Re: [Caml-list] Why does value restriction not apply to the empty list ?

[Richard Jones <rich@annexia.org>]

On Sat, Jan 10, 2009 at 12:34:22PM +0100, Antoine Delignat-Lavaud wrote:
> In Ocaml, the program
> let el = [] in if List.length el > 0 then (List.hd el)+(int_of_string 
> (List.hd el)) else 0 ;;
> yields not type error and returns 0 despite the use of el as both an int 
> list and a string list.
> 
> Thus, I am wondering why does value restriction not apply to the empty 
> list in Ocaml. I don't think it's possible to do a cast with the empty 
> list (it is empty after all) but I don't see any benefit in doing so.

It's a strange one ... when the if statement appears as a toplevel
statement, OCaml infers the type 'a list for the list:

# let el = [] ;;
val el : 'a list = []
# if List.length el > 0 then (List.hd el)+(int_of_string (List.hd el)) else 0;;
- : int = 0
# el ;;
- : 'a list = []

But the same if statement within a function definition causes an error:

# let f el =            
  if List.length el > 0 then (List.hd el)+(int_of_string (List.hd el)) else 0;;
                                                          ^^^^^^^^^^
This expression has type int but is here used with type string

Rich.

-- 
Richard Jones
Red Hat