Hi Fernando. I tried this approach for a toy language as well, and I was unhappy with it. I have found that, with Parsec, it is best to *not* split your parsing completely into "tokenization" and "parsing" phases, but rather to interleave them. Instead of
tokenize :: Parser [MJVal]
make
token :: Parser MJVal
Then use something like the following:
tokenSatisfies :: (MJVal -> Bool) -> Parser MJVal tokenSatisfies f = try $ do t <- token if (f t) then return t else fail "No parse"
program :: Parser Program program = do tokenSatisfies (== Program_) programName <- identifier -- etc. return $ Program i ...
There is a bit of an inefficiency using "try"; you'll reparse the same
token multiple times for each failure branch of a "choice" branch, but
I've found this to be the simplest solution and parsing time rarely
dominates your running time.
-- ryan
On Tue, Nov 10, 2009 at 11:23 AM, Fernando Henrique Sanches
Hello. I'm currently implementing a MicroJava compiler for a college assignment (the implemented language was defined, the implementation language was of free choice). I've sucessfully implemented the lexer using Parsec. It has the type String -> Parser [MJVal], where MJVal are all the possible tokens. However, I don't know how to implement the parser, or at least how to do it keeping it distinguished from the lexer. For example, the MicroJava grammar specifies: Program = "program" ident {ConstDecl | VarDecl | ClassDecl} "{" {MethodDecl} "}". The natural solution (for me) would be: program = do string "program" programName <- identifier ... However, I can't do this because the file is already tokenized, what I have is something like: [Program_, identifier_ "testProgram", lBrace_, ...] for the example program: program testProgram { ... How should I implement the parser separated from the lexer? That is, how should I parse Tokens instead of Strings in the "Haskell way"? Fernando Henrique Sanches
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