C-like macros in OCaml

C-like macros in OCaml

[Raj B]

Hi there

Is there any way I can achieve the use of C-like preprocessor macros  
in OCaml?
i.e.

#define FOO 1

etc.

I am using an array of bits (a bitset) where each bit represents a  
flag, and i would like to
access this set by using names rather than remembering which index in  
the array represents which flag.

Similarly, is there any way of getting a C-like enumeration? e.g.

enum days {Mon = 1, Tue, Wed...}

I guess that OCaml does not provide these by default, but I wonder if  
someone has done some camlp4 magic
to use these.

Thanks
Raj

Re: [Caml-list] C-like macros in OCaml

[Jonathan Bryant]

On Jun 26, 2007, at 6:00 PM, Raj B wrote:
> Similarly, is there any way of getting a C-like enumeration? e.g.
>
> enum days {Mon = 1, Tue, Wed...}

You can use a variant and a pair of functions:

type weekday =
| Monday
| Tuesday
  ...
| Sunday

let weekday_to_int w = match w with
| Monday -> 1
| Tuesday -> 2
  ...
| Sunday -> 7

let int_to_weekday i = match i with
| 1 -> Monday
| 2 -> Tuesday
  ...
| 7 -> Sunday
| _ -> raise (Invalid_argument "int_to_weekday")

--Jonathan

Re: [Caml-list] C-like macros in OCaml

[Richard Jones]

On Tue, Jun 26, 2007 at 05:00:40PM -0500, Raj B wrote:
> Hi there
> 
> Is there any way I can achieve the use of C-like preprocessor macros  
> in OCaml?
> i.e.
> 
> #define FOO 1
> 
> etc.

What about ocamlc -pp /lib/cpp ? :-)

There's also a camlp4 macro preprocessor, but I can't remember now
whether it can do constant defines like that (IIRC it used not, but
now it can, or something like that).

Rich.

-- 
Richard Jones
Red Hat

Re: [Caml-list] C-like macros in OCaml

[Jeremy Yallop]

Richard Jones wrote:
> On Tue, Jun 26, 2007 at 05:00:40PM -0500, Raj B wrote:
>> Hi there
>>
>> Is there any way I can achieve the use of C-like preprocessor macros  
>> in OCaml?
>> i.e.
>>
>> #define FOO 1
>>
>> etc.
> 
> What about ocamlc -pp /lib/cpp ? :-)
> 
> There's also a camlp4 macro preprocessor, but I can't remember now
> whether it can do constant defines like that (IIRC it used not, but
> now it can, or something like that).

It appears to support them.  If you have a file `foo.ml' containing the 
following:

     DEFINE FOO = 1

     let x = FOO

then compiling it with pa_macro under 3.09 gives:

     $ camlp4 pa_o.cmo pa_macro.cmo pr_o.cmo foo.ml
     (* *)

     let x = 1

or with the built-in Camlp4MacroParser under 3.10:

     $ camlp4of foo.ml
     let x = 1

Jeremy.

Re: [Caml-list] C-like macros in OCaml

[Loup Vaillant]

2007/6/27, Jeremy Yallop <jeremy.yallop@ed.ac.uk>:
> Richard Jones wrote:
> > On Tue, Jun 26, 2007 at 05:00:40PM -0500, Raj B wrote:
> >> Hi there
> >>
> >> Is there any way I can achieve the use of C-like preprocessor macros
> >> in OCaml?
> >> i.e.
> >> [...]
> >[...]
> It appears to support them.  If you have a file `foo.ml' containing the
> following:
> [...]

By the way, does camlp4 handle cross modules C-like macros? If it does, how?
For instance, what if I write, in foo.ml :
DEFINE FOO = 1

and then, in bar.ml :
let bar = FOO;;

or else:
open foo;;
let bar = FOO;;

or even :
let bar = Foo.FOO;;

Will I get what I want?

Now, what about regular quotations? And grammar rules? How can I
specify the scope of particular grammar rule (or quotation)? For the
little I know about camlp4, once defined, the scope of a new grammar
rule is global. (Idem for quotations).

Am I right?
Any clue about the way camlp4 keep track of new grammar rules an
quotations? (Or some pointer to the documentation involved)

Thanks
Loup Vaillant

Re: [Caml-list] C-like macros in OCaml

[Christopher L Conway]

On 6/27/07, Loup Vaillant <loup.vaillant@gmail.com> wrote:
> By the way, does camlp4 handle cross modules C-like macros? If it does, how?
> For instance, what if I write, in foo.ml :
> DEFINE FOO = 1
>
> and then, in bar.ml :
> let bar = FOO;;
>
> or else:
> open foo;;
> let bar = FOO;;
>
> or even :
> let bar = Foo.FOO;;
>
> Will I get what I want?

