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September 13 th

September 13 th. Grammars. Grammars. They are a way of describing the syntax of programming languages We will be using BNF grammar (as in Mini-Language Core) Syntax diagrams (in Pascal) BNF grammar Was invented in 1958 as a way of describing Algol 58 by John Backus. BNF grammars .

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September 13 th

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  1. September 13th Grammars

  2. Grammars • They are a way of describing the syntax of programming languages • We will be using • BNF grammar (as in Mini-Language Core) • Syntax diagrams (in Pascal) • BNF grammar • Was invented in 1958 as a way of describing Algol 58 by John Backus

  3. BNF grammars • BNF grammar is context-free grammar • Noam Chomsky – classified grammars • Context-free grammar • Regular grammar • Context-sensitiver grammar • …

  4. Characteristics of grammar • A grammar consists of • A four-tuple • A set of terminals • A set of non-terminals • A set of rules (or productions) • A start (or goal) symbol

  5. Uses of grammars • It can be used to generate legal sentences in a programming language • It can be used to determine whether a given sentence in a programming language is correct • parsing

  6. What did we learn from Mini-Language Core’s BNF • It described the language well enough for most of us to write a program in an unfamiliar language • We could determine whether a given statement we wrote in Mini-Language Core was correct • It really only told us about syntax, not about semantics • There were a number of things that we could not determine from the grammar

  7. Terminology • Lexemes – lowest level syntactic units • Token – is a category of lexemes • index = 2 * count + 17; • Index, count are identifiers • =, *, + are operators • 2, 17 are integer literals • ; is a separator or terminator • Metalanguage – is a language used to describe other languages • Aside: identifiers can reference variables, constants, methods, programs, etc.

  8. Potential Problems • A grammar may be ambiguous • Two different parse trees may be generated for the same expression.

  9. Sections 3.4 and later • Less important in this class – read to be able to answer homework questions but don’t agonize over the material in axiomatic and denotational semantics.

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