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\LARGE\bf Certified Parsing\\[-10mm]

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\section*{Background}

\noindent

Parsing is the act of transforming plain text into some

structure that can be analyzed by computers for further processing.

\emph{yacc} and \emph{lex} no new results can be obtained in this area.

However recent results and novel approaches make it increasingly clear,

that this is not true anymore.

We propose to approach the subject of parsing from a certification point

of view. Increasingly, parsers are part of certified compilers, like \mbox{\emph{CompCert}},

which are guaranteed to be correct and 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. Unfortunately, this means parsers can harbour

errors that potentially invalidate the whole certification and correctness

of the compiler. In this project, we like to change that.

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

for establishing the correctness of some standard lexing and

parsing algorithms. For this, the algorithms still need to be formulated

in way so that it is easy to reason about them. In our earlier work

about lexing and regular languages, the authors showed that this

precludes well-known algorithms based automata. However we showed 

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 a

theorem prover.

\section*{Proposed Work}

A recent development in parsing is Parsing Expression Grammars (PEG), which

are an extension of the standard Context Free Grammars

(CFG)~\cite{Ford04a}. The extension introduces new regular operators, such as

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

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

more powerful. For example disambiguation, formerly expressed by semantic

filters, can now be expressed directly using grammar rules. This means a

simpler and more systematic treatment of ambiguity and more concise grammar

specifications for programming languages.

However, a serious disadvantage of PEG is that it does not allow left

recursion, because parsing algorithms for PEG~\cite{Ford02b} can not deal with

left recursions. Although a new PEG parsing algorithm has been proposed

that can deal with left recursion~\cite{conf/pepm/WarthDM08}, there is no

correctness proof, not even in ``paper-and-pencil'' form. 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 formalize a fixed

point semantics of PEG, based on which an efficient, certified parsing

algorithm can be given given.

There are several existing works we can draw upon:

\begin{enumerate}

\item The works on PEG.

  \begin {enumerate}

  \item An operation semantics for PEG has already been given

    in~\cite{Ford04a}, but it is not adequate to deal with left recursions. But this

    work gives at least a precise description of what the original PEG meant

    for. This will serve an a basis to show the conservativeness of

    the fixed point semantics we are

    going to develop.

  \item The new algorithm~\cite{conf/pepm/WarthDM08} which claimed to be able

    to deal with left recursions. Although there is no correctness proof yet, this

    may provide some useful inspirations for our new algorithm design.

  \end{enumerate}

\item The works on Boolean Grammars~\cite{Okhotin/04a}. Boolean Grammar is

  very closely related to PEG, because it also contains negative and conjunctive

  grammars. The main differences are: First, Boolean Grammar has no ordering on

  productions; Second: Boolean Grammar does not contain STAR operator. There are

  two works about Boolean Grammar which might be useful for this research:

  \begin{enumerate}

  \item A fixed point semantics for Boolean

    Grammar~\cite{journals/iandc/KountouriotisNR09}. The idea to define the

    semantics of negative and conjunctive operators is certainly what we can

    borrow. Therefore, this work gives the basis on which we can add in production

    ordering and STAR operator.

  \item A parsing algorithm for Boolean Grammar based on CYK

    parsing~\cite{journals/iandc/KountouriotisNR09}. The draw back of CYK parsing

    is that: the original grammar specification needs to be transformed into a

    normal form. This transformation may lead to grammar explosion and is

    undesirable. One aim of this research is to see whether this transformation

    can be avoided. For this purpose, other parsing style may provide useful inspirations, for example:

    \begin{enumerate}

    \item Derivative

      Parsing~\cite{Brzozowski64,Almeidaetal10,OwensReppyTuron09,journals/corr/abs-1010-5023}. Christian

      Urban has used derivative methods to establish the correctness of a regular

      expression matcher, as well the the finite partition property of regular

      expression~\cite{WuZhangUrban11}.  There are well founded envisage that the

      derivative methods may provide the foundation to the new parsing algorithms of PEG.

    \item Early parsing~\cite{Earley70,AycHor02}. It is a refinement of CYK

      parsing which does not require the transformation to normal forms, and

      therefore provide one possible direction to adapt the current CYK based

      parsing algorithm of Boolean Grammar for PEG grammar.

    \item The new parsing algorithm proposed by Tom Ridge[???]. Recently,

      T. Ridge has proposed and certified an combinator style parsing algorithm for

      CFG, which borrows some ideas from Early parsing. The proposed algorithm is

      very simple and elegant. We are going to strive for a parsing algorithm as elegant as this one.

    \end{enumerate}

    Which of the above possibilities will finally get into our final solutions

    is an interesting point about this current research.

  \end{enumerate}

\end{enumerate}

Based on these works, we are quite confident that our idea may lead to some concrete results.

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%  \noindent {\bf Objectives:} The overall goals of the project are as follows:

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