Re: [boost] [mpl] Metafunction terminology - revival?

From: David Abrahams (dave_at_[hidden])
Date: 2006-07-25 10:58:04

"David Bergman" <David.Bergman_at_[hidden]> writes:

> Ok, two interpretations:
>
> 1. (Mine) What the MPL documentation calls a "lambda expression" (as of the
> refered page) is either "placeholder expression" or meta function classes
> *generated* by the application of mpl::lambda, i.e., *just* the meta
> function classes generated in that specific way.

It would be perverse, IMO, to call some types lambda expressions only
by virtue of how the type was generated.  If I write down a type that
is exactly identical to mpl::lambda<mpl::identity<_> >::type, but I
don't use mpl::lambda to write that type, by your definition it is not
a lambda expression.  In other words, by your definition:

  mpl::lambda<mpl::identity<_> >::type

is a lambda expression, and yet

  is_same<mpl::lambda<mpl::identity<_> >::type, x>::value

does not imply that x is a lambda expression.

> 2. (Yours) All metafunction classes are "lambda expressions."

2. is correct.

> I am admittedly biased, but I think my interpretation is the most sound one
> ;-) 

> The reason for that is that I would hesitate to call an explicitly
> defined metafunction class, such as
>
> 	struct MyMeta { template<class T> apply { typedef int type; } };
>
> a lambda expression.

That's a circular reason.  You think your interpretation of the
meaning of the term is the most sound because you would hesitate to
apply the term to such a class.

   MyMeta

is a lambda expression.  Consider:

   template <class T> struct always_int { typedef int type; };
   typedef always_int<_1> MyMeta;

now

    MyMeta

is again a lambda expression.

> I still want to tell my "students" that they should implement a
> "metafunctor" rather than a "lambda expression," which I consider to
> be a proper sub set of the former :-)

You are of course free to pick your own terminology, but that's not
the "official MPL" terminology and IMO you will just confuse your
students, which would be a disservice.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com