RE: [Caml-list] still puzzled on generic types

[Walter Cazzola <cazzola@dico.unimi.it>]

On Thu, 29 Sep 2011, David Allsopp wrote:

> I think it's probably something to do with optional arguments not
> behaving as you expect.

uhm, would be interesting to see what is wrong on my use of the optional
arguments.

> But this is a bad use for optional arguments - you should instead use
> a nested function to pass the accumulator values. This works fine:

yes my first attempt was implemented with nested functions but ,,,

> let enumerate l =
>  let rec enumerate acc n = function
>    h::ls -> enumerate ((n, h)::acc) (n + 1) ls
>  | [] -> List.rev acc
>  in
>    enumerate [] 0 l

... my nested function had a different name than the nextee function and
I was passing l to it too. Could you explain me why your code works? in
particular where does it take the list to enumerate? Silly question I
know but it seems magic to me written in such a way

> Concatenating lists is also expensive in terms of the left list so
> your function is about as slow as possible! Much better when aiming
> for tail recursion, accumulate a reversed list and then reverse it in
> the basis case.

Ok, thanks for the note, but efficiency issues are still far in my
learning curve ;-)

Walter

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