[Haskell-cafe] Re: compilation related questions

[wren ng thornton]

minh thu wrote:
> Say you have an AST, with just the information after a parsing pass.
> Then you compute 'derived' values at each node (maybe, I'm not sure if
> it would be practical, type, strictness, ...). To do that, you can
> have a function that, applied to any node, give the desired additional
> information.
> 
> But for some functions, it can be seen clearly that such information
> could have been constructed at the same time that the data structure.
> 
> So it is related to fusion techniques, but with the additional
> possibility of adding fields to the original data structure.


The problem with doing this automatically is that you don't get that 
data for free. Storing the length of finite lists as you construct them 
means that getting the length is constant time, but it introduces a 
space overhead of one Integer per (:). If you call length often enough 
it may be worth it, but generally the book-keeping will be too expensive.

We could use modified strategies where we have two different (:)s, one 
that stores the length and one that doesn't. Then we could apply some 
heuristic like exponential backoff to minimize the space overheads 
subject to some logarithmic bound on the asymptotic cost of calculating 
the length of the list until the first label. Or we could apply dynamic 
memoization techniques and switch a non-length-storing (:) into a 
length-storing (:) whenever we evaluate a call to length, so it'll be 
quicker next time. Or,...


There're a lot of strategies here, but I doubt it's feasible to 
automatically choose a good strategy for a given program (since "good" 
may depend on the vagaries of runtime data). Automatically doing the 
transformations and suggesting them to the user is doable though. You 
may be interested in looking into attribute grammars[1] and finger 
trees[2] for more ideas along this route.


[1] 
http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue4/Why_Attribute_Grammars_Matter

[2] http://apfelmus.nfshost.com/monoid-fingertree.html

-- 
Live well,
~wren