[Caml-list] Mapping onto lists

[Caml-list] Mapping onto lists

[Jay Riddle]

Is there any way to combine these two functions(map and my_map) so
that the following will work?

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 mapping function *)
combined_map 2        [plus 10; plus 100; plus 1000];;
combined_map (plus 2) [10; 100; 100];;

--JayR x8-1335
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RE: [Caml-list] Mapping onto lists

[Frank Atanassow]

> 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.

---
Frank Atanassow, Information & Computing Sciences, Utrecht University
Padualaan 14, PO Box 80.089, 3508TB Utrecht, The Netherlands
Tel +31 (0)30 253-3261 Fax +31 (0)30 251-3791
---

> 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 mapping function *)
> combined_map 2        [plus 10; plus 100; plus 1000];;
> combined_map (plus 2) [10; 100; 100];;

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RE: [Caml-list] Mapping onto lists

[Jun Furuse]

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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