Boost logo

Boost :

Subject: [boost] Phoenix review
From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-09-29 20:38:45

I apologize in advance that I could not dedicate the appropriate amount of
time for my evaluation of the library.

* Design

- The placeholders probably belong in namespace phoenix::placeholders
instead of arg_names, for consistency with std::placeholders.

- It is very odd to include phoenix_bind.hpp and not have the placeholders
available. For that I need phoenix_core.hpp.

- The decision to capture the left side of @= and = by value is debatable.
It is indeed not clear which one is 'better', but we do have Lambda in
Boost, and to deviate from it one needs a stronger supporting argument than

- In general, the subtle differences between Bind, Lambda and Phoenix need
an explanation, even if its only purpose is to clearly show that the
deviation is deliberate and not an accident.

- bind<R> is not supported.

- Smart pointers do not seem to be supported well in conjunction with bind,
even though they seem to work in other contexts.

- What is the equivalent of boost::unlambda in Phoenix? I think I agree with
Giovanni Piero Deretta that lambda[] is an intuitive way to put a 'wall'
between the inside lambda expression and the outside world. It doesn't seem
to; the following fails:

    px::bind( px::lambda[ arg1 + arg2 ], 0, 1 )();

- It is not clear what introspection facilities are provided. Users
sometimes want to derive a signature from a bind expression, or to compute
the minimum arity of a lambda expression. They ought to be able to implement
these queries themselves via a documented introspection interface that will
not break when V3 arrives.

* Implementation

- Phoenix objects do not take rvalues. This is a very common stumbling block
for people wanting to experiment with lambda libraries.

- boost::is_placeholder needs to be specialized for the placeholder types.
It might be interesting to see Phoenix respecting is_placeholder in the
other direction, too, if possible.

- The test coverage seems relatively sparse. I know very well how painful it
is to write tests (for arities from 0 to 9 inclusive), but Phoenix 2.0 needs
and deserves more of them. Take this, for example:

    test::x x_;
    bind(&test::x::m, x_)() = 123;
    bind(&test::x::m, arg1)(x_) = 123;
    BOOST_TEST(x_.m == 123);

It doesn't test whether the first bind worked, and doesn't try to read from
a member using bind.

* Documentation

- The examples assume a number of using directives without this being

- I believe that it would be better to leave the std:: names qualified. As
it stands, it is not immediately obvious whether cout is std::cout or
phoenix::cout (a plausible assumption would be for a phoenix::cout to be
defined as phoenix::ref(std::cout) to avoid capture by value).

- The val(3) example stresses that it returns a nullary function, but val(3)
happily accepts (and discards) arguments - as it should.

- The relationship between phoenix::ref and boost::ref is not made clear, or
even mentioned. Can one use boost::ref instead? In what contexts? std::ref?

(As an aside, there is a slight tendency in the documentation to be a bit
too independent of the rest of Boost for my taste. It mentions Lambda in
passing, but doesn't go into any details; it completely ignores

- The "Forwarding Problem" paper is so 2002. :-)

- The note that the binders are monomorphic should probably be fixed.

- It might be instructive to add an introspection example to the final
chapter that returns the (minimum) arity of a Phoenix expression.

- The documentation is missing a reference section, or at least an index of
the identifiers in the phoenix namespace.

* Vote

This is hard.

On the one hand, it is ridiculous to include
<boost/spirit/include/phoenix_bind.hpp> for phoenix::bind instead of
<boost/phoenix/bind.hpp>, as one would reasonably expect. As Phoenix is
already in Boost, the review merely needs to formally acknowledge the facts.
And the library certainly meets the criteria for acceptance.

On the other hand...

I sense from the state of the documentation that Phoenix being under the
Spirit umbrella and not an "official top level" library has led to a
somewhat isolated development. It is very high quality, above the Boost
median, but somehow doesn't yet feel like a top level library.

Still being on the other hand...

We know that having two libraries with overlapping goals, overlapping
syntax, overlapping identifiers and placeholders, and subtle differences is
confusing and a continuous source of recurring questions. Now we will have

But the fair thing to do is vote for acceptance. Which I will.

Boost list run by bdawes at, gregod at, cpdaniel at, john at