From: Vesa Karvonen (vesa_karvonen_at_[hidden])
Date: 2004-10-31 21:55:21
I have made some optimizations (resulting in an order of magnitude (10x)
improvement in compile-time in some cases on GNU g++) to the library
and implemented a couple of new metafunctions. The latest snapshot can
now be downloaded from the Files Section (both zip and tar.bz for your
Please note that the library is only a proof-of-concept made in about 2
It should be quite possible to provide a more comprehensive library of lazy
metafunctions and provide facilities like the lambda-facility of Boost MPL.
(Also, you shouldn't worry about the upper case naming used in the library.)
Lazy template metaprogramming, as shown by the lazy metaprogramming
library, eliminates the number one stumbling block of template
metaprogramming as described by every introduction or guide to template
metaprogramming (e.g. http://www.boost.org/libs/mpl/doc/#applyif and
Generative Programming section 10.11.1.3 Taking Advantage of Lazy
Behavior (particularly pages 441-443... "You should remember this when
writing static code" ;)).
In lazy template metaprogramming, invocations (::type) are only needed
when a value needs to be explicitly examined (e.g. to extract the head
of a list). Use of the typename-keyword can be practically eliminated and
forwarding (use of inheritance) is the usual way to implement functions.
Explicit invocations can (are) also be used for optimizing lazy
but optimizations are just optimizations. There is no need to optimize
the metaprogram executes too slowly. It is likely that user metacode can
rely on optimized library primitives for the most part.
<--- README-file --->
Lazy Metaprogramming Library
This is a minimal proof-of-concept library that shows how one can
implement lazy template metaprograms.
Most contemporary template metaprogramming libraries are strict,
which means that the result of a metafunction call is evaluated
completely once the metafunction is invoked. Furthermore,
metafunctions are written so that the arguments they receive are
assumed to be completely evaluated.
In addition to strictly evaluated function calls, most strict
programming languages provide lazy control structures, such as
`if' and `while', that evaluate their subexpressions only when
In contemporary template metaprogramming libraries, the separation
between lazy control structures and strict metafunctions is often
quite fuzzy, which can make programming quite confusing and
burdensome. Basically, a programmer using a contemporary template
metaprogramming library has to explicitly annotate the evaluation
order of the metacode by carefully selecting when to invoke and
when not to invoke a metafunction. The invocations can not simply
be completely left out, because strict metafunctions do not accept
The metaprogramming style used in this library is different.
Metafunctions and metadata structures (type lists) in this library
are designed to be lazy. In particular, metafunctions assume that
the arguments they receive are only delayed promises to compute
the value of the argument once explicitly invoked. This change of
style eliminates the need to constantly think about evaluation
order. Metafunction invocations are only needed when a value
explicitly needs to be examined. For example, the metafunction
`HEAD<l>' evaluates to the head of the given list `l'. In order to
do that, `HEAD' needs to explicitly invoke the parameter `l' to
expose the list constructor `CONS' for pattern matching. However,
and most importantly, callers of `HEAD' do not need to invoke the
actual parameter for `HEAD'. (Explicit invocations can also be
used for optimization, but it isn't strictly necessary.)
In theory, the style used in this library could be the ideal way
to implement template metaprograms, because the C++ standard
almost requires template instantiations to be cached, which means
that delayed promises are actually computed only once, saving CPU
time. In other words, C++ implementations are almost required to
perform memoization. In practice, however, the representation of
type information may take a lot of memory and access to the
instantiation cache isn't necessarily implemented efficiently.
This can make lazy metaprograms less efficient in terms of time
and space than their strict counterparts.
Contents of the package
The `inc/lazy_mpl' directory contains the library implementation.
Only a minimal core plus a few higher-order functions on lists are
provided. The list functions imitate the Haskell 98 standard
prelude functions. The `test' directory contains a couple of ad
hoc tests demonstrating the lazy programming style and the use of
To get most out of the code, you should take a look at the
unoptimized implementations of the higher-order list manipulation
metafunctions `MAP', `FOLD[R|L]', `APPEND', `TAKE', `DROP',
`ITERATE', `FILTER', and `ZIP_WITH'. As you can hopefully see,
they are very cleanly expressed. That is how lazy metaprogramming
would look like to a casual user.
This library is only a proof-of-concept. The intention is to show
that lazy template metaprogramming is not only possible, but can
yield both powerful and readable metaprograms.
One important remaining issue would be to implement a lambda
mechanism of some sort, which would make the use of higher-order
metafunctions more convenient.
Another issue would be to study how to best optimize lazy
metafunctions. Many of the metafunctions in the library have been
manually optimized by considering which of their arguments are
strict (always invoked) and then making the implementation take
advantage of the strictness by invoking the arguments early. The
optimizations do not change the way the metafunctions are used
(assuming the optimizations are correct), but can have a
significant effect (order of magnitude) on performance. (I have
not proven that the manually introduced optimizations are
correct---I wouldn't be surprised if some of the optimization were
incorrect.) Hopefully a good selection of optimized lazy
metafunctions can practically eliminate the need for library users
to manually optimize their code. At any rate, optimizations are
only needed when a metaprogram evaluates too slowly.
(C) Copyright Vesa Karvonen 2004.
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.)
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