[Caml-list] short-circuit in Real World Ocaml ch 3 deserves a comment

[Caml-list] short-circuit in Real World Ocaml ch 3 deserves a comment

[Jean Saint-Remy]

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Hi,

I found a missing "does not change" sentence fragment in the third section on Lists and Patterns


"Each:: essentially adds a new block to the proceding picture. Such a block contains two things: a reference to the data in that list element, and a reference to the remainder of the list. This is why :: can extend a list without modifying it; extension allocates a new list element but {here} change any of the existing ones, as you can see: "


The example code using pattern matching appears to be a "let rec" short-circuit which I believe deserves a comment in the book. It appears to be puzzling, we are binding a "zero" to the head of the list in order to drop it? I think it is a fine example of OCaml that should be encouraged, not just an isolated case.

let rec drop_zero ls = match ls with
  | [] -> [] 

  | 0 :: tl -> drop_zero tl
  | hd :: tl -> hd :: drop_zero tl 

;;

With kind regards,

Jean

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Re: [Caml-list] short-circuit in Real World Ocaml ch 3 deserves a comment

[Nicolas Braud-Santoni]

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On 19/11/2013 16:05, Jean Saint-Remy wrote:
> The example code using pattern matching appears to be a "let rec"
> short-circuit which I believe deserves a comment in the book. It
> appears to be puzzling, we are binding a "zero" to the head of the
> list in order to drop it? I think it is a fine example of OCaml that
> should be encouraged, not just an isolated case.
>
> let rec drop_zero ls = match ls with
>   | [] -> []
>   | 0 :: tl -> drop_zero tl
>   | hd :: tl -> hd :: drop_zero tl
> ;;

Hi,

It isn't a « short circuit » : the list is pattern matched, and if it
matches `0 :: tl`, then the result is `drop_zero tl`.
The head of the list (0) is effectively dropped by not including it on
the right hand side.

And, as far as I know, it is rather idiomatic to use pattern matching in
the way (when matching over suitable datatypes).


Regards,
Nicolas


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Re: [Caml-list] short-circuit in Real World Ocaml ch 3 deserves a comment

[Francois Berenger]

On 11/20/2013 12:05 AM, Jean Saint-Remy wrote:
> [...]
> The example code using pattern matching appears to be a "let rec"
> short-circuit which I believe deserves a comment in the book. It appears
> to be puzzling, we are binding a "zero" to the head of the list in order
> to drop it? I think it is a fine example of OCaml that should be
> encouraged, not just an isolated case.
>
> let rec drop_zero ls = match ls with
>    | [] -> []
>    | 0 :: tl -> drop_zero tl
>    | hd :: tl -> hd :: drop_zero tl
> ;;

Are we really speaking of some "real world" OCaml?

This code is not tail recursive and can be replaced
by a one liner using List.filter from the standard library.