Re: [boost] [proto] transform for flattening an expression - how?

From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-18 12:50:17

Larry Evans wrote:
> On 03/18/08 10:30, Eric Niebler wrote:
>> These are domain-specific use cases. The code for that belongs in your 
>> DSEL, not in Proto.
>>
> 
> OK.  But so is:
> 
>    -x + x -> 0
> 
>    0 + x -> x
> 
> domain specific, which is what Markus was asking for.

Right, Markus was just looking for some examples. He didn't want me to 
put those transforms in Proto.

> Now I could just translate:
> 
>    Empty( A | B ) -> Empty<A> * Empty<B>
>    Empty(grammar_terminal) -> 1
>    Empty(epsilon) -> 0
> 
> and I've got what I want and it's in the domain Markus was talking 
> about.  So, if you do it for the arith domain (Markus'), then
> you'd have to have a set of "simplification rule" for that
> domain.  Replace the simplification rules for arith domain
> with simplification rules for domain X, where X is
> boolean expressions or grammar expressions or some other
> domain, and you've generalized what you can simplify.
> 
> My point was that If you can do it for Markus' domain, then
> you can to it for domain X.  IOW, provide a set of simplification
> rules for a particular domain.
> 
> Maybe the next version? ;)

It still doesn't belong in Proto. I see it as analogous to custom 
grammars and Spirit. Just because someone has written a useful grammar 
doesn't mean that Joel should make it part of Spirit. Instead, there is 
a separate Spirit app repository for these bits of useful Spirit code. 
Maybe there should be a Proto app repository, or maybe you and Markus 
can write the expression reduction transform and I can put it in 
libs/proto/example.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com