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copied!<p>Parsing (I claim) is a bit like sorting: a problem that was the focus of a lot of thought in the early days of CS, leading to a set of well-understood solutions with some nice theoretical results.</p> <p>My claim is that the picture that we get (or give, for those of us who teach) in a compilers class is, to some degree, a beautiful answer to the wrong question.</p> <p>To answer your question more directly, an LL(1) grammar can't parse all kinds of things that you might want to parse; the "natural" formulation of an 'if' with an optional 'else', for instance.</p> <p>But wait! Can't I reformulate my grammar as an LL(1) grammar and then patch up the source tree by walking over it afterward? Sure you can! To some degree, this is what makes the question of what kind of grammar your parser uses largely moot.</p> <p>Also, back when I was an undergraduate (1990-94), whitespace-sensitive grammars were clearly the work of the Devil; now, Python and Haskell's designs are bringing whitespace-sensitivity back into the light. Also, Packrat parsing says "to heck with your theoretical purity: I'm just going to define a parser as a set of rules, and I don't care what class my grammar belongs to." (paraphrased)</p> <p>In summary, I would agree with what I believe to be your implied suggestion: in 2009, a clear understanding of the difference between the classes LL(k) and LR(k) is less important in itself than the ability to formulate and debug a grammar that makes your parser generator happy.</p>

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