What's the best way to write a parser by hand? [closed]

Answer 1

Well if you don't mind using another compiler compiler tool like ANTLR I suggest you take a look at Coco/R

I've used it in the past and it was pretty good...

Answer 2

JFlex is a flex implementation for Java, and there is now a C# port of that project http://sourceforge.net/projects/csflex/. There also appears to be a C# port of CUP in progress, which can be found here: http://sourceforge.net/projects/datagraph/

I too would recommend avoiding hand crafting your own solution. I tried this once myself for a very simple language (part of a university project) and it was incredibly time consuming and difficult. It is also exceedingly hard to maintain and change once written.

Using an existing parser generator is the way to go, as the bulk of the hard work has been done and has been well tested over the years.

Answer 3

The only kind of parser that can be handwritten by a sane human being is a recursive-descent. That said, writing bottom-up parser by hand is still possible but is very undesirable.

If you're up for RD parser you have to verify that SQL grammar is not left-recursive (and eliminate the recursion if necessary), and then basically write a function for each grammar rule. See this for further reference.

Answer 4

I would highly recommend the F# language as your language of choice for parsing on the .NET Platform. It's roots in the ML family of languages means it has excellent support for language-oriented programming.

Discriminated unions and pattern-matching allow for a very succinct and powerful specification of your AST. Higher-order functions allow for definition of parse operations and their composition. First-class support for monadic types allows for state management to be handled implicitly greatly simplifying the composition of parsers. Powerful type-inference greatly aides the definition of these (complex) types. And all of this can be specified and executed interactively allowing you to rapidly prototype.

Stephan Tolksdorf has put this into practice with his parser combinator library FParsec

From his examples we see how naturally an AST is specified:

type expr =
    | Val of string
    | Int of int
    | Float of float
    | Decr of expr

type stmt =
    | Assign of string * expr
    | While of expr * stmt
    | Seq of stmt list
    | IfThen of expr * stmt
    | IfThenElse of expr * stmt * stmt
    | Print of expr

type prog = Prog of stmt list

the implementation of the parser (partially elided) is just as succinct:

let stmt, stmtRef = createParserForwardedToRef()

let stmtList = sepBy1 stmt (ch ';')

let assign =
    pipe2 id (str ":=" >>. expr) (fun id e -> Assign(id, e))

let print = str "print" >>. expr |>> Print

let pwhile =
    pipe2 (str "while" >>. expr) (str "do" >>. stmt) (fun e s -> While(e, s))

let seq =
    str "begin" >>. stmtList .>> str "end" |>> Seq

let ifthen =
    pipe3 (str "if" >>. expr) (str "then" >>. stmt) (opt (str "else" >>. stmt))
          (fun e s1 optS2 ->
               match optS2 with
               | None    -> IfThen(e, s1)
               | Some s2 -> IfThenElse(e, s1, s2))

do stmtRef:= choice [ifthen; pwhile; seq; print; assign]


let prog =
    ws >>. stmtList .>> eof |>> Prog

On the second line, as an example, stmt and ch are parsers and sepBy1 is a monadic parser combinator that takes two simple parsers and returns a combination parser. In this case sepBy1 p sep returns a parser that parses one or more occurrences of p separated by sep. You can thus see how quickly a powerful parser can be combined from simple parsers. F#'s support for overridden operators also allow for concise infix notation e.g. the sequencing combinator and the choice combinator can be specified as >>. and <|>.

Best of luck,

Danny