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From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-04-09 11:55:42

On Wed, Apr 9, 2008 at 11:16 AM, shunsuke <pstade.mb_at_[hidden]> wrote:
> Giovanni Piero Deretta wrote:
> > I'm attaching some comments after a not-so-brief reading of the egg
> > documentations.
> >
> I'm attaching the answers to your egg.txt.

And here are my replies to some of your answers.

> > === Introduction
> >
> > - "Egg what for?" (shouldn't it be "What is Egg for?"): why do I need to
> > use the ugly BOOST_EGG_PIPABLE syntax? why doesn't the following
> > syntax work?:
> >
> > result_of_pipable<T_plus>::type const my_plus = {};
> (As you pointed,) Those macros are unneeded in the case of stateless functions.
> So, I propose a new `static_` form:
> The adapting form in review version:
> result_of_curry2<F>::type const c = BOOST_EGG_CURRY2({}); // stateless
> is changed to a new `static_` form *without macros*:
> static_< result_of<T_curry2(F)> >::type const c = {{}};
> How do you think about this new form?

Much better, but I do not particulary like the static_ name
(expecially because of the trailing '_'). Statically_initialized is
too long, so if you can't come up with anything better and shorter, I
can live with static.

Btw, can you explain me (again, please :P) why

  result_of<T_curry2(F)> >::type const c = {{{}};

isn't enough? I guess I should see static_ implementation.

What about collapsing static_ and result_of in a single class, and
allowing arbitrary composition:

  apply<compose, apply<compose, my_fun1, my_fun2>, my_fun3>::type c = {{}};

The apply name is already taken by MPL, but maybe something of the sort.

> > - Notation:
> >
> > Please get rid of this notation. The double underscore syntax hurts
> > my eyes :). Also it is very hard to have to always refer to this
> > section when reading the documentation. The last four entries are
> > obvious enough, and so is decltype. For the rest you should find a
> > different, self explicative notation.
> Because the document of FunctionAdaptor section is going to
> be simplified, I think I can remove most of them.


> > 3. "Unintentional ADL". It is claimed (in the solution) that egg
> > solves the problem of unintended adl by making make_filtered an
> > object.
> > Be aware that gcc performs ADL even on function objects.
> Which version of gcc?

AFAIK all of them:

namespace a {

    struct foo_t {};

    struct bar_t {
        void operator()(foo_t){};
    } bar;

int main() {
    a::foo_t foo;


This compiles at least with gcc-4.1.3 and gcc-3.3.6 (it doesn't with
comeau online)
The biggest problem is of course is that gcc fails to compile this:

namespace a {

    struct foo_t {};

    struct bar {

void bar(...);

int main() {
    a::foo_t foo;


I.e. ADL finds even non callable entities! Comeau online compiles it cleanly.

> > - "Binding functions"
> >
> > "boost::result_of isn't used for now". Why?
> Because of a bug described in the `egg::result_of_` section.

Ah, now understand why I'm missing some documentation details! the
documentation doesn't always clearly show which words are links. for
example in my browser result_of_ is not visually distinguished from
the rest of the paragraph. I do not know if it is a problem of my
browser of you need to fix your CSS.

> > The description of lazy supposes that the reader is familiar with
> > boost lambda which is a non trivial library. Of course egg is not
> > exactly a beginner library, so it might be expected.
> > The visual distinction between make_filtered and make_Filtered might
> > not be caught immediately. May be change the name?
> > Also, I do not like this convention of having the lazy variant
> > having an uppercase letter in the middle. Is this just a
> > random example or is meant to be a useful convention? (BTW, I think
> > that egg documentation should have a good section about function
> > object naming conventions).
> I don't find a convertion for lazy and unlazy functions yet.

I do not either, other than puting them in a 'lazy' namespace. This is
one of the reasons I like optionally lazy function objects :).

> > Also I do not like the T_* convention to name function types. I
> > liked best the old op_* convention.
> LexicallyTypedObject concept is not restricted to FunctionObjects.
> "T_" is short of "typeof".
> This concept is a workaround for "missig decltype" in C++03.

What about using the prefix 'typeof_' or a postfix '_type'? I really
dislike that uppercase T.
Maybe even a postfix _t would be more than enough.

> > - Getting initializers
> >
> > "As mentioned before, the static initialization is important". Why?
> > You still do not explain it.
> Click "before".

Right, missed the link. Probably this part should go in an advanced
section or at least an had-hoc section on static initialization.
Certainlin not in the quick start.