Probably not, but that depends on what you want. ;-) I believe that
pa_macro DEFINEs have module scope. To get the same DEFINE in multiple
files, you can use -D arguments on the command line, but pa_macro
doesn't allow these arguments to have values (i.e., you can only test
IFDEF/IFNDEF).* If you want arguments, see pa_macro_arg:
http://caml.inria.fr/cgi-bin/hump.en.cgi?contrib=569

Chris

* I submitted a patch for this, but I don't think it's made its way
into a release. I didn't realize the 3.10/new camlp4 push was on when
I made it.

> Now, what about regular quotations? And grammar rules? How can I
> specify the scope of particular grammar rule (or quotation)? For the
> little I know about camlp4, once defined, the scope of a new grammar
> rule is global. (Idem for quotations).
>
> Am I right?
> Any clue about the way camlp4 keep track of new grammar rules an
> quotations? (Or some pointer to the documentation involved)
>
> Thanks
> Loup Vaillant
>
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Re: [Caml-list] C-like macros in OCaml

[Loup Vaillant]

2007/6/27, Christopher L Conway <cconway@cs.nyu.edu>:
> On 6/27/07, Loup Vaillant <loup.vaillant@gmail.com> wrote:
> > By the way, does camlp4 handle cross modules C-like macros? If it does, how?
> > For instance, what if I write, in foo.ml :
> > [...]
> > Will I get what I want?
>
> Probably not, but that depends on what you want. ;-) I believe that
> pa_macro DEFINEs have module scope.

Too bad. Actually, I want to implement a Lisp syntax for Ocaml, which
means I also want Lisp macros (regular and reader ones). Lisp macros
are often used extensively (Paul Graham talks about over 20% of the
code). That is why I want properly scoped macros : no global
namespace, and yet the possibility to use them between compilation
unit.

The only solution I came up with was producing at parsing type some
file for each .ml file containing a macro, so the preprocessor can
check these when parsing another file using this macro.

Hence my question : doesn't camlp4 provide a better solution to this
kind of problem? I know very little about the inner core of camlp4,
but I am sure I can reuse some ideas.


> To get the same DEFINE in multiple
> files, you can use -D arguments on the command line, but pa_macro
> doesn't allow these arguments to have values (i.e., you can only test
> IFDEF/IFNDEF).* If you want arguments, see pa_macro_arg:
> http://caml.inria.fr/cgi-bin/hump.en.cgi?contrib=569

I will take a look, thanks for the pointer.

Loup

Re: [Caml-list] C-like macros in OCaml

[Nicolas Pouillard]

On 6/27/07, Christopher L Conway <cconway@cs.nyu.edu> wrote:
> On 6/27/07, Loup Vaillant <loup.vaillant@gmail.com> wrote:
> > By the way, does camlp4 handle cross modules C-like macros? If it does, how?
> > For instance, what if I write, in foo.ml :
> > DEFINE FOO = 1
> >
> > and then, in bar.ml :
> > let bar = FOO;;
> >
> > or else:
> > open foo;;
> > let bar = FOO;;
> >
> > or even :
> > let bar = Foo.FOO;;
> >
> > Will I get what I want?
>
> Probably not, but that depends on what you want. ;-) I believe that
> pa_macro DEFINEs have module scope. To get the same DEFINE in multiple
> files, you can use -D arguments on the command line, but pa_macro
> doesn't allow these arguments to have values (i.e., you can only test
> IFDEF/IFNDEF).* If you want arguments, see pa_macro_arg:
> http://caml.inria.fr/cgi-bin/hump.en.cgi?contrib=569
>
> Chris
>
> * I submitted a patch for this, but I don't think it's made its way
> into a release. I didn't realize the 3.10/new camlp4 push was on when
> I made it.

And your patch has been applied.

-- 
Nicolas Pouillard

Re: [Caml-list] C-like macros in OCaml

[David Allsopp]

On Jun 26, 2007, at 6:06 PM, Jonathan Bryant wrote:
> On Jun 26, 2007, at 6:00 PM, Raj B wrote:
> > Similarly, is there any way of getting a C-like enumeration? e.g.
> >
> > enum days {Mon = 1, Tue, Wed...}
>
> You can use a variant and a pair of functions:
>
> type weekday =
> | Monday
> | Tuesday
>   ...
> | Sunday

You can also use a variant and Obj.magic which turns the two functions into:


let weekday_to_int (w : weekday) = Obj.magic w + 1

let int_to_weekday i =
    if i >= 1 && i <= 7
    then (Obj.magic (i - 1) : weekday)
    else raise ...

But this is really dirty and a potential source of bugs... but the result is
closer to the "#define" way.
 

David