Re: [boost] [Fit] Review

Subject: Re: [boost] [Fit] Review
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-03-10 13:46:05

On Thursday, March 10, 2016 at 12:34:26 AM UTC-6, Vicente J. Botet Escriba 
wrote:
>
> Le 10/03/2016 01:12, Paul Fultz II a écrit : 
> > 
> > On Wednesday, March 9, 2016 at 8:50:01 AM UTC-6, Zach Laine wrote: 
> >> 
> >> There are a number of adaptors that I find have obscure names: 
> >> 
> >> by() is a projection; why not call it project()? 
> >> 
> > I chose `by` because of how it is used linguistically. That is you 
> write: 
> > `sort(v.begin(), v.end(), by(&Employee::Name, _<_))` which I read "sort 
> by 
> > employee name". Also, if I write `by(decay, construct<std::tuple>())`, I 
> > read 
> > "construct tuple by decay". In haskell, it uses `on` for this adaptor, 
> which 
> > is what I originally called it. `project` is a more straighforward name. 
> `by` is a comparator while `projet` is a projection. 
>

By is a projection in the example of `construct`.
 

> >   
> > 
> >> compress() is defined in terms of "fold", with a link to the Wikipedia 
> >> page 
> >> for same; why not call it foldl()?  Similarly, reverse_compress() 
> should 
> >> be 
> >> foldr(). 
> >> 
> > Not `foldr`. This is because `foldr` is symetrical. For example, `foldl` 
> and 
> > `foldr` should produce the same results: 
> > 
> >      foldl(_+_, std::string())("Hello", "-", "world"); // "Hello-world" 
> >      foldr(_+_, std::string())("Hello", "-", "world"); // "Hello-world" 
> It would be good to use a binary function that has two different types 
> to see the difference in behavior. 
>

Yes, you could do `flip(_+_)` to get the same effect. However, I want the
callback to be consistent with compress reverse_compress, so it always is
called with f(state, element). Also, `fold` and `reverse_fold` are commonly
used names in C++, so I would like to keep it more consistent with those
usage. If the user wants `fold_right`, it can be easily written, like this:
`auto fold_right = compose(compress, flip)`.
 

> > However, compress and reverse_compress work like this: 
> > 
> >      compress(_+_, std::string())("Hello", "-", "world"); // 
> "Hello-world" 
> >      reverse_compress(_+_, std::string())("Hello", "-", "world"); // 
> > "world-Hello" 
> > 
> > I was reluctant to call it `fold` as it seems to imply some data 
> structure 
> > to 
> > fold over, whereas this is simply an adaptor. I used the word compress 
> as 
> > its 
> > a another name used for fold. However, it seems more people would prefer 
> to 
> > use `fold` and `reverse_fold`. 
> You are folding the parameter pack. I like those. and I would like also 
> fold_left, fold_right. 
> >   
> > 
> >> flow() is a super obscure name!  Why is this not reverse_compose() or 
> >> similar? 
> >> 
> > I actually got the name from underscore.js. The reason why I chose this 
> name 
> > instead of `reverse_compose` is because when this is used with pipable 
> > functions it seems confusing: 
> > 
> >      reverse_compose( 
> >          filter([](int i) { return i < 3; }), 
> >          transfrom([](int i) { return i*i; }) 
> >      )(numbers); 
> > 
> > With the word 'reverse' there, it almost looks as if it computes the 
> > pipeline 
> > in reverse order, which it doesn't. So I would prefer a name without 
> reverse 
> > in it. 
> Would pipe works better? 
>

pipe could work, I don't know how others feel about such a name.
 

> >   
> > 
> >> indirect() could more easily be understood if it were called deref() or 
> >> dereference(). 
> >> 
> > Well, `indirect` is how it is commonly called in libraries such as 
> > Boost.Range, range-v3, and PStade libraries. So I would like to keep the 
> > name 
> > consistent with other similar constructs. 
> >   
> Could you tell us what indirect is in range-v3, what is the signature 
> and the semantics. I don't see it is used for the same purposes, but 
> maybe I'm wrong. 
>

It dereferences the values in a range. They both are functors that will 
dereference the parametrized type.
 

