On Wed, Dec 19, 2012 at 6:50 PM, Jeremy Yallop <yallop@gmail.com> wrote:

On 19 December 2012 22:23, Philippe Wang <mail@philippewang.info> wrote:
> I have a (somehow silly) answer to your question:
>
> let gen_make_circ n =
> Printf.printf "let make_circ = function [] -> []\n";

That's an interesting idea, Philippe. Here's an approach along the
same lines using MetaOCaml.

let rec docycle l =
.!(.< let rec s =
.~(let rec loop = function
| [] -> .< s >.
| x :: xs -> .< x :: .~(loop xs) >.
in loop l)
in s >.)

The essential idea is the same as in your code: dynamically generating
a suitable 'let rec' expression. However, MetaOCaml also helpfully
guarantees that the generated code is well-formed and well-typed, and
makes it possible to compile and run the generated code without
leaving the language.

Here's the code in action:

# docycle;;
- : 'a list -> 'a list = <fun>
# docycle [1;2;3];;
- : int list = [1; 2; 3; 1; 2; 3; 1; 2; 3; 1; 2; 3; 1; 2; 3; 1; 2;
3; 1; ...]
# docycle [1;2;3;4;5];;
- : int list = [1; 2; 3; 4; 5; 1; 2; 3; 4; 5; 1; 2; 3; 4; 5; 1; 2;
3; 4; ...]

(For the sake of simplicity I haven't handled the case where the input
list is empty, but that's not difficult to do.)

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