ANTLR Source to Output - antlr

Question

I'm trying to implement something like a Code Contracts feature for JavaScript as an assignment for one of my courses.
The problem I'm having is that I can't seem to find a way to output the source file directly to the console without modifying the entire grammar.
Does anybody knows a way to achieve this?
Thanks in advance.
Here's an example of what I'm trying to do:
function DoClear(num, arr, text){
Contract.Requires<RangeError>(num > 0);
Contract.Requires(num < 1000);
Contract.Requires<TypeError>(arr instanceOf Array);
Contract.Requires<RangeError>(arr.length > 0 && arr.length <= 9);
Contract.Requires<ReferenceError>(text != null);
Contract.Ensures<RangeError>(text.length === 0);
// method body
[...]
return text;
}
function DoClear(num, arr, text){
if (!(num > 0))
throw RangeError;
if (!(num < 1000))
throw Error;
if (!(arr instanceOf Array))
throw TypeError;
if (!(arr.length > 0 && arr.length <= 9))
throw RangeError;
if (!(text != null))
throw ReferenceError
// method body
[...]
if (!(text.length === 0))
throw RangeError
else
return text;
}

Answer 1

There are a few (minor) things you'll want to consider:
ignore string literals that might contain your special contract-syntax;
ignore multi- and single line comments that might contain your special Contract syntax;
ignore code like this: var Requires = "Contract.Requires<RangeError>"; (i.e. regular JavaScript code that "looks like" your contract-syntax);
It's pretty straight forward to take the points above into account and also simply create single tokens for an entire contract-line. You'll be making your life hard when tokenizing the following into 4 different tokens Contract.Requires<RangeError>(num > 0):
Contract
Requires
<RangeError>
(num > 0)
So it's easiest to create a single token from it, and at the parsing phase, split the token on ".", "<" or ">" with a maximum of 4 tokens (leaving expressions containing ".", "<" or ">" as they are).
A quick demo of what I described above might look like this:
grammar CCJS;
parse
: atom+ EOF
;
atom
: code_contract
| (Comment | String | Any) {System.out.print($text);}
;
code_contract
: Contract
{
String[] tokens = $text.split("[.<>]", 4);
System.out.print("if (!" + tokens[3] + ") throw " + tokens[2]);
}
;
Contract
#init{
boolean hasType = false;
}
#after{
if(!hasType) {
// inject a generic Error if this contract has no type
setText(getText().replaceFirst("\\(", "<Error>("));
}
}
: 'Contract.' ('Requires' | 'Ensures') ('<' ('a'..'z' | 'A'..'Z')+ '>' {hasType=true;})? '(' ~';'+
;
Comment
: '//' ~('\r' | '\n')*
| '/*' .* '*/'
;
String
: '"' (~('\\' | '"' | '\r' | '\n') | '\\' . )* '"'
;
Any
: .
;
which you can test with the following class:
import org.antlr.runtime.*;
public class Main {
public static void main(String[] args) throws Exception {
String src =
"/* \n" +
" Contract.Requires to be ignored \n" +
"*/ \n" +
"function DoClear(num, arr, text){ \n" +
" Contract.Requires<RangeError>(num > 0); \n" +
" Contract.Requires(num < 1000); \n" +
" Contract.Requires<TypeError>(arr instanceOf Array); \n" +
" Contract.Requires<RangeError>(arr.length > 0 && arr.length <= 9); \n" +
" Contract.Requires<ReferenceError>(text != null); \n" +
" Contract.Ensures<RangeError>(text.length === 0); \n" +
" \n" +
" // method body \n" +
" // and ignore single line comments, Contract.Ensures \n" +
" var s = \"Contract.Requires\"; // also ignore strings \n" +
" \n" +
" return text; \n" +
"} \n";
CCJSLexer lexer = new CCJSLexer(new ANTLRStringStream(src));
CCJSParser parser = new CCJSParser(new CommonTokenStream(lexer));
parser.parse();
}
}
If you run the Main class above, the following will be printed to the console:
/*
Contract.Requires to be ignored
*/
function DoClear(num, arr, text){
if (!(num > 0)) throw RangeError;
if (!(num < 1000)) throw Error;
if (!(arr instanceOf Array)) throw TypeError;
if (!(arr.length > 0 && arr.length <= 9)) throw RangeError;
if (!(text != null)) throw ReferenceError;
if (!(text.length === 0)) throw RangeError;
// method body
// and ignore single line comments, Contract.Ensures
var s = "Contract.Requires"; // also ignore strings
return text;
}
BUT ...
... I realize that it isn't what you're exactly looking for: the RangeError is not placed at the end of your function. And that's going to be tough one: a function might have multiple returns, and is likely to have multiple code blocks { ... } making it difficult to know where the } is that ends the function. So you don't know where exactly to inject this RangeError-check. At least, not with a naive approach as I demonstrated.
The only reliable way to implement such a thing is to get a decent JavaScript grammar, add your own contract-rules to it, rewrite the AST the parser produces, and finally emit the new AST in a friendly-formatted way: not a trivial task, to say the least!
There are various ECMA/JS grammars on the ANTLR Wiki, but tread with care: they are user-committed grammars and may contain errors (probably will in this case[1]!).
If you choose to place the RangeError there where it should be rewritte, like so:
function DoClear(num, arr, text){
Contract.Requires<RangeError>(num > 0);
...
// method body
...
Contract.Ensures<RangeError>(text.length === 0);
return text;
}
which would result in:
function DoClear(num, arr, text){
if (!(num > 0)) throw RangeError;
...
// method body
...
if (!(text.length === 0))
throw RangeError
return text;
}
then you need not parse the entire method body, and you might get away with a hack as I proposed.
Best of luck!
[1] the last time I checked these ECMA/JS script grammars, none of them handled regex literals, /pattern/, properly, making them in my opinion suspect.