Question

I’m making a simple parser for some Java-like language (just for learning purposes). I’m having trouble determining whether a statement is a variable declaration. This may be a problem with my lexer (which is really sloppy). If the lexer sees some text, it simply labels it an identifier, even if that text is a keyword or a type. The job of telling those apart is given to the parser.

This has worked so far, but now I’m trying to parse variable declarations, like this one here:

int x = 3;

The problem is I don’t know how to determine whether this is a variable declaration. If I just look at the first token and find that it’s an “identifier”, that doesn’t tell me anything, since this line of code also starts with an identifier:

System.out.print("hi");

And statements like this are handled by another part of the parser.

Another solution I thought about was checking to see if the first token is a type. For example, I could have a method that looks something like:

boolean isType(String t) {
    if( t.equals("int")  ||
        t.equals("long") ||
        t.equals("char") ||
        /* et cetera */ )
        return true;
    else return false;
} 

The problem with this is that it only allows for a certain set of types. Since my little language is compiled to Java bytecode, I need it to recognize arbitrary classes as types.

So my question is: is it possible to determine whether a statement is a variable declaration or not, without knowing all the possible variable types?

Answer 1

I’ve had to do something of this flavor for a class, about 4 years ago. Although I don’t remember all the details of the “official” way to do it, but

What I would do, is I would look ahead, at the future symbols to determine whether or not it’s a variable declaration, so as Benjamin Gruenbaum said, if you see a legal identifier(at the beginning of a line) followed by another legal identifier, than the first one is probably a variable declaration.