* Petr Pudlák <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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
