Syntax error analyser

This repository provides different tools to work on the error messages of a menhir-generated parser.

The main tool is lrgrep. It takes:

• a compiled Menhir grammar (a .cmly file, produced by passing --cmly flag to Menhir)
• a list of rules (usually a .mlyl file).

If the list of rule is well-formed, it produces an OCaml module that can match the rules against the state of a parser at runtime.

By carefully crafting the rules, one can provide fine-grained message to explain syntax errors.

The repository is is structured as follow:

• the main tool, lrgrep, can be found in src/main.ml
• support implements the compact table representation shared by the generator and the generated analysers via the lrgrep.runtime library
• in ocaml, we try to apply this methodology to OCaml grammar:
  • parser_raw.mly and lexer_raw.mll define an OCaml 4.13 compatible grammar with syntax error reporting removed
  • parse_errors.mlyl define the error rules for this grammar
  • the frontend binary is an alternative parser that can be used with ocamlc/ocamlopt 4.14 (using the -pp <path-to-frontend.exe> flag)
  • the interpreter binary is a tool that takes an incorrect input and prints detailed information on the parsing process at the point of failure, useful for devising good error patterns
• lib implements various algorithms used by other tools

Working on OCaml grammar

For now, the main focus is on the ocaml sub-directory, and ocaml/parse_errors.mlyl specifically.

My current workflow is as follow:

• starts from an example, an OCaml code with a syntax error for which the message is quite bad
• by reading the grammar and the output of the interpreter, get an idea of what the parsing situation looks like around the error point
• craft an error rule, and debug it using by passing -pp frontend to ocamlc

Setting up the tools

All the work is done using OCaml 4.14. Make sure you are using the right switch:

$ ocamlc -version
4.14.1

Clone the repository and install dependencies:

$ git clone https://github.com/let-def/lrgrep.git
$ cd lrgrep
$ opam install menhir fix cmon

At this point, make should succeed (contact me if not) and produce the three binaries: lrgrep.exe, frontend.bc and interpreter.exe.

It is usually better to test with the bytecode frontend as it leads to shorter iteration cycles.

Quick test

Try the new frontend with some simple examples:

$ ocamlc -c -pp _build/default/ocaml/frontend.bc test_ok.ml

This first example compiled successfully.

$ ocamlc -c -pp _build/default/ocaml/frontend.bc test_ko_01.ml
ocamlc -pp _build/default/ocaml/frontend.bc test_ko_01.ml
File "test_ko_01.ml", line 4, characters 0-3:
4 | let z = 7
    ^^^
Error: Spurious semi-colon at 2:9

File "test_ko_01.ml", line 1:
Error: Error while running external preprocessor
Command line: _build/default/ocaml/frontend.bc 'test_ko_01.ml' > /tmp/ocamlppbbc3f9

In this one however, there is a syntax error. Luckily, this case is covered by a rule: while the error happens on line 4, it is likely caused by the semi-colon at the end of line 2.

Using the frontend for compiling ocaml files

By using the OCAMLPARAM environment variable, we can instruct all execution of ocaml compilers in the current shell to use our frontend.

$ ./demo/setup_shell.sh
export 'OCAMLPARAM=pp=$PWD/lrgrep/_build/default/ocaml/frontend.bc,_'
# setup_shell commands produces a suitable OCAMLPARAM value
$ eval `./demo/setup_shell.sh`
$ ocamlc test_ko_01.ml
...
Error: Spurious semi-colon at 2:9
...
# In the updated environment, the new frontend is picked up automatically

Now you are ready to iterate on ocaml/parse_errors.mlyl to produce new rules.

Note: unset OCAMLPARAM to switch back to the normal frontend

Devising new rules

Once you made sure your setup is working (make is (re-)building the frontend and ocamlc is using it), you can proceed to DEVISING-RULES.md to get started with the error DSL and the associated workflow.

Getting started with LRGrep codebase

I am trying to document the code. Each of the src, lib, ocaml, and support directories contain a README.md that briefly explains the purpose of this directory.

External dependencies that are worth knowing:

• MenhirSdk is a part of the Menhir parser generator that allows external tool to post-process compiled grammars
• Cmon is a pretty-printer for recursive values
• Fix is a library for computing fixed points; it also provides a convenient representation of finite sets
• LRijkstra is taken from Menhir and implements the algorithm described in "Faster Reachability Analysis for LR(1) Parsers", though we apply it for a slightly different purpose than the one described in the articles