Proto for Boost mailing list page: [proto] My own lambda for MSM / wish list

Subject: [proto] My own lambda for MSM / wish list
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2011-03-14 08:39:41

Hi Thomas,

>> Row < source_state , event , target_state, action , guard >
>I suggest you to look into spirit how semantic actions are dealt with, it reminds me of exactly this:

Ok I will, thanks.

>Phoenix actors are POD, they are default constractible. Do you have a testcase where this is not the case?

Unfortunately no more, it was quite some time ago, maybe I did it with
V2. I got something which was not usable by BOOST_TYPEOF.
I'll try it again.

>> std::set<int, BOOST_MSM_LPP_LAMBDA_EXPR(_1 < _2) > s2;
>As Eric noted, decltype is the answer. I can't see why this should not be possible within phoenix.

Wait, I'm confused, it IS possible, or it SHOULD be possible? Can I
pass any valid phoenix expression? Not just a simple operator
expression, but an if/else, while, or an expression made of several
statements?
I want an arbitrary phoenix expression.
That I need a decltype is clear and what I'm already doing. But an
expression containing references, pointers, or even an int would be of
no use for a decltype.

>> std::transform(vec.begin(),vec.end(),vec.begin(), ++_1 + foo_(_1) );
>To answer your question, phoenix actors return their last statement. In your case the result of ++_1 + foo_(_1) is returned.

Hmmm now I'm lost. Just after you write that if_/else_ returns nothing.

>> std::transform(vec.begin(),vec.end(),vec.begin(),
>> if_(_1)[ ++_1,return_(++_1)].else_ [return_(++_1)] );
>Ok, if_(...)[...] and if(...)[...].else_[...] are statements in phoenix, meaning they return nothing (aka void).
>The answer to your question is to simply use phoenix::if_else which is in the operator module because it emulates the ?: operator.

Ok but this is just a partial answer, right? What about while / do or
whatever I might need?
I would like a clear statement: If I have an expression composed of
any number of statements of any type and any order inside the full
extent of phoenix (say if_().else_[], while_(), ++_1,fun_()), what is
the return value?
>From what you write I understand "the last statement except in some
cases". Which ones?

>As Eric already noted, Phoenix captures everything by value. There is no special syntax needed to capture other variables in scope (if you want to capture them by value, meaning that they will get copied into the phoenix expression).
>Your solution is similar to what is there in phoenix already: Local variables and the let/lambda expressions. Your examples will have your exact "better syntax":
>
>lambda[ _a = q ] [ _1 + _a /* first"captured" */]
>
>lambda[ _a = ref(q) ] [ _a++ /* first "captured" */ ]
>
>lambda[ _a = cref(q) ] [ _1 + _a /* first "captured" */ ]

Ah perfect! This is what I'm looking for.

>> Ok, these ones I didn't implement them, but I can dream.
>I am dreaming with you! :)

Let's not dream much longer then ;-)
We have here people who can do it.

>By looking at these examples i can not see what's there that is not provided by Phoenix already.

Then it's perfect and what I want, but I'll need to give it a closer
look. Mostly I want:
- return values
- the ability to decltype anything. For MSM it's about the same as the
std::set case.

If you allow a short criticism, I'd say that phoenix is great and
maybe even offers all I want, but the doc is doing it a great
disservice. I got so frustrated I started my own implementation and
seeing my lack of time, it means something ;-)
What I think we need is:
- more different examples showing what it can do, not the trivial
examples it currently took over from V2
- much much more about internals so that I don't come to the idea of
implementing a solution for my own case

Thanks,
Christophe