Actually the point of my remark is that would we be able to write meta programs in imperative style instead of FP style compilation would be much faster.
The slowness of metaprogram compilation has almost *nothing* to do with the FP style of metaprogramming, and almost everything to do with the accidental nature of the metaprogram evaluation engine (using template instantiation).
And why do we have so many template to instantiate? Exactly my point: because we couldn't change type definition we could only create new one. How many helper templates gets instantiated to remove const types from the type-list? Should be none. So you are right: it has everything to do with nature of metaprogram evaluation engine, which is FP based one.
In fact, you *can* write metaprograms in an imperative style, given an appropriate framework designed to interpret them correctly. It'll be much slower than what we use today.
You have any grounds for this statement?
And Spirit framework would benefit from it the most.
I doubt that. Parsing in particular has little to gain from the use of mutable data.
I meant from compilation time standpoint. Gennadiy.