Sunday, 8 April 2007

The Atomiser, Part V

The Atomiser can now walk its own Abstract Syntax Tree and pluck lists of valid atoms from its own source code.

Next on the list is to

  • Have the Erlang compiler tell us when we use an atom that is not in the valid list.

Sounds easy: let's get to it!

We want to turn our placeholder atom clause in the walk_ast function into something a little more useful:

walk_ast([{atom,Line,Atom}|RestAST], Atoms) ->
    walk_ast(RestAST, atom_check(Atom, Line, Atoms));

Our new atom_check function will compare the supplied atom against the given atoms dictionary. If the atom exists in the dictionary then a new dictionary will be returned, with that atom's value updated to 'found' (taking the place of the atom's line-of-definition integer). If the atom does not exist in the dictionary then a warning message will be displayed and the original dictionary will be returned:

atom_check(Atom, Line, Atoms) ->
    case dict:find(Atom, Atoms) of
        {ok, found} ->
        {ok, _LineDefinedOn} ->
            dict:store(Atom, found, Atoms);
        error ->
                "Line ~B: Atom ~w unexpected.~n",
                [Line, Atom]),

...and we are almost done.

A bit of a shock - but a good sign - is the number of unknown atoms that appear when you first run the Atomiser on itself. All of the atoms we use in our walk_ast pattern tuples show up as unknown atoms, so let's add some proper valid atom lists to the top of the file. First, the atoms we use in our patterns:

-atoms([atom, attribute, call, 'case', clause, clauses, compile]).
-atoms([cons, eof, export, file, 'fun', function, match, module]).
-atoms([nil, remote, string, tuple, var]).

And then some other atoms we are use elsewhere in the code:

-atoms([atoms, error, found, ok]).

And... all the function names are showing up as unknown atoms, too?

Executive decision time!

Since internal function-calls are already picked up by the compiler when they do not match a function in the module, I think it should be safe to modify the 'call' line to:

?WALK_AST({call,_Line,_Fun,Args}, [Args]);

This will skip validating the called function name.

And now,

1> c(atomiser), l(atomiser).


No comments:

Post a Comment

Obligatory legal stuff

Unless otherwise noted, all code appearing on this blog is released into the public domain and provided "as-is", without any warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and noninfringement. In no event shall the author(s) be liable for any claim, damages, or other liability, whether in an action of contract, tort or otherwise, arising from, out of or in connection with the software or the use or other dealings in the software.