Re: [Haskell-cafe] Lazy HTML parsing with HXT, HaXML/polyparse, what else?

On Mon, 14 May 2007, Malcolm Wallace wrote:

Henning Thielemann wrote:

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*Text.ParserCombinators.PolyLazy> runParser (exactly 4 (satisfy Char.isAlpha)) ("abc104"++undefined) ("*** Exception: Parse.satisfy: failed

How can I rewrite the above example that it returns ("abc*** Exception: Parse.satisfy: failed

The problem in your example is that the 'exactly' combinator forces parsing of 'n' items before returning any of them. If you roll your own, then you can return partial results:

> let one = return (:) `apply` satisfy (Char.isAlpha) in runParser (one `apply` (one `apply` (one `apply` (one `apply` return [])))) ("abc104"++undefined) ("abc*** Exception: Parse.satisfy: failed

Equivalently:

> let one f = ((return (:)) `apply` satisfy (Char.isAlpha)) `apply` f in runParser (one (one (one (one (return []))))) ("abc104"++undefined) ("abc*** Exception: Parse.satisfy: failed

I wonder whether 'apply' merges two separate ideas: Applying a generated function to some parser generated value and forcing some parser to always succeed. From the documentation of 'apply' I assumed that 'apply f x' fails if 'f' or 'x' fails. In contrast to that it seems to succeed if only 'f' succeeds. Wouldn't it be better to have an explicit 'force' which declares a parser to never fail - and to return 'undefined' if this assumption is wrong. I have seen this 'force' in the MIDI loader of Haskore: http://darcs.haskell.org/haskore/src/Haskore/General/Parser.hs It would hold: apply f x == do g <- f; fmap g (force x)