make them stand out more prominently, some entity comments
where started with a line of starts. Unfortunately, doxygen
(and javadoc) only recogise comments which are started exactly
with /**
This caused quite some comments to be ignored by doxygen.
Credits to Hendrik Boom for spotting this problem!
A workaround is to end the line of stars with *//**
the buildsystem will now pick up and link
all test cases according to the layer, e.g.
backend tests will automatically be linked
against the backend + library solely.
2013-01-07 05:43:01 +01:00
Renamed from tests/lib/meta/function-closure-test.cpp (Browse further)
