Re: [Boost-users] [boost] [Fit]Â formal review starts today

Subject: Re: [Boost-users] [boost] [Fit] formal review starts today
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-03-07 23:41:54

On 3/7/2016 10:02 PM, Paul Fultz II wrote:
>
>
> On Monday, March 7, 2016 at 2:52:09 AM UTC-6, Edward Diener wrote:
>
>     On 3/7/2016 12:15 AM, Paul Fultz II wrote:
>      >
>      >
>      > On Sunday, March 6, 2016 at 9:42:19 PM UTC-6, Edward Diener wrote:
>      >>
>      >> On 3/3/2016 6:43 AM, Vicente J. Botet Escriba wrote:
>      >>> Dear Boost community, sorry for the late anounce
>      >>>
>      >>> The formal review of Paul Fultz II's Fit library starts today,
>     2nd March
>      >>> and ends on 13th March.
>      >>>
>      >>> Fit is a header-only C++11/C++14 library that provides
>     utilities for
>      >>> functions and function objects.
>      >>
>      >> These are some comments/queries about the Fit documentation.
>      >>
>      >> Introduction
>      >>
>      >> The introduction says that Fit "provides utilities for functions
>     and
>      >> function objects." But it seems as if Fit works only with lambda
>      >> functions and function objects.
>      >
>      >
>      > Fit works with any generalized Callable:
>      >
>      > http://en.cppreference.com/w/cpp/concept/Callable
>     <http://en.cppreference.com/w/cpp/concept/Callable>
>
>     Please document this. A generalized Callable is a much bigger set of
>     functionality than just lambda functions and function objects.
>
>      >
>      >
>      >> The term 'function' normally encompasses
>      >> a much larger definition in C++ which includes global functions,
>     static
>      >> functions, member functions, and lambda functions.
>      >
>      >
>      > Which is what Callable encompasses, but I used "function" since more
>      > people are familiar with that than Callable.
>
>     A 'function' and a Callable in C++ are two entirely different things.
>     Use the one that is precise and don't worry what people are familiar
>     with. Explain what a Callable is early on ( I know you explain it later
>     ) if you feel that people do not know what it means.
>
>      >
>      >
>      >> Fit needs to be more
>      >> precise in what it says it works with. It repeatedly refers to
>     function
>      >> objects and lambda functions as 'functions'. I think this
>     vagueness of
>      >> terminality is really confusing in the documentation.
>      >>
>      >
>      > Perhaps, I should refer to the Callable concept early on then.
>
>     See above.
>
>      >
>      >
>      >>
>      >> Quick Start:Function Objects
>      >>
>      >> "We can make it behave like a regular function if we construct
>     the class
>      >> as a global variable."
>      >>
>      >> What about a non-global
>      >>
>      >> sum_f sum = sum_f();
>      >>
>      >> makes 'sum' not behave like a regular function other than the
>     fact that
>      >> the variable 'sum' may eventually go out of scope ?
>      >>
>      >
>      > In C++, a regular function is always global, there is no such
>     thing as local
>      > function(sans gcc extensions).
>
>     My point is that the non-global 'sum' in my example above behaves just
>     as much like a regular function as your global 'sum'. You may want to
>     promote the idea of global function objects but I think that this is
>     personally a bad idea. IMO global variables of any kind are to be
>     avoided.
>
> It is quite common in several modern C++ libraries to declare functions as
> global objects. There are many advantages to this. Why do you believe it
> should be avoided? Especially since it has the same effect as a free
> function.
> DO you believe free function should be avoided as well?

Global objects, whether functions or data, increases the chances for 
name clashes. Also accessing global data increases the chances for 
errors in a multi-threaded environment.

Free functions can be in a namespace, as the standard library functions 
are in the 'std' namespace. This helps greatly in avoiding name conflicts.

>
>
>
>      >
>      >
>      >>
>      >> Quick Start:Lambdas
>      >>
>      >> Why do we need both BOOST_FIT_STATIC_LAMBDA and
>      >> BOOST_FIT_STATIC_LAMBDA_FUNCTION ? I would seem that
>      >> BOOST_FIT_STATIC_LAMBDA_FUNCTION would be adequate and
>      >> BOOST_FIT_STATIC_LAMBDA is just redundant, mimicking lambda
>     syntax to no
>      >> purpose.
>      >>
>      >
>      > BOOST_FIT_STATIC_LAMBDA_FUNCTION and BOOST_FIT_STATIC_FUNCTION
>     both define a
>      > function at global scope, and can only be used at global scope,
>     whereas
>      > BOOST_FIT_STATIC_LAMBDA can be used to constexpr initialize local
>     variables
>      > as
>      > well.
>
>     It might be good to add that when the end-user first encounters
>     BOOST_FIT_STATIC_LAMBDA, because the name does not suggest
>     initialization of local objects. Furthermore I cannot imagine why one
>     would want to use it to initialize a local object,
>
>
> Really? You just said that you would prefer avoiding global functions.

I mean that for local objects I do not see the reason why I would ever 
use BOOST_FIT_STATIC_LAMBDA instead of initializing local objects in the 
normal C++ way(s). If this is not the case you need to explain what 
problem(s) BOOST_FIT_STATIC_LAMBDA solve in initializing local data 
which ordinary C++ initialization of local data could encounter. If 
there aren't any problems BOOST_FIT_STATIC_LAMBDA solves in initializing 
local data I see no purpose for its existence.

