10 May 2008

Parsing Dot with F#: Part 4 - Conclusion

In this very last part, I'll just show how to put everything together: the types, the lexer and the parser. First, we need to generate some F# files from the dotlexer.fsl  and the dotparser.fsy files. This is easily accomplished by running fslex and fsyacc, respectively. This creates dotlexer.fs, dotparser.fs and dotparser.fsi. If, like me, you're working in Visual Studio, you can add these files to your project, but take care that you put everything in the right order or it won't compile (I actually lost quite a bit of time with this): first the definition of the abstract syntax type, then the parser files, then the lexer file. Finally, you can call the parser:

let parseFromString(text) =
    let lexbuf = Lexing.from_string text
        DotParser.start DotLexer.token lexbuf
    with e ->
        let pos = lexbuf.EndPos
        printf "error near line %d, character %d\n%s\n" pos.Line pos.Column (e.ToString())
        Graph (new List<_>())

This is fairly straightforward: the Lexing module defines functions to create a lexing buffer from a string. Feed this buffer into the DotParser.start function generated by fsyacc. This function will return the expected type, as defined in the .fsy file.

That's really all there is to it - the rest is some exception catching and printing an error for debugging purposes. One thing I'd like to warn you about is that debugging this stuff can be quite tedious, due to the fact that first the F# files are generated by the fsyacc and the fslex tools, and afterwards compiled. Although the compiler does a fairly good job of showing where the actual errors are in the fsy and fsl files, it is sometimes hard to track down what exactly is the problem.

Finally, you can download the example project.

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