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

From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-18 11:30:10

Larry Evans wrote:
> On 03/17/08 23:45, Larry Evans wrote:
>> On 03/17/08 17:55, Eric Niebler wrote:
>>> Larry Evans wrote:
>>>> Me too!  
>>> I couldn't tell from your email -- what are you me too-ing?
>> flattening a sequence of binary operators into one list
>> with a tag indicating the operator to be applied between
>> elements in the list.

See my reply to Markus. Use proto::flatten() and proto::unpack_expr().

> And also the means for simplifying an expression somewhat
> like described by Markus:
> 
>> I want to detect simultaneous occurences of
>> x and -x and have them dropped from the typelist of my sum.
> 
> Use case1:
> 
>    translating a grammar expression:
> 
>       x | x
> 
>    to:
> 
>        x
> 
> Use case2:
> 
>    Step 1:
>       translating grammar expression:
>          A | B
>       to an expression representing if it can derive empty
>       string:
>          empty<A> | empty<B>
>       which, if A is a the epsilon symbol then emptt<A> == true_
>       and the whole expression simplifies to:
> 
>           true_

These are domain-specific use cases. The code for that belongs in your 
DSEL, not in Proto.

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