[Caml-list] Closure & Ref

[Caml-list] Closure & Ref

[chris.danx]

Hi,

I was toying with ocaml just now and have successfully written a 
function that takes and int that produces a function that takes an int 
to add to the original.

let prodAdd x =
    let value = ref x in
       fun y -> !value + y;;

Now I want to do a function that takes a ref to a list and returns a 
function that adds items to the list and produce a function that returns 
another that returns the list.  How do I do that?

let prod_list_acc a =
   fun x -> a := x :: !a; true;;

let return_acc a =
   fun () -> !a;;

but that gives a "unit -> int list" =.  How do you get a copy of the 
list values?


This might seem like a crazy thing to do but I am toying with an idea 
relating to a paper I read on traits.  Instead of having classes at all 
you can just have traits, closures and mutable values.


Cheers,
Chris

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Re: [Caml-list] Closure & Ref

[Dustin Sallings]

On Nov 17, 2003, at 11:37, chris.danx wrote:

> let prodAdd x =
>    let value = ref x in
>       fun y -> !value + y;;

	This is very similar to

let prodAdd x y = x + y;;

	The only difference is that I'm making a reference with the value of x.

# let prodAdd x = let value = ref x in fun y -> !value + y;;
val prodAdd : int -> int -> int = <fun>
# let prodAdd2 x y = x + y;;
val prodAdd2 : int -> int -> int = <fun>
# prodAdd 1 2;;
- : int = 3
# prodAdd2 1 2;;
- : int = 3
# prodAdd 1;;
- : int -> int = <fun>
# prodAdd2 1;;
- : int -> int = <fun>


> Now I want to do a function that takes a ref to a list and returns a 
> function that adds items to the list and produce a function that 
> returns another that returns the list.  How do I do that?
>
> let prod_list_acc a =
>   fun x -> a := x :: !a; true;;
>
> let return_acc a =
>   fun () -> !a;;
>
> but that gives a "unit -> int list" =.  How do you get a copy of the 
> list values?
>
>
> This might seem like a crazy thing to do but I am toying with an idea 
> relating to a paper I read on traits.  Instead of having classes at 
> all you can just have traits, closures and mutable values.

	That's not a very functional style.  It does seem to work for me with 
the following syntax, though:

  # let list_acc a x = a := x :: !a; ();;
val list_acc : 'a list ref -> 'a -> unit = <fun>
# let return_list a = !a;;
val return_list : 'a ref -> 'a = <fun>
# return_list a;;
- : int list = [1; 2; 3; 4; 5]
# list_acc a 6;;
- : unit = ()
# return_list a;;
- : int list = [6; 1; 2; 3; 4; 5]


-- 
Dustin Sallings

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Re: [Caml-list] Closure & Ref

[Dustin Sallings]

On Nov 17, 2003, at 13:02, Dustin Sallings wrote:

>> let prodAdd x =
>>    let value = ref x in
>>       fun y -> !value + y;;
>
> 	This is very similar to
>
> let prodAdd x y = x + y;;
>
> 	The only difference is that I'm making a reference with the value of 
> x.

	I don't know what happened to this sentence.  It should read something 
more like, ``The only difference is that you're making a reference with 
the value of x unnecessarily.''

-- 
Dustin Sallings

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Re: [Caml-list] Closure & Ref

[Brian Hurt]

On Mon, 17 Nov 2003, chris.danx wrote:

> Hi,
> 
> I was toying with ocaml just now and have successfully written a 
> function that takes and int that produces a function that takes an int 
> to add to the original.
> 
> let prodAdd x =
>     let value = ref x in
>        fun y -> !value + y;;

Since you're not setting the reference, why have one?  Instead, try:

let prodAdd x = fun y -> x + y

But we can do it simpler than that:

let prodAdd x y = x + y

And use partial function application.  (prodAdd 4) returns a function 
which adds 4 to whatever int parameter passed to it.

> 
> Now I want to do a function that takes a ref to a list and returns a 
> function that adds items to the list and produce a function that returns 
> another that returns the list.  How do I do that?
> 
> let prod_list_acc a =
>    fun x -> a := x :: !a; true;;
> 
> let return_acc a =
>    fun () -> !a;;
> 
> but that gives a "unit -> int list" =.  How do you get a copy of the 
> list values?

