CSE 3341Principles of Programming

Languages

Neelam Soundarajan Computer Sc. & Eng.

Dreese Labs 579e-mail: neelam@cse

CSE 3341/655; Part 1 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.

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How do we study a language?

Syntax: What do legal programs in L look like?

Semantics:What do the various instructions of L do (when exec.)?

Programming Methodology:How are you supposed to use the features of L?For solving what kinds of problems?

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Compilers

Compiler

M

L→MC

PL

(in L)PM

InputFor P

PM Output

M

L→MC : Compiler, in M, for translating from L to M

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Compilers (contd.)M

L→MC

M

L’→MCL

L’→MC

M

L→MC

M

L’→LCL

L’→LC

M

L’→LC

PL’ PL

M

L→MC

PL PM

C

Eiffel→CC

M

Eiffel→CCE.g.: From to

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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

Compilers: Intermediate Langs.

I

L→IC

M

I→MC

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Interpreters

M

LI

L

L’IPL’

Data

For P

Output

From P

PL M

LI

Data

For P

Output

From P

LM

I : Interpreter for L (written in M)

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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!

Compilers & Interpreters

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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.

Syntax

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Derivation trees/Parse trees

How do you derive “655” from this grammar?

<no>

<no><digit>

<digit> <no>6

5 5

(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.

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Example: Grammar of expressions

<exp>::= <no> | <id> | <exp> + <exp> | <exp> * <exp>

<id> ::= X | Y | Z

Parse tree for X + Y :

<exp>

<exp><exp>

<id><id>

X Y

+

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Grammar of expressions (contd.)

Parse tree for X + Y * Z:

<exp>

<id>

X

<exp><exp> +

* <exp><exp>

<id>

Y

<id>

Z

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Grammar of expressions (contd.)

Another tree for X + Y * Z:

<exp>

<id>

Z

<exp><exp> *

<exp><exp> +

<id>

X

<id>

Y

Which is the right tree?The grammar is ambiguous.

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<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?

Another grammar for expressions

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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?

Grammars (contd.)

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**Replace this page** with a discussion of regular expressions

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>

Grammars (contd.)