> > - Object generators
> >
> > It is completely un-obvious what are you trying to explain
> > here. Even clicking on the link about object generators doesn't
> > really enlighten me much: if the name 'always' parallels the mpl
> > counterpart it should be a function that always returns the same
> > value. But why you call it 'always_return' and not just 'always'?
> >
> > Ah, I see, you later name the actual object 'always'. But why the
> > adapted class (or would be better to call it 'facaded'?) was
> > postifixed _result? If it were just a metafunction to compute the
> > result type I would understand, but the 'call' method makes it
> > something more than an mpl metafunction.
> always_result means a return type of always.

sure, but it also contains the body of 'always' (i.e. the call
member), so it is a misleading name.

> > "bind expressions my be less readable. nestN offers a better syntax"
> >
> > egg::nest2(plus)(5, egg::nest2(_1_(_1))(_1_(_2), _0_(_1)))
> > (i6)(minus, i7);
> >
> > Is that a joke?!?!? :) I use both boost.bind and lambda.bind
> > extensively and while I might not like the syntax, at least is clear
> > what it does. the _N_(_M) syntas not only is *extremely* ugly, but
> > I have absolutely no idea of what are you trying to do! Please
> > explain.
> >
> > The haskel-like lambda syntax doesn't help. I can barely read simple
> > haskel expressions, certainly not nested lambdas. I expect most C++
> > programmers (even very sophisticate ones) wouldn't either.
> nestN returns a function which returns a function which returns a function...
> Probably I should change the notation into C++0x lambda expression?

That would probably help a little. Much more prose (like a step by
step analisys of what that expression means) would be much better.

> > I find the prefix T_ extremely ugly. I preferred what I
> > think egg used previously, i.e. the op_ notation.
> This convention is not restricted to FunctionObjects.
> `T_` is short of "typeof".
> This convention is a workaound for "missing decltype" in C++03.

I know, I just find the syntax ugly.

> > Even better would be
> > to segregate function object types to their own namespace, a-la fusion
> > or proto. (i.e. the functional namespace).
> See this:
> namespace poost {
> namespace op {
> struct foo {};
> }
> op::foo const foo = {};
> namespace nested {
> namespace op {
> struct bar {};
> }
> op::bar const bar = {};
> void test()
> {
> op::foo my_foo = foo; // doesn't compile
> }
> }
> }
> Thus, I've rejected segregated-namespace-way.

void test() {
   poost::op::foo my_foo = foo; // it compiles!

Seems a very simple fix (and arguably the right thing in the first place!)

> > - Pipable and Ambi: These concepts are intermixed and too fuzzy
> > IMHO. All the details of Ambi and Pibable, should be collapsed on a
> > simpler Pibaple concept
> I made some efforts to define these concept.
> I believe this way is the simplest (for now).
> > (and BTW, Ambi is not a self descriptive name):
> I will be happy if a better name for ambi is found in this review.
> > " A pipable function object 'f' is an unary polymorphic function
> > object for which the expression 'x | f' is equivalent to f(x) ".
> >
> > Making a function object pipable should just be a matter of deriving
> > it from an (empty) egg::pipable, which would manifest the intention of
> > the object to participate in pipelines.
> I'm not sure it has an advantage over higer-order functions.

Less intellectual overhead :). I need to know less things to
appreciate what 'pipable' means. Concept should be as simple as
possible IMHO. (you can of course add "... but not simpler" :) )

> > Oven should provide a free
> > function operator| (that will be found via adl, and sfinae'd on
> > is_base_and_derived<pibable, Rhs>).
> Sorry, I couldn't understand this sentence.
> Why is Oven stated here?

I meant Egg of course. What I wanted to say is:

Egg provides a templated operator| in egg namespace. There is a single
implementation for this operator and is:

namespace pibable_ {

struct pipable {}; // ADL hook

template<typename Lhs, typename Rhs>
boost::lazy_enable_if<bost::is_base_and_derived<pipable, Rhs>,
boost::result_of<Rhs(Lhs&)> >::type
operator|(Lhs& lhs, Rhs rhs) {
   return rhs(lhs);

template<typename Lhs, typename Rhs>
boost::lazy_enable_if<bost::is_base_and_derived<pipable, Rhs>,
boost::result_of<Rhs(const Lhs&)> >::type
operator|(const Lhs& lhs, Rhs rhs) {
   return rhs(lhs);

Any unary function object that wants to model the pipable concept must
derive from pipable_::pipable. This will trigger ADL and will find the
operator | when necessary. (In c++0x of course you would put
operator| in the global namespace and make it a template function
constrained on the (non auto) concept Pipable)

This of course only work with unary callable entities. to make a
non-unary entity unary, apply the appropriate curry/bind grease :).
This will also get rid of the need for Ambi.

