AN S-EXPRESSION PARSER built with Sedlex and Menhir
This is an example of the construction of an error-reporting parser
and lexer using sedlex and menhir. The parser runs interactively
when connected to a terminal. It is intended ONLY to serve as
an example of the use of sedlex and menhir, but may be helpful
if you need an uncomplicated parser for S-expressions that can
parse them efficiently both interactively from terminals
as well as from files.
To construct the parser, the lexer, and an example of their use:
dune build example.exe
EFFICIENT INTERACTIVE LEXING
The supplied sedlexing.{ml,mli} have been adapted so that the lexer
can be made to respond /immediately/ to characters appearing on the
input channel, when the channel is connected to a terminal, yet can
run efficiently (without recourse to the operating system for every
single input character) when connected to a file.
Such a lexer is constructed from a lexbuf made from a channel that
can be tested to see if it's a tty.
let chan = open_in "/dev/stdin" in
let istty = Unix.isatty @@ Unix.descr_of_in_channel chan in
let lexbuf = Sedlexing.Utf8.from_channel ~chunk_size:(if istty then 1 else 1024) chan in
Sedlexing.set_filename lexbuf "/dev/stdin";
if istty then
Sedlexing.set_prompter lexbuf (fun () -> Format.printf "> %!");
[NB: the standard sedlexing.{ml,mli} can also be used as per other
published examples and their documentation, but in this case the lexer
responds to input only when the end-of-file key is pressed at the terminal.
This is because the "chunk-size" for refilling the standard form of
buffer is constant (512), and the refilling loop requires an eof
before it terminates.]
UPDATING ERROR MESSAGES
The supplied sources are all up to date wrt. the grammar file sexpParser.mly
The Makefile has four /consequential/ targets.
diff: generates sexpParserMessages.new and runs a diff between it and
the existing sexpParserMessages. This is the default target, invoked
on make.
* If no differences are reported then there is no need
for further action.
* If there are differences only in state numbers, then
there is no need for further action.
If there is a difference that contains something like
<YOUR SYNTAX ERROR MESSAGE HERE>
then you will need to edit the .new file to change such
generic messages into concrete descriptions of the error.
When you have done so, then
make cp
to copy the .new file to the sexpParserMessages file, then
make ml
to generate source file sexpParserMessages.ml
new: Generates sexpParserMessages.new incrementally from
sexpParserMessages and sexpParser.mly
The .new file may need editing to bring its error
messages up to date with the parser specification
(see the diff target)
raw: Generates sexpParserMessages.raw ab-initio from sexpParser.mly
The .raw file will certainly need editing to bring its error
messages up to date with the parser specification.
ml: Generates the sexpParserMessages.ml source file from the
parser specification and the messages file.