Very strange camlp4 error when using records

Very strange camlp4 error when using records

[Richard Jones]

I have a strange case where the generated code looks completely
correct - indeed, I can pretty-print the code, save it to a file, and
compile it using ocamlc just fine.  However when using the camlp4
preprocessor directly (ocamlc -pp "camlp4o ./pa_test2.cmo" ...), it
fails to compile.

$ ocamlc -pp "camlp4o pa_extend.cmo q_MLast.cmo" -I +camlp4 -c pa_test2.ml
$ ocamlc -pp "camlp4o ./pa_test2.cmo" -I +camlp4 -c test2.ml
File "test2.ml", line 1, characters 11-18:
Unbound value t.a
$ camlp4o ./pa_test2.cmo pr_o.cmo test2.ml
type t = { a : int }
let rec t t = Array.iter print_endline [| string_of_int t.a |]
      (* generated code above compiles just fine in toplevel or ocamlc *)

-- pa_test2.ml -----------------------------------------------
let rec generate loc name = function
  | <:ctyp< int >> ->
      <:expr< string_of_int $lid:name$ >>
  | <:ctyp< { $list:fields$ } >> ->
      let fields = List.map (
	fun (loc, fname, mut, tdef) ->
	  let name = name ^ "." ^ fname in
	  generate loc name tdef
      ) fields in
      <:expr< Array.iter print_endline [| $list:fields$ |] >>
  | _ -> assert false

DELETE_RULE
  Pcaml.str_item: "type"; LIST1 Pcaml.type_declaration SEP "and"
END

EXTEND
  GLOBAL: Pcaml.str_item;

  Pcaml.str_item:
    [[ "type"; tds = LIST1 Pcaml.type_declaration SEP "and" ->
	 let typedefs = <:str_item< type $list:tds$ >> in
	 let values = List.map (
	   fun ((loc, name), tparams, tdef, cl) ->
	     let body = generate loc name tdef in
	     <:patt< $lid:name$ >>, <:expr< fun $lid:name$ -> $body$ >>
	 ) tds in
	 <:str_item<
	   declare
	     $typedefs$;
	     value rec $list:values$;
           end
         >> ]];
END
--------------------------------------------------------------

-- test2.ml --------------------------------------------------
type t = { a : int }
--------------------------------------------------------------

Ideas?

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

Re: [Caml-list] Very strange camlp4 error when using records

[Martin Jambon]

On Fri, 13 May 2005, Richard Jones wrote:

[...]

> $ ocamlc -pp "camlp4o pa_extend.cmo q_MLast.cmo" -I +camlp4 -c pa_test2.ml
> $ ocamlc -pp "camlp4o ./pa_test2.cmo" -I +camlp4 -c test2.ml
> File "test2.ml", line 1, characters 11-18:
> Unbound value t.a
> $ camlp4o ./pa_test2.cmo pr_o.cmo test2.ml
> type t = { a : int }
> let rec t t = Array.iter print_endline [| string_of_int t.a |]
>       (* generated code above compiles just fine in toplevel or ocamlc *)

You have to be careful that the strings that you introduce in the AST as
"lid" (lowercase identifier), "uid" (uppercase identifier), "str"
(escaped string) or "chr" (escaped char) are *not checked*.

What you do indirectly in your AST generator ("generate") is:

<:expr< string_of_int $lid: "t.a" >>
                             ^^^
                             that's not a lid
                             but it is pretty-printed as if it was one
Instead of:

<:expr< string_of_int $lid:"t"$ . $lid:"a"$ >>

(this is also OK: <:expr< string_of_int t.a >> )

This why you get an error message that you would never get in hand-written
OCaml (it would be "Unbound value t" or "Unbound record field label a").


Martin


> -- pa_test2.ml -----------------------------------------------
> let rec generate loc name = function
>   | <:ctyp< int >> ->
>       <:expr< string_of_int $lid:name$ >>
>   | <:ctyp< { $list:fields$ } >> ->
>       let fields = List.map (
> 	fun (loc, fname, mut, tdef) ->
> 	  let name = name ^ "." ^ fname in
> 	  generate loc name tdef
>       ) fields in
>       <:expr< Array.iter print_endline [| $list:fields$ |] >>
>   | _ -> assert false
>
> DELETE_RULE
>   Pcaml.str_item: "type"; LIST1 Pcaml.type_declaration SEP "and"
> END
>
> EXTEND
>   GLOBAL: Pcaml.str_item;
>
>   Pcaml.str_item:
>     [[ "type"; tds = LIST1 Pcaml.type_declaration SEP "and" ->
> 	 let typedefs = <:str_item< type $list:tds$ >> in
> 	 let values = List.map (
> 	   fun ((loc, name), tparams, tdef, cl) ->
> 	     let body = generate loc name tdef in
> 	     <:patt< $lid:name$ >>, <:expr< fun $lid:name$ -> $body$ >>
> 	 ) tds in
> 	 <:str_item<
> 	   declare
> 	     $typedefs$;
> 	     value rec $list:values$;
>            end
>          >> ]];
> END
> --------------------------------------------------------------
>
> -- test2.ml --------------------------------------------------
> type t = { a : int }
> --------------------------------------------------------------
>
> Ideas?
>
> Rich.
>
> --
> Richard Jones, CTO Merjis Ltd.
> Merjis - web marketing and technology - http://merjis.com
> Team Notepad - intranets and extranets for business - http://team-notepad.com
>
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>

--
Martin Jambon, PhD
http://martin.jambon.free.fr