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From: Jon Harrop <jon@ffconsultancy.com>
Organization: Flying Frog Consultancy Ltd.
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Preferred Way to Split a List
Date: Tue, 30 Oct 2007 07:50:24 +0000
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X-Spam: no; 0.00; combinator:01 val:01 val:01 frog:98 wrote:01 rec:01 caml-list:01 int:01 int:01 arbitrary:02 arbitrary:02 arg:03 tend:03 element:03 let:03 

On Monday 29 October 2007 23:33, Robert Fischer wrote:
> What is the preferred way to split a list into two, at an arbitrary
> point?  There's lots of ways you could do it, but I'm not sure if
> there's a standard best practice for this.

I tend to write a combinator that nests an arbitrary number of function 
applications:

# let rec nest n f x =
    if n=0 then x else nest (n-1) f (f x);;
val nest : int -> ('a -> 'a) -> 'a -> 'a = <fun>

and then apply this to a function that moves head elements:

# let aux = function
    | front, h::back -> h::front, back
    | _ -> invalid_arg "aux";;
val aux : 'a list * 'a list -> 'a list * 'a list = <fun>

Then I write a "chop" function in terms of those two:

# let chop n list =
    nest n aux ([], list);;
val chop : int -> 'a list -> 'a list * 'a list = <fun>

For example, splitting after the fourth element:

# chop 4 [1;2;3;4;5;6;7;8;9];;
- : int list * int list = ([4; 3; 2; 1], [5; 6; 7; 8; 9])

Note that the front list is reversed.

PS: Ignore any responses that even mention Obj.
-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/products/?e