Now you're setting the reference.  But prod_list_acc and return_acc need 
to share the reference.

Now, I for one, hate globals.  This is the result of programming for years 
in C.  What I would do is write a function which returns a tuple of two 
functions, and accumlator function and a current list function, like:

let make_listacc () =
	let r: int list ref = ref [] in
	let acc x = r := x :: !r
	and lst () = !r
	in acc, lst
;;

This allows you to have multiple different lists being constructed 
independently.  Note, the above code is actually more generic than it 
looks- I had to add an explicit type statement to make it "come out 
correct".  Without the explicit type information:

let make_listacc () =
        let r = ref [] in
        let acc x = r := x :: !r
        and lst () = !r
        in acc, lst
;;

The function is both clearer than the original, and creates lists of any 
type, not just ints.  If you hear me bitching about C++ and Java making 
generics "special and extraordinary", this is a classic example of what 
I'm kvetching about.

You'd use make_listacc like:

	let my_acc, my_list = make_listacc() in
	my_acc 3;
	my_acc 4;
	my_acc 5;
	my_list ()
	;;

The above code returns [5;4;3] (note, the list is built backward!).

The most common way to access members of a list is to use what is called 
"list comprehensions".  Don't let the names fool you- these are just 
functions that walk the list and do something on every element of the 
list.  Look at List.iter and List.fold_left.  So let's say I want to take 
a list of ints and sum them.  I could simply do:
	List.fold_left (fun x y -> x + y) 0 lst

Note that + is just a function, so I could just as easily have done:
	List.fold_left (+) 0 lst
to do the same thing.

The second most common way to access members of a list is to write a short 
recursive function.  Say I wanted to know if the list contained a given 
number.  I might write:
	let rec has_num x = function
		| [] -> false
		| h :: t -> if (h == x) then true else has_num x t
	;;

If you want to access specific members of a list (other than the head), I
wouldn't recommend using a list, but instead some other datastructure (an
array or hash table being the obvious choices).


-- 
"Usenet is like a herd of performing elephants with diarrhea -- massive,
difficult to redirect, awe-inspiring, entertaining, and a source of
mind-boggling amounts of excrement when you least expect it."
                                - Gene Spafford 
Brian


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Re: [Caml-list] Closure & Ref

[chris.danx]

Brian Hurt wrote:

> Since you're not setting the reference, why have one?  Instead, try:
> 
> let prodAdd x = fun y -> x + y
> 
> But we can do it simpler than that:
> 
> let prodAdd x y = x + y
> 
> And use partial function application.  (prodAdd 4) returns a function 
> which adds 4 to whatever int parameter passed to it.

I didn't know OCaml had partial function application, thanks!  I have 
programmed in Haskell before, but never got seriously into it (the 
things I wanted to do like IO where "advanced" in Haskell, whereas 
they're basic to me.  That's one reason I want to learn OCaml, it 
doesn't so hard to do trivial interaction).


> This allows you to have multiple different lists being constructed 
> independently.  Note, the above code is actually more generic than it 
> looks- I had to add an explicit type statement to make it "come out 
> correct".  Without the explicit type information:
> 
> let make_listacc () =
>         let r = ref [] in
>         let acc x = r := x :: !r
>         and lst () = !r
>         in acc, lst
> ;;
> 
> The function is both clearer than the original, and creates lists of any 
> type, not just ints.  If you hear me bitching about C++ and Java making 
> generics "special and extraordinary", this is a classic example of what 
> I'm kvetching about.

I hear you.  From my previous spell in FP I really got to like the ease 
of "generics" which are inconvient in C++ and a pain in Ada.  GUIs and 
IO was just easier.

[snip]

> If you want to access specific members of a list (other than the head), I
> wouldn't recommend using a list, but instead some other datastructure (an
> array or hash table being the obvious choices).

It was just a toy example to see if I could get that aspect of this idea 
to work in OCaml.  Hmm...

Thanks!

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