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camlp4: question about functor-style syntax extensions
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Date: -- (:)
From: Bruno De Fraine <Bruno.De.Fraine@v...>
Subject: camlp4: question about functor-style syntax extensions
Hello,

Here's a minimal syntax extension in the functor-style, which seems  
to be preferred style with the new camlp4. It defines a PI constant  
which can be used inside expressions:

   module Id = struct let name = "pi" and version = "3.14" end

   open Camlp4.Sig

   module Pi(Syntax : Camlp4Syntax) = struct
     include Syntax
     EXTEND Gram
       expr: LEVEL "simple"
       [[ "PI" -> <:expr< $flo:"3.14159265358979312"$ >> ]];
     END
   end

   let module M = Camlp4.Register.OCamlSyntaxExtension(Id)(Pi) in ()

As mentioned in the documentation, an extension (such as Pi) must be  
functor: Camlp4Syntax -> Camlp4Syntax, and  
Register.OCamlSyntaxExtension requires this type. This is why I have  
to "include" (instead of "open") the original syntax: to produce a  
valid output syntax. My question is: how is this output syntax ever  
used? (Note that the EXTEND-statement does not make any structural  
changes to the syntax module, just dynamic changes as a side-effect  
upon functor application.)

To put all my cards on the table: I believe the output syntax is  
never used. For example, you can sabotage one of the main grammar  
entries by finishing the definition of Pi with:

   let top_phrase : Ast.str_item option Gram.Entry.t = Obj.magic 0

And the extension keeps working all the same from the toplevel. In  
fact, a look at the code of OCamlSyntaxExtension in Register.ml  
confirms it is never used:

   module OCamlSyntaxExtension
     (Id : Sig.Id) (Maker : functor (Syn : Sig.Camlp4Syntax) ->  
Sig.Camlp4Syntax) =
   struct
     declare_dyn_module Id.name (fun _ -> let module M = Maker Syntax  
in ());
   end;

The output syntax M is thrown away, i.e. the syntax extension relies  
entirely on side-effects of the functor application. I think the type  
required for a syntax extension could just as well have been a  
functor that return an empty module: Camlp4Syntax -> sig end

Why bother making this remark if you can just include the original  
syntax at the beginning and it works? I believe there are two  
important reasons. The first is didactical: the signature  
Camlp4Syntax -> Camlp4Syntax suggests that the syntax extension works  
by structurally transforming one syntax into another, while this is  
not what is going on. This situation makes the workings of camlp4 all  
the more difficult to understand for novice (and perhaps seasoned)  
camlp4 developers. The second reason is practical: you can easily  
define something in your extension that clashes with a name from  
Camlp4Syntax (e.g. "expr"), and then the compiler will complain if  
the types do not agree. You can assure you export the exact original  
definitions by putting "include Syntax" at the end of the extension  
instead of the beginning, but then you still need an "open Syntax" at  
the beginning to have the useful modules (like Ast, Gram, etc.)  
available. All of this is an annoying redundant idiom given that the  
output Syntax is not used.

Regards,
Bruno


--
Bruno De Fraine
Vrije Universiteit Brussel
Faculty of Applied Sciences, DINF - SSEL
Room 4K208, Pleinlaan 2, B-1050 Brussels
tel: +32 (0)2 629 29 75
fax: +32 (0)2 629 28 70
e-mail: Bruno.De.Fraine@vub.ac.be