programming languages seminar, spring 20041 grammar adaptation (ralf lämmel, cwi) presentation and...

Programming Languages Seminar, Spring 2004

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Grammar Adaptation (Ralf Lämmel, CWI)

Presentation and slides by:Faizan Javed

March 9th, 2004


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1. Introduction

Grammar Adaptation:1. Model Restructuring (fold unfold) and

Local Changes (removal of phrases, restriction)

2. Performed by grammar programmers manually, otherwise.

3. Transformations relevant for grammar development, maintenance, re-engineering and recovery.


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1. Introduction (contd.)

Grammar Recovery1. Concerned with derivation of a

languages’ grammar.2. Grammar transformations can be used

to facilitate grammar recovery.3. Important for software re-engineering

- Y2K problem, Euro-conversion problem


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1. Introduction (contd..)

Grammar recovery is hard:1. ancient languages (ex: Cobol dialects)2. in-house languages and language

extensions COBOL example [Lämmel, Verhoef]:

1. Grammar transformations made the process:- accessible, traceable and measurable.


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1. Introduction (contd…) Sample Adaptation (VS Cobol II):1. Problems faced were errors, omissions

and use of informal comments.

2. Reason: lack of use of formal methods

3. Example: VS Cobol II REDEFINES clause


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1. Introduction (contd….) REDEFINES clause is

actually of type: “REDEFINES” data-name

Sample defines a structure of a data item with a REDEFINES clause.

Use “delete” transformation operator:

delete level-number (data-name | “FILLER")? in REDEFINES-clause


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1. Introduction (contd..)

Transformational approach benefits:

1. Adds Traceability: changes can be recorded

2. Reusable adaptation scripts: useful for dialects of the same grammar

3. Relaxed notions for non-semantics preserving operators


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2. Grammar Fragments Grammar fragments are a variant of

context-free grammars used for grammar transformations. In grammar adaptation, grammars “evolve”.

“Evolving” grammars rather than “reduced” grammars: reduced grammars assume that each production can be used in some derivation of a terminal string from the start symbol.


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2.1 Standard Context-Free grammars

Standard CFG definition: <N, T, s, P>

1. N and T are disjoint finite sets of nonterminals and terminals respectively.

2. s is the start symbol,3. P is a finite set of productions or

(context-free) rules with4. A production <n, u> P with n 2 N and u 2 (N U T)* is also written as n → u.


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2.2 Deviation Start symbol not required for incomplete

grammars No explicit declaration of non-terminals and

terminals – grammar represented just by productions

Non-terminals from and terminals from Bottom non-terminals: non-terminals which are not

terminated This deviation known as grammar fragments. Domain defined as:


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2.2 Deviation (contd..) Relevant set of grammar symbols:


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2.2 Deviation (contd..) Bottom non-terminals ( ):1. Non-terminal lacking a definition, OR2. Indicates a connectivity problem; defined

with a different left-hand side.

Top non-terminals ( ):1. Non-terminals defined, but not used.2. Start symbols usually meet this criteria.


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2.3 Semantics Language generated by a common CFG

defined as: Terminal strings can also be generated

from an arbitrary non-terminal n; the semantics of n w.r.t. a grammar fragment are:


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2.3 Semantics (contd..) Terminal strings not sufficient since

grammar fragments are not necessarily terminated (ex: productions with bottom non-terminals)

Incomplete grammar :


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2.3 Semantics (contd..) Sentential forms provide an upper

bound on the denotation.

However, sentential forms don’t provide a basis to state the semantics preservation of fold/unfold modulations.


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2.3 Semantics (contd..) Observable non-terminals: restrict

sentential forms so that only particular non-terminals are observable.

means semantics restricted to sentential forms consisting solely of bottom non-terminals and terminals.


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2.3 Semantics (contd..) Looping non-terminals: non-terminal “b” lacks

a base case

“Ultimate” denotation of n w.r.t :


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3. Formal Reasoning

Need to compare grammars, and characterize the properties of grammar transformations.

