Re: [Caml-list] How to add () to function parameters

[Till Varoquaux <till@pps.jussieu.fr>]

Oh, and I nearly forgot: In practice you shouldn't really have that
many functions taking more than 5 unlabeled arguments lying around so
I would bite the bullet and define cps1 through 5 the straightforward
way....

Till

On Fri, Oct 16, 2009 at 11:15 AM, Till Varoquaux <till@pps.jussieu.fr> wrote:
> Well I can basically see two solutions (plus countless complications
> that I won't go into.)
>
> We want to define a function
>
> val cps3: f:('a -> 'b -> 'c -> 'd) -> ('d -> 'e) -> 'a -> 'b -> 'c -> 'e =
>
> that takes a three argument function a returns the same function in CPS style.
>
> The functional unparsing/danvy way [1]:
>
>> let (++) f g = fun x -> f (g x)
>> let i k f arg = k (f arg)
>> let cps ty ~f k = ty k f
>> let cps3 ~f = cps (i++i++i) ~f
>
> brute force style:
>
>> let e acc ~f cont = acc cont f
>> let i = fun acc g -> g (fun cont v arg -> acc cont (v arg))
>> let cps = fun z ->
>>   let acc = (fun cont x -> cont x) in
>>   z acc
>> let cps3 ~f = cps i i i e ~f
>
> The first style is an acquired taste quite the same way that monad
> are. With some getting use to and abstracting your types in a sensible
> way you can enclose things quite nicely and define elegant
> printf/scanf kind of functions.
>
> I strongly discourage you to use the second style. It is a very
> reworked mlton.fold [2] style solution. mlton's fold is a lot more
> esoteric and leads to types that I have never been able to abstract
> properly.
>
> Till
>
> [1] http://www.brics.dk/RS/98/12/
> [2] http://mlton.org/Fold
>
> On Wed, Oct 14, 2009 at 6:44 AM, Till Crueger <Till.Crueger@gmx.net> wrote:
>> Hi,
>>
>> I am looking for a way to add a unit parameter to a function that takes an
>> arbitrary number of parameters. If the number of parameters is known this is
>> fairly easy and I can just do:
>>
>> let lift1 f a =
>>   fun () ->
>>      f a;;
>>
>> let lift2 f a b =
>>   fun () ->
>>     f a b;;
>>
>> (all these create one closure per lifting)
>> etc...
>>
>> However it is a bit of a hassle to have to code each of these lifts... So
>> what I am looking for is a way to extend this pattern to all numbers. So far
>> I got to the point that I can do the following:
>>
>> let lift_once f a =
>>   fun () ->
>>      f a;;
>>
>> let lift_more f a =
>>   fun () ->
>>      f () a;;
>>
>> So for a function f taking two parameters a and b I can do
>>
>> lift_more (lift_once f a) b
>> (two closures created)
>>
>> and for a function taking the parameters a, b and c I can do
>>
>> lift_more (lift_more (lift_once f a) b) c
>> (three closures created)
>>
>> to get the lifted functions.
>>
>> However this solution gets quite ugly with all the parentheses. Also there
>> are a lot of closures being produced and evaluated for any single lifting. I
>> had a look at the Jane Street blog post about variable argument functions
>> (http://ocaml.janestcapital.com/?q=node/22), which seems to do similar
>> things. However I have never been really good with CPS, so I don't know if
>> those techniques can be applied to this problem.
>>
>> Is there any way to do this, which does not get this ugly. Also the
>> resulting lifted function should not contain too many closures.
>>
>> Thanks for your help,
>>   Till
>>
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>