[Caml-list] Re: Generating a module implementation from a module signature using camlp4

[Mike McClurg <mike.mcclurg@gmail.com>]

On Fri, Oct 12, 2012 at 8:39 PM, Hongbo Zhang <bobzhang1988@gmail.com> wrote:
> On 10/12/12 2:03 PM, Mike McClurg wrote:
>> On Fri, Oct 12, 2012 at 6:55 PM, Wojciech Meyer
>> <wojciech.meyer@googlemail.com> wrote:
>>>
>>> Hi Mike,
>>>
>>> You can explicitly pretty print the camlp4 quotations:
>>>
>>> let foo nm =
>>>    <:str_item<module $uid:nm$ = struct let $lid:String.lowercase nm$ = ()
>>> end>>
>>>
>>> module OCamlPrinter = Camlp4.PreCast.Printers.OCaml;;
>>> OCamlPrinter.print_implem (foo "OK")
>>
>> Hi Wojciech,
>>
>> Thanks for that advice. I attempted something like this earlier, but
>> even with your printer module I get the following:
>>
>> Error: This expression has type AstFilters.Ast.str_item
>>         but an expression was expected of type Camlp4.PreCast.Ast.str_item
>>
>> Is there any way to unify the modules AstFilters.Ast and
>> Camlp4.PreCast.Ast? In my earlier attempt I tried some module Foo =
>> Blah with module ... trickery, but I couldn't seem to unify the types
>> properly.
>
> They are actually the same, unfortunately the interface does not expose the
> equality, the ugly solution is to use
> Obj.magic

Hey, that works! I probably wouldn't have thought to use Obj.magic. I
think it's probably OCaml's goto statement. It's not that it should
never be used, but there are so few cases where it's appropriate that
we condition ourselves not to think about it. Well, at least I have.

> Another solution to your problem is to use map to traverse sig_item, but
> maintain a state in your object to collect them as str_items

Yes, that's basically what I want to do. I just needed a way to print
those str_items directly, because sig_item filters can only return
sig_items.

Thanks for the help Hongbo and Wojciech.

Mike