[Caml-list] '_a out of nowhere? (list.reverse)

[Caml-list] '_a out of nowhere? (list.reverse)

[Seth J. Fogarty]

# let cons x y = x::y
# and flip f x y = f y x ;;
val cons : 'a -> 'a list -> 'a list = <fun>
val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c = <fun>
# let reverse = List.fold_left (flip cons) [];;
val reverse : '_a list -> '_a list = <fun>
# reverse [1;2;3];;
- : int list = [3; 2; 1]
# reverse ['a';'b';'c'];;
This expression has type char but is here used with type int

I've encountered similar things with references, but never with something as
basic as a fold_left. Can someone tell me why this is happening? This, to me,
is very wrong behavior. This is on Debian's OCaml 3.07.


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Re: [Caml-list] '_a out of nowhere? (list.reverse)

[Daniel Bünzli]

         Objective Caml version 3.07+2

# let cons x y = x::y
   and flip f x = f y x;;
Unbound value y
# let cons x y = x::y
   and flip f x y = f y x;;
val cons : 'a -> 'a list -> 'a list = <fun>
val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c = <fun>
# let reverse = fun l -> List.fold_left (flip cons) [] l;;
val reverse : 'a list -> 'a list = <fun>
# reverse [1;2;3];;
- : int list = [3; 2; 1]
# reverse ['a';'b';'c'];;
- : char list = ['c'; 'b'; 'a']


It's a faq,

<http://caml.inria.fr/FAQ/FAQ_EXPERT- 
eng.html#variables_de_types_faibles>

Daniel

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Re: [Caml-list] '_a out of nowhere? (list.reverse)

[Stephane Legrand]

On Wed, Apr 14, 2004 at 02:05:32PM -0500, Seth J. Fogarty wrote:
> # let cons x y = x::y
> # and flip f x y = f y x ;;
> val cons : 'a -> 'a list -> 'a list = <fun>
> val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c = <fun>
> # let reverse = List.fold_left (flip cons) [];;
> val reverse : '_a list -> '_a list = <fun>
> # reverse [1;2;3];;
> - : int list = [3; 2; 1]
> # reverse ['a';'b';'c'];;
> This expression has type char but is here used with type int
> 
> I've encountered similar things with references, but never with something as
> basic as a fold_left. Can someone tell me why this is happening? This, to me,
> is very wrong behavior. This is on Debian's OCaml 3.07.

Try this:

# let cons x y = x::y
# and flip f x y = f y x ;;
# let reverse = List.fold_left (flip cons) [];;
# reverse ['a';'b';'c'];;
# reverse [1;2;3];;

In your example, when you first use reverse with an integer list,
you definitely have a reverse with :
 
  val reverse : int list -> int list = <fun>


Regards,
Stephane.

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