> But it seems not to work correclty: I ask for 'a -> 'a list
>
> http://search.ocaml.jp/?q=%27a+-%3E+%27a+list
>
> and it returns 'a list -> 'a list functions.

I'm an author of OCaml API Search.
Thanks for using.

I borrowed searching module from OCamlBrowser http://caml.inria.fr/pub/docs/manual-ocaml/manual028.html 
.
OCamlBrowser is older than hoogle, so there are some difference.