As an extension (I'm not proposing it, it is just for the sake of
discussion), '|' could be a synonym for compose (IIRC it is spelled
'$' in haskell):

 if lhs and rhs are callable entites (in practice you detect pipability):
   a | b
 is the same as:
   compose(b, a);

 else if only b is a callable entity:
   a | b
 is the same as
   compose(b, always(a));

 else the expression is illegal.

[IIRC in haskel values are in practice treated as nullary functions,
so the use of 'always' here would mimic the functional comunity usage]

This means that you can create pipelines of transforms:

   map(my_range, my_first_view|my_second_view)

Would be the same as:

   map(my_range, compose(my_second_view, my_first_view));


   map(my_range, my_first_view) | protect(lazy(_1, my_second_view))

[where map returns a pair of transform_iterators]. As an alternative
for the ugly protect + lazy, read further.

> > Adapting function objects of arbitrary arity to make them pibable is
> > really orthogonal. Lambda would works just fine, but egg could provide
> > (if already doesn't)adaptors to add curriability functionality to a
> > generic function object:
> >
> > struct op_foo : curriable<op_foo> {
> > typedef whatever result_type;
> > template<class A1, class A2, class A3>
> > result_type operator()(A1, A2, A3);
> > } foo;
> >
> > x | foo(_, 10, "bar");
> >
> > Note that you need to explicitly mark the missing argument. And in
> > principle you can have more than one missing argument:
> >
> > f(_, 10, _)("baz", "bar");
> Sorry, I couldn't understand this proposal.
> What does `x | foo(_, 10, "bar")` mean?

hum, let me see, in egg syntax it should be:

  compose(lazy(foo)(_1, 10, "bar), always(x))

but see below:

> Also, f(_, 10, _)("baz", "bar"); seems supported by egg::lazy.

yes, the only difference is that the result is not a lambda expression
(as if there was an implicit protect):
see the difference between:

  lazy(foo)(lazy(bar)(_1, _2)); // ll::bind(foo, ll::bind(bar, _1))


  lazy(foo)(protect(lazy(bar)(_1, _2))); // ll::bind(foo,
protect(ll::bind(bar, _1)))

with my syntax (actually this is lifted directly from the generalized
currying in FC++), you could spell the latter:

  foo(bar(_,_)); // s

this is important if egg were (as one would expect) to register its
operator| with boost::lambda:

 map(my_range, _1 | lazy(is_less)(_1, 10)); // does not work!

 map(my_range, _1 | protect(lazy(is_less)(_1, 10))) ; //ok

 map(my_range, _1 | is_less(_, 10)); // also ok

A nice name for the generalized curry operation is of course curry:

  auto is_less = curry(is_less_non_curriable);

 assert( is_less(_, 10)(7) == true );

This is trivially implementable with lazy + protect, but an ah-hoc
implementation might be simpler and easier on the compiler (no need
for full blown lambda support). Also, i spell the missing parameters
'_' because that's what FC++ used, 'deferred' might also be a good

What do you think?

> > - Major Function Object: this name is not descriptive. Make it Lambda
> > Compatible Polymorphic Function Object.
> A specific name should not be contained in this concept name
> so that MajorFunctionObject can support yet another lambda library.ù

Hum, the support hook shuld only be boost::result_of and its protocol.

> > Or better, modify boost.lambda
> > so that any Polymorphic function object work with it (shouldn't be
> > hard), so that we do not need an ad-hoc concept.
> It seems hard.

I do not think so, in fact I think that there are patches around. On
the other hand, I'm not the one doing it, so I shouldn't complain.

> > - Static Function Object: again, you are mixing a concept, with
> > pertains types, with restriction on object usages or initialization. I
> > do not think that you can encode the fact that 'f' is statically
> > initialized in a concept. Also, this 'concept' badly need a rationale!
> I'm going to modify this concept.
> The "stateless" requirement will be added,
> and the static-initialization guarantee will be removed.
> Do you think it is right?

I do not know if it is right, but I have only needed static
initialization for stateless function objects. Others might have
different experiences (and they should better speak now ;) ).

> > - Writing you strategies: why should I want to write my own strategy?
> > What can I do exactly?
> See the Eric's request below.
> > In valid expression you introduce apply_little, but you do not
> > document it anywhere it seems (its use seems obvious, though).
> It isn't placed in Valid expressions table?

yes, my fault, but those tables are almost unreadable to me, sorry :(

> > Where
> > is the custom forward strategy in the valid expression box? How
> > should be interpreted.
> Sorry, I couldn't understand this question.

Me neither :). I have no idea of what I was asking.

> > === Function builders
> >
> > - function: finally we find the function class! I object in another
> > class in boost called function. What about function_adaptor?
> This is not an adaptor.

Hum, IMHO,yes. It is just adapt a class from the internal Minor
function object convention to the external Major function object.

