At 2002-06-29 14:43, Wolfgang Jeltsch wrote: >To simplify things a bit, let's take a simpler Parser type which doesn't >use monad state transformers but simple state transformers instead. This >makes the type ParserState simpler. You can think of a parser state as >an infinite list of substate-token pairs now. The tokens denote the >input stream. The substates may be used to encapsulate parser states of >a subordinate parsing process. When a token is read, the corresponding >token-substate pair is discarded.
Sounds very complicated. Wouldn't a simple stream transformer work? newtype MyParser baseMonad token output = MkMyParser (baseMonad token -> baseMonad output); > (>>=): sequencing of parsers > return: returning values > fail: generation of always failing parsers > mzero: always failing parser > mplus: parsing using alternative parsers > fmap: applying a function to the result of a parser > (>>>): providing high-level tokens and parsing using these tokens > identity: reading a single token > pure: reading a single token and applying a function to it I think I can do all of those for MyParser. There's a one-token look-ahead variant: newtype MyLookaheadParser m t a = MkMyLookaheadParser (m t -> Maybe t -> m (Maybe t,a)); >(I don't allow >finite lists here to make some things a bit easier an more elegant. You >can easily produce an infinite list from a finite one by applying >something like (++ repeat Nothing) . map Just to the finite list.) Wouldn't it be easier to allow finite token lists and have reading past the last token be the same as mzero? -- Ashley Yakeley, Seattle WA _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
