Commit 7dce8dffdaecbd4e67fced1a6eb3278ca6d8cf0d - Parc

Add documentation for `ParcAssertDemo.hs`. Chris Pressey 8 months ago

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72	72	combinator can be used to apply arithmetic operators like `+` and `*`
73	73	when parsing an arithmetic expression.
74	74
	75	### `ParcAssert.hs`
	76
	77	With what we have so far, if we merely accumulate state as we parse,
	78	we can parse only context-free languages.
	79
	80	But if we enable the parser to succeed or fail based on some predicate
	81	applied to the parsing state, we can parse context-sensitive languages.
	82
	83	(Actually, if we don't restrict ourselves to sufficiently simple
	84	predicates, we can parse languages quite beyond the context-sensitive
	85	range. For example, we could use a predicate which checks if the
	86	string passed to it is a valid sentence in Presburger arithmetic.
	87	Formulating a set of parser combinators which actually (and provably)
	88	is limited to parsing context-sensitive languages seems like a hard
	89	(maybe open) problem.)
	90
	91	[`ParcAssertDemo.hs`](ParcAssertDemo.hs) uses `ParcSt2St` and adds a
	92	combinator called `assert` that takes such a predicate on the parsing
	93	state, and produces a parser which succeeds only when that predicate
	94	is true on the current parsing state, failing otherwise.
	95
	96	The demo module gives an example of a parser for a classic
	97	context-free language: strings of the form `a`^_n_ `b`^_n_ `c`^_n_.
	98
75	99	### ... and more?
76	100
77	101	With luck, there will be further experiments added here over time.