[Caml-list] Problem with Arg.Tuple

[Caml-list] Problem with Arg.Tuple

[Gabriel Cardoso]

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Dear list,

I have empty arguments when using Arg.Tuple with references. This
minimalist example illustrates my problem :

let f a b =
  print_endline a;
  print_endline b

let s = ref ""

let speclist = [(
  "-a",
  Arg.Tuple [
    Arg.Set_string s;
    Arg.String (f !s) (* !s is empty !!! *)
    (* Arg.String (fun ss -> f !s ss) (\* Works just fine ... why ? *\) *)
  ],
  "doc"
)]

let _ = Arg.parse speclist (fun s -> ()) "usage"

--------------------
$ ocamlc test.ml
$ /a.out -a x y

y
------------------------

When uncommenting the line 12 :
------------------------
$ ocamlc test.ml
$ ./a.out -a x y
x
y
-------------------------

Any idea what am I doing wrong ?

Thanks in advance !

Gabriel

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Re: [Caml-list] Problem with Arg.Tuple

[Samuel Hornus]

On Mar13, 2012, at 11:49 , Gabriel Cardoso wrote:

> Dear list, 
> 
> I have empty arguments when using Arg.Tuple with references. This minimalist example illustrates my problem :
> 
> let f a b =
>  print_endline a;
>  print_endline b
> 
> let s = ref ""
> 
> let speclist = [(
>  "-a",
>  Arg.Tuple [
>    Arg.Set_string s;
>    Arg.String (f !s) (* !s is empty !!! *)

Here, the value of !s is empty when the (f !s) is evaluated…
you shoud pass the whole reference as argument :
Arg.String(f s)
and 
let f a b = print_endline !a; print_endline b

>    (* Arg.String (fun ss -> f !s ss) (\* Works just fine ... why ? *\) *)

why ? because the !s is evaluated only later, when the anonymous function (fun ss ->…) is applied.
-- 
Sam

Re: [Caml-list] Problem with Arg.Tuple

[Gabriel Cardoso]

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Thanks, I get it now !

Cheers

Gabriel

2012/3/13 Samuel Hornus <samuel.hornus@inria.fr>

>
> On Mar13, 2012, at 11:49 , Gabriel Cardoso wrote:
>
> > Dear list,
> >
> > I have empty arguments when using Arg.Tuple with references. This
> minimalist example illustrates my problem :
> >
> > let f a b =
> >   print_endline a;
> >   print_endline b
> >
> > let s = ref ""
> >
> > let speclist = [(
> >   "-a",
> >   Arg.Tuple [
> >     Arg.Set_string s;
> >     Arg.String (f !s) (* !s is empty !!! *)
>
> Here, the value of !s is empty when the (f !s) is evaluated…
> you shoud pass the whole reference as argument :
> Arg.String(f s)
> and
> let f a b = print_endline !a; print_endline b
>
> >     (* Arg.String (fun ss -> f !s ss) (\* Works just fine ... why ? *\)
> *)
>
> why ? because the !s here is evaluated only later, whenthe anonymous
> function is applied.
> --
> Sam

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