> IIRC, Boost.Phoenix also uses "function".

Yes, I saw it too. But Phoenix is not a first class boost library yet
:). when it will be reviewed, I'll make the same comment.

> > BTW, do you want to encourage stateful
> > function objects? Since I learned 'bind' I have never again written
> > a stateful function object.
> I rarely use Boost.Bind and Boost.Lambda in production code.

Why not? I do all the time.

> > It would be great if egg had a way to
> > express the result type of bind (in fact I think that lazy does
> > exactly that).
> If a Boost.Lambda expression is simple, it is feasible.
> The full support would be challenging work.

Agree. It would enought it just supported function composition (i.e.
no overloaded operators).

> > "it can be considered a port of callable". I do not know what
> > 'callable' is. Please provide a link/reference.
> Click 'callable'.

Ah, I see. Again, the word here doesn't show up as a link untill i
mouse over it.

> > - implicit. "Implicit adds implicit conversion support to a cast from
> > function". What does that means? What are the use cases?
> It automatically passes initialized types to cast functions.

could you provide an example?
> > Arguments except for the last one are captured by-copy. When you
> > need to capture by-reference, pass arguments using boost::ref."
> > Why the last one is an exception (I guess your use case, and mine too,
> > is to pass ranges and containers as last arguments)
> No, it is not range problem.
> Currying is "binding", per se.
> It must copy arguments to make an intermidate FunctionObjects.
> The last argument is not needed to be copied fortunately.

I figured it out later :).

> > In the example, mabye 'my_minus_base' would be a better name for
> > the struct?
> I prefer prefix, but I'm sure I don't know english well. :-)

Oh, well, me neither :P

> > - Uncurry. Uh? What is this useful for? BTW, a more useful facility
> > would be the ability to traverse all objects stored in a curry
> > closure (is closure the right term?), like the undocumented
> > boost.bind apply_visitor.
> I couldn't find boost.bind apply_visitor.

The actual name is visit_each (which I think is the protocol used by
apply_visitor). There is some reference in bind/storage.hpp. The
actual api is not documented, but I guess that the intent is that
boost::visit_each(vistitor, bind-expression); should iterate on all
arguments closed in the bind expression. Searching the boost mailing
list reveals that bind_visitor has been broken for a while. I do not
know if it is still true (I have never used it - yet).

> > - lazy. Useful, 'nuf said. I would also like to have optionally lazy
> > function objects, but it seems that I'm the only one who likes them
> > :).
> In fact, I know another one who likes them.


> > - mono. I have never needed this, but I understand that it might be
> > useful sometimes.
> IIRC, Dan Marsden's Boost.Traversal makes use of one.

I was missing the fact that it allows one to inspect the argument types.

> > - nestN. This is still very very obscure to me. The more I read the
> > description the more confused I become. I understand that the
> > problems are nested lambda expressions, but I do not understand how
> > your soultion work, what the syntax is and why it must be so
> > complicated
> A comparison between "bind" and "nestN" has been requested,
> so I will add some explanations of details.


> > (did you see how phoenix handles the problem via local
> > variables to lambda?)
> Because Egg is not "yet another lambda library",
> I'm not sure it should provide some cool syntax.

well, at least readable :)

> If reviewers regard it as an inadequate component,
> I will simply remove nestN from Egg.
> BTW, how can we write nested lambda using phoenix?

  lambda[ for_each(_1, lambda(_a = _1)[ front(_a) += _1] )(range)

[at least It should work, given appropriate definitions of for_each and front]

> >
> > - bll_N. Is this just an alias for lambda placeholders?
> > why i can't just use _N and boost::lambda::placeholderN_type?
> bll_N is a model of LexicallyTypedObject
> so that you don't need to remember the name "placeholderN_type".

so now I have to remember two types instead of one :)

> > - construct_braced{1,2}. Eh? what is this for?
> >
> > - construct_variadic. Same as above.
> egg::generator needs this to initialize pod types.

Is it a general purpose utility of just an implementation detail?

> > Doesn't having to specify the signature partially defeats the
> > purpose? Why can't I just do:
> >
> > BOOST_AUTO(f, unfuse(fuse(_1 + _2)))
> >
> > and keep my lovely polimorphic-nes? i.e. would it be possible for
> > egg to (optionally) register all its type with boost.typeof?
> Egg could do, but I think it is nearly impossible for Boost.Lambda.

Unless lambda registers its types of course. Anyways, It would be
enough for me if this worked:

BOOST_AUTO(foobar, compose(foo, bar));

> > Of course the user would be responsible for registering its own
> > function objects.
> I can't imagine users register their function objects.

Well, if an user wants to use typeof it needs to registers its types anyway.

Enough for now. More comments to come.


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