[Caml-list] Menhir grammar with sequences delimited by same token

[Dario Teixeira <dario.teixeira@nleyten.com>]

Hi,

(Sending this to Caml-list because Menhir-list is currently down.)

I've come across an interesting parsing problem, one for which I
wonder if there is a succinct solution in Menhir.  Suppose I want
to parse a markup which uses the same token for delimiting *both*
the beginning and the termination of a bold sequence (and likewise
for an emph sequence).  Basically this:

   inline:
     | TEXT               {Ast.Text $1}
     | BOLD inline* BOLD  {Ast.Bold $2}
     | EMPH inline* EMPH  {Ast.Emph $2}


Which of course has a shift/reduce conflict: if the token stream is
[BOLD; TEXT; BOLD; ...], what should the parser do upon encountering
the second BOLD -- start a new nesting level, or close the current
one?  I can force the latter behaviour by rearranging the grammar
so that an inline sequence within BOLDs cannot contain BOLD itself,
and likewise for EMPH:

   inline:
     | TEXT                        {Ast.Text $1}
     | BOLD inline_sans_bold* BOLD {Ast.Bold $2}
     | EMPH inline_sans_emph* EMPH {Ast.Emph $2}

   inline_sans_bold:
     | TEXT                        {Ast.Text $1}
     | EMPH inline_sans_emph* EMPH {Ast.Emph $2}

   inline_sans_emph:
     | TEXT                        {Ast.Text $1}
     | BOLD inline_sans_bold* BOLD {Ast.Bold $2}


For this simple example this approach is feasible, but blows up
into silliness for a real-world case where besides BOLD and EMPH I
have many other similar tokens.  Does Menhir offer a more succinct
solution to this problem?  (I reckon using the priority mechanism
somehow, but exactly how eludes me.)

Thanks in advance for your time!
Best regards,
Dario Teixeira