Thanks Roman and Andres for the tip. I knew the trick with accumulating a function, but I had never imagined it could work so efficiently. I thought the problem with using foldr would be that the function would be neither tail recursive nor efficient and so I hadn't even tried. Apparently that was wrong. After your suggestion I checked its performance and how it compiles to core and to my surprise GHC optimizes the whole thing into a most-efficient tail recursive function!
Best regards, Petr 2013/2/18 Roman Cheplyaka <r...@ro-che.info> > * Petr Pudlįk <petr....@gmail.com> [2013-02-18 17:10:26+0100] > > Dear Haskellers, > > > > while playing with folds and trying to implement `!!` by folding, I came > to > > the conclusion that: > > > > - `foldr` is unsuitable because it counts the elements from the end, > while > > `!!` needs counting from the start (and it's not tail recursive). > > - `foldl` is also unsuitable, because it always traverses the whole list. > > Every structurally-recursive function is definable through foldr, > because foldr *is* the structural recursion (aka catamorphism) operation > for lists. > > Here the trick is to make the accumulator a function. This way you can > pass a value from left to right. > > Something like > > foldr (\x rest n -> ...) id list 0 > > I'll leave filling in the dots as an exercise. > > Roman >
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