Monadic Compositional Parsing with Context Using Maltese as a Case Study

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Monadic CompositionalParsing with ContextUsing
Maltese as a Case Study

• Gordon J. Pace
• September 2004

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Monadic CompositionalParsing with ContextUsing
Maltese as a Case Study

• A Users Perspective.
• For the technical details, read the paper!

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Combinator-Based Programming

• A Recipe
• Provide a class/type of objects you will be
  talking about
• Provide a few basic objects of that type
• Provide a few combinators to combine objects of
  the type into more complex ones.
• Mix well and put on low heat for 35 min.

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Combinator-Based Programming

• type Parser a

A basic type A parser which consumes a text
stream, returning an object of type a
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

Some basic parsers
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

And some combinators which produce new parsers
out of old ones.
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

A parser which always fails
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

Given a value, this parser leaves the input
stream untouched, returning the given value
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

A parser which returns the first character on the
input stream
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

Non-deterministic choice between two parsers
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Combinator-Based Programming

• fail Parser a
• return a - Parser a
• one Parser Char
• Parser a - Parser a - Parser a
• mthen Parser a - Parser a - Parser a

Sequential composition of two parsers
Actually, Im cheating by somewhat simplifying
the type
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New parsers are easy to derive

• p1 p2
• do
• x1
• x2
• return (x1,x2)

Parse a pair of objects in sequence
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New parsers are easy to derive

• matchSat
• (Char - Bool) - Parser Char
• matchSat cond
• do
• x
• if cond x
• then return x
• else fail

Parse a character, if it satisfies a given
condition
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New parsers are easy to derive
Parse a particular character

• matchChar c matchSat (c)
• matchString return
• matchString (ccs)
• do
• matchChar c
• matchString cs
• return (ccs)

Parse a particular string
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New parsers are easy to derive

• star Parser a - Parser a
• star p
• return
• do
• (x,xs) star p
• return (xxs)

Kleene star
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New parsers are easy to derive

• p1 p2
• do
• w1
• plus (parseSat isSpace)
• w2
• return (w1,w2)

Space separated words
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Context The Maltese Article

• Basic form of definite article is il.
• Nouns starting with a sun letter (xemxin)
  transform the l to match the first letter of the
  noun.
• Nouns starting with 2 or 3 consonants, the first
  being x or s, drop the initial i, and add an
  initial i to the noun.
• Nouns starting with a vowel drop the initial i
  from the article.
• The initial i is also dropped when the preceding
  word starts with a vowel.

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Context The Maltese Article
Context

• Basic form of definite article is il.
• Nouns starting with a sun letter (xemxin)
  transform the l to match the first letter of the
  noun.
• Nouns starting with 2 or 3 consonants, the first
  being x or s, drop the initial i, and add an
  initial i to the noun.
• Nouns starting with a vowel drop the initial i
  from the article.
• The initial i is also dropped when the preceding
  word starts with a vowel.

Context
Context
Context
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Context The Maltese Article
Context
How can we write a compositional parser one
that parses a stand-alone article?

• Basic form of definite article is il.
• Nouns starting with a sun letter (xemxin)
  transform the l to match the first letter of the
  noun.
• Nouns starting with 2 or 3 consonants, the first
  being x or s, drop the initial i, and add an
  initial i to the noun.
• Nouns starting with a vowel drop the initial i
  from the article.
• The initial i is also dropped when the preceding
  word starts with a vowel.

Context
Context
Context
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Looking at the Context

• We would like to write
• definiteNoun
• article noun

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Looking at the Context

• We can look at local context using other parsers
• article
• do
• initial_i
• c
• parseChar -
• matchFuture (parseChar c)

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Looking at the Context

• We can look at local context using other parsers
• initial_i
• matchPast (
• (parseSat isVowel wordSep)
• )ifnot
• parseChar i

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More Context First Form Verbs
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More Context First Form Verbs
Context
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Setting the Context

• We would like to write something like
• sentence
• noun verb noun

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Setting the Context

• We set a context by setting attributes
• setAttribute (name, value)
• getAttribute name
• renameAttribute (name, name)

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Setting the Context
Clashing values cause the parser to fail

• We set a context by setting attributes
• setAttribute (name, value)
• getAttribute name
• renameAttribute (name, name)

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Setting the Context

• parsePersonSingular
• do
• setAttribute ("SubjectNumber", "Singular")
• c
• case c of
• 'n' - setAttribute ("SubjectPerson" 1)
• 'j' - setAttributes ("SubjectPerson",3"),
  ...
• 't' - setAttribute ("SubjectPerson",2")
• setAttributes ("SubjectPerson",3"),
  ...

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Setting the Context

• parseSubject
• do
• n
• renameAttributes
• ("Noun"x, "Subject"x)
• x
• parseObject ...
• parseSentence
• parseSubject parseVerb parseObject

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Contribution

• Local context parsed using the same parser
  combinators
• Global, shared information shared via attributes
• All this done compositionally, and
• By enriching standard parser combinators.

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Conclusions

• Optimise it, apply it to larger NL subset,
  compare with other, more traditional approaches.
• Optimise the new context-aware combinators.
• Extend it to work both as a language generator
  and parser.