Re: [boost] ML Interpreter for Template Metaprogramming

From: Vesa Karvonen (vesa_karvonen_at_[hidden])
Date: 2002-10-03 08:27:29

From: "David Abrahams":
>How sophisticated is the pattern-matching for function arguments?

It is rather simple. It can currently match any expression composed of 
symbols and pair<>s (list<>s are composed of pair<>s). The match is done by 
recursively inspecting the pattern looking for a symbol (during 
compilation).

The language is modelled after ML so that all "native" functions take a 
single parameter and return a single value. Pattern matching is used for 
supporting multiple parameters and multiple return values.

The current prototype interpreter also defines the following expressions 
(which I forgot to introduce in the previous e-mail (well, they were not 
used in any of the examples)):

expr : let< pattern-expr, value-expr, body-expr >
     | q< C++-TYPE >

`let<>' is useful for factoring out computations. It binds the value of 
value-expr to the symbols in pattern-expr and then evaluates body-expr in an 
environment augmented with the pattern.

`q<>' is like `(quote expr)' in Lisp, and is currently used for introducing 
"foreign" data to the code, but it might be possible to get rid of it, by 
making it so that expressions can be identified and only expressions are 
compiled and evaluated.

Currently there is no mechanism to call "foreign" template metafunctions, 
but it should not be difficult to introduce such a feature.

...

While I was building the interpreter, I realized that it would also be 
possible to make an imperative language, where it would be possible to 
rebind the values in the environment. For example, it is possible to make a 
template based interpreter for a language that supports the following kind 
of programming (but I wouldn't go there):

typedef fn<N,
           let<pair< I0, I1 >, pair< i<0>, i<1> >,
               while_<not_eq<N,i<0> >,
                      let<TMP, I1,
                          set<I0, I1>,                 // rebind I0
                          set<I1, plus<I0,TMP> >,      // rebind I1
                          set<N, minus<N,i<1> > > > >, // rebind N
               I0> > fib_fn;

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