[Haskell-cafe] Re: compilation related questions
barsoap at web.de
Fri Apr 10 08:40:00 EDT 2009
minh thu <noteed at gmail.com> wrote:
> 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.
Well, the point fusion is about is _not_ to construct lists.
naiveDropEnd xs = take (length xs - 1) xs
, which, due to length being a catamorphism, traverses xs twice.
It can be, in fact, reduced to the more sensible
dropEnd (x:) = 
dropEnd (x:xs) = x:dropEnd xs
, but that requires getting rid of the fold by replacing Integers
length' = map (\_ -> ())
pred = tail
succ = (():)
zarroo = 
Now we can write
notSoNaiveDropEnd xs = take' (pred $ length' xs) xs
, which can be fused into a single y-combinator.
Morale of the story: Folds are the bane of laziness. Having some magic
in place than can choose a lazily constructed (and fused) Peano
instance for Num in the right places would be awesomely cool.
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