RE: [Caml-list] Mapping onto lists

[Jun Furuse <Jun.Furuse@inria.fr>]

Hello,

From: "Frank Atanassow" <franka@cs.uu.nl>
> > Is there any way to combine these two functions(map and my_map) so
> > that the following will work?
> 
> In short, no, but perhaps it will help to point out that:
> 
>   my_map x = map (fun f -> f x)
> 
> Maybe it is possible in G'Caml, though.

Yes. It is a simple overloading example. :-)

let rec fmap v = function
  | [] -> []
  | f::fs -> f v :: fmap v fs
;;

generic rec combined_map = case
| ($a -> $b) -> $a list -> $b list => List.map
| $a -> ($a -> $b) list -> $b list => fmap
;;

# combined_map ((+) 2) [10; 100; 100];;
- : int list = [12; 102; 102]

# combined_map 2 [(+) 10; (+) 100; (+) 1000];;
- : int list = [12; 102; 1002]

The code of the original post:
>> let plus x y = x + y;;
>> 
>> let rec map f = function
>>     [] -> []
>>   | head::tail -> let r = f head in r :: map f tail;;
>> 
>> map (plus 2) [10; 100; 100];;
>> - : int list = [12; 102; 1002]
>> 
>> let rec my_map f = function
>> 	   [] -> []
>>     | head::tail -> let r = head f in r ::    my_map f tail;;
>> 
>> my_map 10 [plus 2; plus 4; plus 7];;
>> - : int list = [12; 14; 17] 
>> 
>> combined_map 2        [plus 10; plus 100; plus 1000];;
>> combined_map (plus 2) [10; 100; 100];;

--
JPF
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