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CSE 3341 Principles of Programming Languages

CSE 3341 Principles of Programming Languages. Neelam Soundarajan Computer Sc. & Eng. Dreese Labs 579 e-mail: neelam@cse. Goals of the Course. Main Goal: Discuss key concepts underlying PLs Sub-Goals: Alternative programming paradigms Implementation issues

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CSE 3341 Principles of Programming Languages

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  1. CSE 3341Principles of Programming Languages Neelam Soundarajan Computer Sc. & Eng. Dreese Labs 579 e-mail: neelam@cse

  2. Goals of the Course • Main Goal: • Discuss key concepts underlying PLs • Sub-Goals: • Alternative programming paradigms • Implementation issues • At end of course: Given a feature, you should be able to: • Decide whether you like it or not, and why • Have an idea how to implement it • Decide what kinds of problems it is suited for. CSE 3341/655; Part 1

  3. How do we study a language? • Syntax: What do legal programs in Llook like? • Semantics:What do the various instructions of Ldo (when exec.)? • Programming Methodology: How are you supposed to use the features of L? For solving what kinds of problems? CSE 3341/655; Part 1

  4. PL (in L) PM Compiler M M C C L→M L→M PM Output Input For P : Compiler, in M, for translating from L to M Compilers CSE 3341/655; Part 1

  5. M L M M M L M M C C C C C C C C C C L’→M Eiffel→C L→M L’→M L→M L→M L’→L L’→L Eiffel→C L’→L PL PM PL’ PL C M to E.g.: From Compilers (contd.) CSE 3341/655; Part 1

  6. I M C C L→I I→M Compilers: Intermediate Langs. • Identify a language I • For each L, write • For each M, write New machines are easy to handle; New languages are easy to handle; Common intermediate language: C CSE 3341/655; Part 1

  7. Output From P PL M M I I L L Data For P M : Interpreter for L (written in M) I PL’ L Output From P Data For P L I L’ Interpreters CSE 3341/655; Part 1

  8. Compilers & Interpreters • What is an assembler?A simulator?The JVM? • JIT: “Just-in-time” compilation • Running theme for the course:Runtime versus compile-time • "Interpreter interprets each line into binary code which can be run on different platforms": Not true! CSE 3341/655; Part 1

  9. Syntax • BNF (“Backus Normal Form”): Notation for describing syntax of languages precisely. • Example: Set of all non-negative integers: <no> ::= <digit> | <digit> <no> <digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 |7 | 8 | 9 • Set of all non-neg. nos. not starting with 0:<nlzno> ::= <nlzdigit> | <nlzdigit> <nlzno><nlzdigit>::= 1 | 2 | 3 | 4 | 5 | 6 |7 | 8 | 9 ?? • <, >, ::=, | are reserved (“meta”) symbols;<digit>, <no> : Non-terminals; 0, 1, 2, ...: Terminals • To define a BNF grammar: Specify terminal and non-termnial symbols; Define the production for each non-terminal. CSE 3341/655; Part 1

  10. <no> <digit> 6 <no> <digit> 5 5 Derivation trees/Parse trees How do you derive “655” from this grammar? <no> (There is a bug in this tree!) The string derived (or parsed) by the tree: Append together the labels at the leaves in left-to- right order. CSE 3341/655; Part 1

  11. <exp> <exp> <exp> + <id> <id> Y X Example: Grammar of expressions <exp> ::= <no> | <id> | <exp> + <exp> | <exp> * <exp> <id> ::= X | Y | Z Parse tree for X + Y : CSE 3341/655; Part 1

  12. <exp> <exp> <exp> <exp> + <id> <id> <id> * X Y Z Grammar of expressions (contd.) Parse tree for X + Y * Z: <exp> CSE 3341/655; Part 1

  13. <exp> <exp> <exp> <exp> <exp> * <id> <id> <id> + Z Y X Grammar of expressions (contd.) Another tree for X + Y * Z: Which is the right tree? The grammar is ambiguous. CSE 3341/655; Part 1

  14. Another grammar for expressions • <exp> ::= <fac> | <fac> + <exp><fac> ::= <no> | <id> | <no> * <fac> | <id> * <fac> • This grammar is not ambiguous • Reintroduce ambiguity among +’s and *’s (but not between + and *) • Parenthesized expressions? CSE 3341/655; Part 1

  15. Grammars (contd.) • Exercise: Make precedence left to right or right to left • Ambiguous grammar for numbers: <no> ::= <digit> | <no> <no> • Even the following is not a good grammar: <no> ::= <digit> | <digit> <no>Problem: Wrong semantics.Q: Can you fix it? CSE 3341/655; Part 1

  16. Grammars (contd.) • Syntax graphs: Pictorial representation of BNF grammars. • Extended BNF (see Ch. 2.1.2 of book): [ ... ]: optional item { ... }: repetition (0 or more times) * : repetition (0 or more times) + : repetition (1 or more times) • <if> ::= if <cond> then <stmt> {<else-if>} [else <stmts>] end-if;<else-if> ::= elseIf <cond> then <stmts> CSE 3341/655; Part 1

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