Certain relations on the grammar fragments are defined.


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3.1 Equivalent grammars Equivalent grammars:

Useful in fold/unfold manipulations


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3.1 Equivalent Grammars Equivalent grammars example:

Using equivalence properties,


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3.2 Beyond Equivalence Equivalence often too restrictive to

characterize related grammars! Some “relaxations” on the relations

between two grammars:


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3.2 Beyond Equivalence (contd.)

Equivalence modulo renaming:

In above fig, it holds that


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Sub-grammar relation:

In above fig., it holds that but not vice versa.


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Enrichment relation:

In above fig, it holds that


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Instance relation:

In above fig., it holds that


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3.3 Grammar Transformers Partial grammar transformers: Grammar relations

can be used to define various preservation properties for grammar transformations.


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4. Transformation Framework

Define a framework for grammar transformations offering transformation primitives and combinators.

Discuss supplementary concepts like focus, constraints and symbolic operands.


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4.1 Primitives id: identity function fail: undefined grammar transformation reset: returning empty set of rules add/subtract: add/subtract a rule from a

grammar replace: replace a phrase by a phrase in a

grammar substitute: substitute non-terminal by a non-

terminal


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4.1 Primitives (contd.)


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4.2 Combinators


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4.3 Constraints


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4.4 Symbolic operands Introduces higher level of abstraction? Examples:1. definition of n : Denotes RHS of n.

Useful in unfolding operation; no need to point out definition explicitly.

2. all: focus operand. Denotes all non-terminals defined in a grammar. Useful if an operator expecting a focus parameter should be applied globally.


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5.1 Operator Suite - Overview Stepwise adaptation: application of a

sequence of transformation operators from the operator suite, T1;..;Tm.

Transformational grammar programmer: uses only operators of the suite, not the combinators or the primitives.

Three groups of operators: 1. Refactoring 2. Construction3. Destruction


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5.2 Refactoring Restructure grammar so that:1. Comprehensibility is improved2. Subsequent adaptation steps are easier to

perform

Semantics-preserving in the narrow sense.

Use of pre- and post- conditions in operators.


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5.2 Refactoring (contd.)


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5.3 Construction Facilitate grammar substitution,

extension and completion.


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5.3 Construction (contd.) Unify example:1. Useful if a bottom non-terminal should be

resolved interms of an existing definition2. Or, if two bottom non-terminals intentionally

coincide.


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5.3 Construction (contd.) Useful in grammar completion and

connection:1. Missing rules added by include.2. Too restrictive phrases generalised by

generalise.3. Missing definition of non-terminals

established using resolve.4. Non-terminals unified with unify.


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5.4 Destruction Essentially inverse of construction

(except delete operator)


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5.4 Destruction (contd.) Separate

example:


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5.4 Destruction (contd.)

Useful for correction or revision:1. Too general phrases can be

restricted.2. Superfluous rules or definitions can be

excluded or rejected, respectively.3. Accidentally unified phrases can be

separated by introducing new non-terminals in certain occurrences.


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5.5 Discussion


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5.5 Discussion (contd.) Some semantics preservation

examples:1. preserve: strictly preserving –

equivalence of phrases implies equivalent grammars.

2. unfold: strictly preserving – unfold defined interms of preserve.

3. introduce: introducing – adds a rule for a new non-terminal


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6.0 Conclusion Towards proper grammar re-engineering:

-viewed as coding work-not a research focus anymore…more research needed!-paper contributes by defining the foundations of an adaptive style of grammar development.

Semantics preservation and relaxation:-refactoring operators are semantics preserving only.-paper introduces a set of weaker preservation notions, suitable to characterise revisions and extensions.

Perspective:- more global notions than just preservation – can the grammar be improved with the transformation sequence?-Open research problem: how clients of a grammar such as compiler compiler inputs, rewrite rules have to be adapted if the grammar serving as contract changes.

programming languages seminar, spring 20041 grammar adaptation (ralf lämmel, cwi) presentation and...

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