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\LARGE\bf Novel Certified Parsers\\[-10mm]

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\section*{Background}

\noindent

Parsers transform plain text into some abstract structure that can be analyzed by

computers for further processing.  One might think that parsers have been

studied to death, and after \emph{yacc} and \emph{lex} no new results can be

obtained in this area.  However recent developments and novel approaches make

it increasingly clear, that this is not true anymore~\cite{Might11}. And

there is a real practical need for new results: for example the future HTML5 

Standard abandons a well-defined grammar specification, in favour of a bespoke

parser given as pseudo code.

This work targets parsers from a certification point of view. Increasingly,

parsers are part of certified compilers, like

\mbox{\emph{CompCert}}~\cite{Leroy09}, which are guaranteed to be

bug-free. Such certified compilers are crucial in areas where software just

cannot fail. However, so far the parsers of these compilers have been left out

of the certification.  This is because parsing algorithms are often ad hoc and

their semantics is not clearly specified. This means, unfortunately, parsers

can harbour errors that potentially invalidate the whole certification and

correctness of the compiler. In this project, we aim to change this situation

with the help of the theorem prover Isabelle/HOL.

Only in the last few years, theorem provers have become powerful enough for

establishing the correctness of some standard lexing and parsing

algorithms. For this, the algorithms still need to be formulated in a way so

that it is easy to reason about them. In our earlier work about lexing and

regular languages, we showed that this precludes well-known algorithms based

automata~\cite{WuZhangUrban11}. However we showed also that regular languages can be formulated and

reasoned about entirely in terms regular expressions, which can be easily

represented in theorem provers. This work uses the device of derivatives of

regular expressions. We like to extend this device to parsers and grammars.

The aim is to come up with elegant and practical useful parsing algorithms

whose correctness can be certified in Isabelle/HOL.

\section*{Proposed Work}

A recent development in the field of parsing are Parsing Expression Grammars

(PEGs)~\cite{Ford04}, which

are an extension of the well-known Context Free Grammars

(CFGs). This extension introduces new regular operators, such as

negation and conjunction, on the right-hand side of grammar rules, as well as

priority orderings for rules. With these extensions, PEG parsing becomes much

more powerful and more useful in practise. For example disambiguation, formerly expressed by semantic

filters, can now be expressed directly using grammar rules. 

However, there is a serious limitation of PEGs, which affects potential

applications: grammars involving left recursion are not

allowed~\cite{Ford02}. Although one PEG parsing algorithm has already been

proposed that can deal with left recursion~\cite{WarthDM08}, there

is no correctness proof for it, not even one with ``pencil and paper''. One aim of this

research is to solve this sorry state-of-affairs by either certifying this

algorithm or inventing a new one. For this we will first devise a

fixed-point semantics of PEGs, against which we can certify a parser. For this

semantics we take as starting point the paper~\cite{Ford04}, which does not

treat left-recursion, but gives an operational semantics for PEG

parsing. There are also good indications that we can adapt work on Boolean

Grammars~\cite{Okhotin04}, which are similar to PEGs and for which the

paper~\cite{KountouriotisNR09} gives a fixed-point semantics 

to negation operators, but not to the Kleene star.

For the parsing algorithm, we might be able to build upon

the classic Cocke-Younger-Kasami (CYK)

algorithms~\cite{KountouriotisNR09} and

Early~\cite{AycHor02, Earley70} parsers. The defect of CYK algorithms, however,

is that the grammar specifications given by the user need to be transformed

into a normal form. This transformation may potentially lead to rule explosion

and hence inefficient parsing. We will investigate whether this transformation

can be avoided.  The problem with Early-style parsers is that they need to be 

extended to PEG parsing in order to be helpful for us.

Finally, we want to investigate whether derivatives of regular expressions

~\cite{Brzozowski64,Might11,OwensReppyTuron09}

can be generalised to PEG parsing. In earlier work, we showed that lexing based on

derivatives gives rise to very elegant regular expression matchers that can be

certified in a theorem prover with ease.  We will study whether the idea of

taking a derivative of a regular expression can be extended to rules in

grammars. The problem that needs to be addressed is again how to deal with 

left-recursive grammar rules.

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