>
> ref(flv) is a callable that wraps an lv 
> indirect(ptr) is a callable that wraps a ptr and deref it before calling 
>

Well, it can be used with anything that is dereferenceable. So it works with
boost::optional as well.
 

>
> deref/dereference will only relate the dereference action, but not the 
> call. 
>
> deref_on_call is too long? 
>

Yes it is.
 

> >> I don't feel as strongly about partial(), but I think it might be 
> clearer 
> >> if 
> >> it were called partial_apply() or curry(). 
> >> 
> > Hmm, I don't think a name with `apply` in it is good name for an 
> adaptor. 
> Maybe you could explain why curry is not a good name here? What is the 
> difference between partial an currying a function. 
> >   
> > 
> >> conditional() should be called invoke_first(), call_first_of(), or 
> >> similar. 
> >>   I 
> >> find it too easy to confuse with if_(). 
> >> 
> I agree conditional is not a good name. 
>
>
> > I see, I was trying to describe an adaptor where you could put functions 
> > with 
> > conditions in it. Other people, seem to prefer a name like `linear`, so 
> I 
> > might use that instead. 
> This function is related to match. The difference is that one select the 
> best matching overload and those must be exclusive and the other the 
> first matching overload and the match can be inclusive. I would like to 
> see the semantics on the name, but I have not a concrete proposal. 
>
> We need to take in account that this is an adaptor, so there is no call, 
> so `invoke_first` or call_first_off will not work. Those functions 
> creates another function object that applies a different algorithm to 
> select the overloaded functions when called. 
>
> I've used 
>      overload -> match 
>      first_overload -> conditional 
>

I would prefer `match` and `linear`.
 

>
> I prefer those, but I'm not yet happy with. Naming is difficult. 
> >> 
> >>   
> >> 
> >> 
> >>    * Documentation 
> >> 
> >> The Quick Start section is good at showing a compelling motivating case 
> >> for 
> >> using the library.  The result at the end of the section looks like a 
> very 
> >> good start on a "Scrap Your Boilerplate" solution for simple, 
> dump-style 
> >> printing.  It's relatively easy to read and write. 
> >> 
> >> The documentation is what needs the most work, from what I've seen. 
>  The 
> >> Quick 
> >> Start is quite nice, but then there are some things that are 
> >> underexplained 
> >> in 
> >> the subsequent Overview and Basic Concepts sections.  For instance, why 
> >> are 
> >> these macros used everywhere and what do they do?  They should be used 
> for 
> >> the 
> >> first time after showing what the expanded code looks like. 
> > 
> > I don't think the docs should show the expansion of the macros, that is 
> part 
> > of the implementation and not interface. I could show an "idea" what is 
> is 
> > expanding to, with explanation of what else it is doing beyond the 
> simple 
> > explanation. 
> I agree hat it is absolutely needed to show the exact expansion, but it 
> is clear that the user wants to know what is behind the scenes.

 
I strongly disagree. Documentation is about documenting the interface, not 
the
implementation. Mainly because the implementation could change or vary 
between
platforms while the interface would remain the same. If the user wants to 
know
about the implementation, they can look at the source code or in the case of
macros, look at the preprocessed output.
 

>
> >   
> > 
> >> This will 
> >> justify 
> >> their use for those users that want them, and show those that don't 
> what 
> >> they 
> >> can write instead. 
> >> 
> > 
> > I could show an alternative without the macros, but I would prefer to 
> put 
> > that 
> > in the Advance section. In the introduction of the library, I would 
> rather 
> > show the constructs that can be easily used without a need to explain a 
> > bunch 
> > of caveats. 
> I will put all of them in the advanced section. I would no mention them 
> in the introduction as the user can create the HOF locally or use a 
> factory. 
>

Being able to declare the functions as global variables is a key feature of
the library. The example in the quick start guide shows how to implement 
many
things without the need for a lot of template boilerplate. For those users
that prefer not to use global function objects(and I have yet seen a
compelling reason not to use them) can look at the 'Advanced' section.

I realize now, that I need to spend more discussing the advantages of using
global function objects(more composability), and address some of the
misperceived issues with these global function objects(they can be in a
namespace to avoid name conflicts and are not mutable so it won't be a 
problem
in mutilthreaded environments).
 

>
> Best, 
> Vicente 
>
>
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