[Haskell-cafe] uu-parsinglib - Greedy Parser

oleg at okmij.org oleg at okmij.org
Thu Jan 15 06:03:13 UTC 2015

Marco Vassena wrote:
> The bigger picture is that I am trying to figure out what 
> are the core constructs needed to define a parser, therefore I want to
> have a rather abstract interface. In my set of core constructs there are:
> 	<$> : (a -> b) -> f a -> f b
> 	<*> : f (a -> b) -> f a -> f b
> 	<|> : f a -> f a -> f a  -- (symmetric choice)
> 	pure : a -> f a
> 	fail : f a 
> 	pToken : f Char 
> Is it possible to define a parser that applies the longest 
> matching rule using these constructs only?

No, these are not sufficient. Longest-matching rule (aka maximal
munch) is defined as applying a parser for as long as it
succeeds. Therefore, we need a way to determine if a parser succeeded:
we need what is called a (monadic) reflection. In simpler words, we
need an operation like the soft-cut in Prolog. It can be defined in
terms of a primitive called `msplit', described in the LogicT paper:

The following article talks about maximal munch in more detail, in
terms of logic programming:

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