Re: [Boost-users] [Proto] Extracting types of sub-expression in transform

Subject: Re: [Boost-users] [Proto] Extracting types of sub-expression in transform
From: Ivan Godard (igodard_at_[hidden])
Date: 2008-10-19 05:46:30

    Eric Niebler wrote:

    I hadn't even considered the notion of a type system for a DSEL, but it 
    makes perfect sense. Unfortunately, the only tutorial I found for van 
    Wijngaarden grammars is here:

    http://homepages.cwi.nl/~steven/vw.html

    Perhaps it's because I'm tired, but I'm not making heads or tails of it 
    at the moment. Can you point me to a gentler introduction?

    (Way back when I started with expression templates, I thought that 
    calling them an "embedded language" was just a cute metaphor, but much 
    can be gained from taking the serious view that these really *are* 
    languages, deserving of grammars, semantic actions, ... type systems, 
    too. Once we fully embrace that view, there's decades worth of 
    programming language theory we can leverage.)

and:

    Joel Falcou wrote:

    Eric Niebler a ?crit :

>     > (Way back when I started with expression templates, I thought that 
>     > calling them an "embedded language" was just a cute metaphor, but much 
>     > can be gained from taking the serious view that these really *are* 
>     > languages, deserving of grammars, semantic actions, ... type systems, 
>     > too. Once we fully embrace that view, there's decades worth of 
>     > programming language theory we can leverage.)
>     >
>         
    You just have described parts of my research plans  ;) 
    My current work try to get a way using ET and Concept in C++ to have a 
    condensed way to describe operationnal and/or denotanionnal semantic for 
    C++ DSEL. Guess I'll indeed have to swallow up this typing system rules 
    myself.

    Ivan : Thanks for the references.
      

VWG was invented for and used to define Algol68. Personally, I find it 
easiest to understand a formal system when I have a worked out example 
in front of me. If you're the same way, I suggest starting with:

    http://en.wikipedia.org/wiki/Algol68
    http://burks.brighton.ac.uk/burks/language/other/a68rr/rrtoc.htm
    Lindsey, C.H. and van der Meulen, S.G., /Informal Introduction to
    ALGOL 68/, North-Holland, 1971 (also sometimes known as "Algol68
    without Tears")

I'm biased here - I was on the working group that produced the second 
reference above - but I think that the only way to introduce semantics 
into a practical and general DSEL system is via formal means. However, 
both VWG and its exemplar in Algol68 have a very high intellectual 
tractability barrier, akin to the barrier facing newcomers to Functional 
Programming or Metaprogramming. But a casual note in Boost has already 
turned up three people who at least see the problem. That's a start :-)

Ivan