>
>     so you might want to
>     explain the benefit of doing so as opposed to the normal syntax for
>     creating a lambda function.
>
>
> Maybe the quick start guide isn't the place for this in the first place.

I would hope to convince you that a good Overview of your library should 
come first and then all the Quick Start guides, tutorials, examples etc. 
would make much more sense once your end-user understands what your 
library really does in terms of at least the main general functionality.

>
>
>      > In fact, BOOST_FIT_LIFT uses this since it is not always clear what
>      > context the user might call BOOST_FIT_LIFT.
>      >
>      >
>      >>
>      >> Quick Start:Overloading
>      >>
>      >> The overloading adaptors show two or more lambda functions. Can
>     they
>      >> also work with function objects ? Or a mix of lambda functions and
>      >> function objects ? In fact all the reamining Quick Start topics
>     show
>      >> examples with lambda functions. Do they also work with function
>     objects ?
>      >>
>      >
>      > Yes it can be used with function objects. I probably should show
>     an example
>      > of
>      > that as well. I used the lambdas because of the terseness of them.
>
>     Evidently the adaptors can be used with any Callable. I think you
>     should
>     make that point very strongly and show that in examples also.
>
>
> Agreed.
>
>
>      >
>      >
>      >>
>      >> Quick Start:Variadic
>      >>
>      >> I do not understand what 'We can also make this print function
>     varidiac,
>      >> so it prints every argument passed into it.' means ?
>      >>
>      >
>      > I'll try to explain that better, but basically it will print each
>     argument,
>      > so:
>      >
>      > print("hello", 5); // Will print "hello" and 5
>
>     I wouldn't associate that with the word 'Variadic' but I do think you
>     need to explain that more clearly.
>
>
> I always understood variadic to mean taking a variable number of
> arguments. What do you understand variadic to mean?

Your terminology is fine, but I always think of variadic in terms of 
variadic templates or variadic macros. After all your not making the 
print function variadic in the sense of taking a variable number of 
arguments. The simple_print functionality is any of 3 different lambda 
functions, with 'conditional' picking the first one that is callable and 
'fix' passing in the callable as the first parameter. So I really don't 
understand what 'by' does here in printing each argument irregardless of 
your 'Variadic' terminology. You need invoking examples and then show 
what the output would be for each invocation, and then it would be 
easier for me and others to understand what 'by' does in this situation.

>
>
>      >
>      >
>      >>
>      >> I do not think the Quick Start explains very much since it is
>     dealing
>      >> with adaptors of which we know almost nothing and the
>     explanation for
>      >> these adaptors and what they actually do is very terse.
>      >
>      >
>      > Probably can expand the explanation of adaptors a little more.
>
>     Good idea. What do adaptors create ?
>
>
> They create a function.
>
>     Different function objects I would
>     imagine ? In that case how about the explanation that adaptors take
>     Callables as input and generate function object types that adapt the
>     original functionality of one or more Callables to some other purpose.
>
>
> The examples are already show taking Callables. Are you suggesting I
> show an example taking a member function or something?

Global/static function, member function, std::function, boost::function, 
Boost Phoenix and/or Boost lambda function objects, instantiated 
template functions. My point is that you should show your library 
functionality working with as many types of Callables as possible. 
Instead your doc suggests that function objects and lambda functions are 
the only type which can be passed to your library's functionality and 
you enforce this view by your examples.

>
>      >
>      >
>      >>
>      >> In the 'Signatures' section of the Overview I read:
>      >>
>      >> "All the functions are global function objects except where an
>     explicit
>      >> template parameter is required." I honestly don't know what this is
>      >> supposed to mean. Does this refer to when function objects are
>     referred
>      >> to as parameters to the adaptors, functions, and utilities of
>     the library
>      >> ?
>      >>
>      >
>      > I don't understand what you are asking. It means that the
>     function is
>      > written
>      > like this in the documentation:
>      >
>      > template<class IntegralConstant>
>      > constexpr auto if_(IntegralConstant);
>      >
>      > But its actually a function object like this in the code:
>      >
>      > struct if_f
>      > {
>      >      template<class IntegralConstant>
>      >      constexpr auto operator()(IntegralConstant) const;
>      > };
>      > const constexpr if_f if_ = {};
>      >
>      > However, `if_c` is written like this in the documentation:
>      >
>      > template<bool B, class F>
>      > constexpr auto if_c(F);
>      >
>      > It requires the bool `B` template parameter explicity. So in the
>     code it is
>      > written as a function and not as a function object.
>
>     I don't understand to what you are referring when you say 'function'.
>     Are you talking about adaptors in your library, functions in your
>     library, or what ?
>
>
> I am talking about all functions that are defined in the library, that
> includes adaptors as well. I am not sure how to make that clearer.
>
>     Please try to understand that your use of the word
>     'function' is very broad but that the word 'function' in C++ has a much
>     narrower meaning.
>
>
> By function, I mean something like in the example:
>
> template<class IntegralConstant>
> constexpr auto if_(IntegralConstant);
>
> I think everyone agrees that is a function in C++.

I call that a function template. My definition for 'function' is not the 
same as yours. I know you feel differently but please realize that the 
term 'function', to mean any and every type of callable in C++, is not a 
universally agreed on terminology. I would bet that for the vast 
majority of C++ programmers a function is:

ReturnType FunctionName(ZeroOrMoreFunctionParameters);