From: Hendrik Tews <tews@tcs.inf.tu-dresden.de>
To: caml-list@inria.fr
Subject: Re: [Caml-list] parsing included files recursively using ocamllex and ocamlyacc
Date: Mon, 22 Sep 2003 10:41:38 +0200 [thread overview]
Message-ID: <16238.46530.168784.624111@ithif51.inf.tu-dresden.de> (raw)
In-Reply-To: <3F6C665E.5030508@socialtools.net>
Hi,
Benjamin Geer writes:
Subject: [Caml-list] parsing included files recursively using ocamllex and ocamlyacc
I'm writing an interpreter for a small language (to be released as an
open source library), using ocamllex and ocamlyacc. I'd like this
language to support an instruction that, at compile time, recursively
includes source code from another file.
I use the following approach:
- recognize the include directive in the lexer
- built an abstract lexer that wraps the original lexer and that
+ maintains a stack of lexers with some additional state
+ treats INCLUDE and EOF tokens
- the parser is called on this abstract lexer
This is all straightforward, the only subtle thing is that the
abstract lexer has to substitute the lexbuf argument coming from
the parser with the top of its internal lexer stack. Except for
the first call: In this case the lexbuf argument describes the
top-level input stream. You use this first lexbuf to initialize
the lexer stack.
The disadvantage of this approach is that you cannot have tokens
that span over several files.
In the following I give a few code samples.
The lexer contains the following rule:
| "#include" [' ' '\t'] '"'
{ let f = string lexbuf
in
INCLUDE( f, get_loc lexbuf )
}
(get_loc is not relevant; it tracks line and character numbers.
string matches everything until the next ``"'')
The grammar does not mention the INCLUDE token, instead the
parser is called as
Grammar.file Abstract_lexer.token
The Abstract_lexer module contains the following:
exception Include_error
(* to signal an error with the include directive *)
let empty s =
try
ignore(Stack.top s); false
with
| Stack.Empty -> true
(* Stack.empty, which was missing in earlier releases *)
let d s =
if debug_level _DEBUG_LEXER
then begin
prerr_endline s;
flush stderr
end
(* give diagnostic output *)
let get_current_directory () = match !current_top_level_input with
| None -> assert(false)
| Some f -> Filename.dirname f
(* compute the base directory for relative includes *)
type lexing_pos =
{
lexbuf : Lexing.lexbuf;
util_state : Parser_util.state_type;
closing_action : unit -> unit
}
(* the record I save on the lexer stack. The state field saves
line and character numbers. The closing action is for
close_in <include file. I prefer a closure here because the
toplevel input might be a string.
*)
let lexer_stack = (Stack.create () : lexing_pos Stack.t)
(* this is the stack of open lexers *)
let initialize top_file_name =
current_top_level_input := Some top_file_name;
Parser_util.reset_line top_file_name;
Stack.clear lexer_stack
(* initialize this module for the next toplevel input *)
let token_from_top () =
Lexer.token (Stack.top lexer_stack).lexbuf
(* read a token from the current lexer;
might raise Stack.Empty if there is no current lexer, which
happens on the first token of a toplevel input
*)
(* divert into the next include file *)
let divert lexbuf file closing_action =
(* give some diagnostics *)
let _ = d ("Diverting into " ^ file ) in
let _ =
(* if we are leaving an input stream to process an include, we
have to save line and character numbers
*)
if not (empty lexer_stack) then
let including_lex_pos =
{ (Stack.pop lexer_stack) with
util_state = Parser_util.get_state()
}
in
Stack.push including_lex_pos lexer_stack
in
(* reset line and character numbers *)
let _ = Parser_util.reset_line file in
(* and put the new lexer on top of the stack
let included_lex_pos =
{ lexbuf = lexbuf;
util_state = Parser_util.get_state();
closing_action = closing_action
}
in
Stack.push included_lex_pos lexer_stack
(* The main function we provide: Read the next token, but treat
EOF's and INCLUDE directives magically. This function is
recursive on INCLUDE's, on EOF's of include files, and on the
first token.
*)
let rec token lexbuf =
try
(* ignore the lexbuf argument, use the lexer on top of the stack *)
(match token_from_top() with
(* process an INLCUDE *)
| INCLUDE(filename, loc) ->
let relocated_name =
Filename.concat (get_current_directory()) filename in
let included_channel =
try
open_in relocated_name
with
| Sys_error msg ->
begin
error_message loc msg;
raise Include_error
end
in
begin
divert (Lexing.from_channel included_channel)
relocated_name (fun () -> close_in included_channel);
token lexbuf
end
(* process EOF's *)
| EOF ->
begin
(Stack.pop lexer_stack).closing_action ();
if empty lexer_stack
(* This was EOF on the top level! )
then EOF
(* It's an EOF in an include file *)
else
let top = Stack.top lexer_stack in
let top_state = top.util_state in
let (line, line_start, file) = top_state
in begin
Parser_util.set_state top_state;
d ("Continuing lexing in file " ^ file ^
" at line " ^
(string_of_int line) ^
" char " ^
(string_of_int
((Lexing.lexeme_end top.lexbuf) - line_start)));
token lexbuf
end
end
(* no include, no EOF -> pass it on
| othertoken -> othertoken
*)
)
with
(* catch the empty stack exception on the first token of the
toplevel input; initialize the stack and retry
*)
| Stack.Empty ->
begin
divert lexbuf
(remove_option !current_top_level_input)
(* the toplevel is not closed here *)
(fun () -> ());
token lexbuf
end
Bye,
Hendrik
-------------------
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prev parent reply other threads:[~2003-09-22 8:41 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-09-20 14:38 Benjamin Geer
2003-09-20 15:15 ` Michal Moskal
2003-09-20 19:07 ` Benjamin Geer
2003-09-22 7:22 ` Eckart Goehler
2003-09-22 8:41 ` Hendrik Tews [this message]
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