programming languages fundamentals

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Programming Languages Fundamentals

Cao Hoaøng Truï

Khoa Coâng Ngheä Thoâng TinÑaïi Hoïc Baùch Khoa TP. HCM

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Contact

• Trần Giang Sơn

• [email protected]

• http://www.cse.hcmut.edu.vn/~tgson

mailto:[email protected]

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References

• Ngôn ngữ lập trình – các mô hình và nguyên lý, Cao Hoàng Trụ, Khoa Khoa học máy tính, Đại học Bách Khoa, 1992

• Programming languages: design and implementation, second edition, Terrence W.Partt, 1990. Printice - Hall International Editions

• Fundamentals of programming languages, Ellis Horowits, 1983, Computer Science Press

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Contents

• Evolution and classification

• Formal syntax and semantics

• Compilation and interpretation

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Machine Language

CPU

I/O

Memory

0101001001101011

1101111001001001

0001101010101010

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Machine Language

Operation Code Operands

Instruction:

10110011010010010011010110110001

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Assembly Language

A := B + Cif A = 0 then body

MOV r0, B ; move B into register r0ADD r0, C ; addMOV A, r0 ; storeBNE L1 ; branch if result not equal 0body

L1:

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Language Levels

Natural Language

Machine Language

Low-Level

High-Level

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What Makes a Good Language?

• Clarity, simplicity, unity of language concepts

• Clarity of program syntax

• Naturalness for the application

• Support for abstraction

• Ease of program verification

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What Makes a Good Language?

• Programming environment

• Portability of programs

• Cost of use program execution program translation program creation, testing, use program maintenance

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Language Classification

• Imperative von Neumann Fortran, Pascal, Basic, C object-oriented Smalltalk, Eiffel, C++, Java

• Declarative functional Lisp, ML, Haskell dataflow Id, Val logic Prolog, VisiCalc

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Von Neumann Languages

• Most familiar and successful

• Imperative statements

• Modification of variables

Fortran, Pascal, Basic, C, …

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Object-Oriented Languages

• Imperative statements

• Message passing among objects

Smalltalk, Eiffel, C++, Java

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Functional Languages

• Recursive definition of functions (lambda calculus)

• Expressions of function composition

Lisp, ML, Haskell

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Logic Languages

• Logical facts and rules (predicate logic)

• Computation as theorem proving

Prolog, VisiCalc

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Dataflow Languages

• Computation as token flow among nodes

• Inherently parallel model

Id, Val

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Contents




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Formal Syntax and Semantics

• Computer languages must be precise

• Both their form (syntax) and meaning (semantics) must be specified without ambiguity

• Both programmers and computers can tell what a program is supposed to do

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Formal Syntax

• Abstract syntax

• Context-free grammars

• Backus-Naur formalism (BNF)

• Syntax diagrams

• Derivations and parse trees

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Abstract Syntax

• Syntactic class

• Syntactic form

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Example: Expressions

• Syntactic class:E expressionI identifierC constantO operator

• Syntactic form:E = I | C | E O E | (E)

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a * (2 + b)

E O E

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a - b - c

E O E

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Abstract Syntax

• Advantage: simple

• Disadvantages:No terminal symbols definedAmbiguous

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Context-Free Grammars

• Start symbol

• Non-terminals

• Terminals

• Productions A 1 | 2 | … | n

(Noam Chomsky, 1959)

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Example: Unsigned Integers

6 2 5 7 3

<digit> <unsigned_integer>

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Example: Unsigned Integers

• Start symbol <unsigned_integer> • Non-terminals <unsigned_integer>, <digit>

• Terminals 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

• Productions<unsigned_integer> <digit> |

<digit> <unsigned_integer>

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Backus-Naur Formalism

<unsigned_integer> ::= <digit> | <digit> <unsigned_integer>

(John Backus, 1960)

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<factor>

12 * 3 + 4

<term>

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• Start symbol <expression>

• Non-terminals <expression>, <term>, <factor>,

<unsigned_integer>, <term_op>,

<factor_op>

• Terminals 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, +, , *, /

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Syntax Diagrams

term

expression term_op term

expression

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Derivations

<expression> <expression> <term_op> <term> <term> + <factor> <term> <factor_op> <factor> + <unsigned_integer> <factor> <unsigned_integer> + 4 <unsigned_integer> 3 + 4 12 3 + 4

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Parse Trees

<expression>

<expression>

<term_op>

<term>

<term>

<unsigned_integer>

<factor>

<factor_op>

<factor>

<term>

<unsigned_integer>

<unsigned_integer>

<factor>

+

12

3 4

*

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Parse Trees

<expression>

<expression>

<term_op>

<term>

<term>

<unsigned_integer>

<factor>

<factor_op>

<factor>

<term>

<unsigned_integer>

<unsigned_integer>

<factor>

+

12

3 4

*

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Formal Semantics

• Operational semantics

• Denotational semantics

• Axiomatic semantics

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Operational Semantics

• A virtual computer to execute a program.

• A set of formally defined operations to specify how the internal state of the virtual computer may change.

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Denotational Semantics

• Each program construct is a function that maps an input to an output.

• A program is a composition of functions.

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Axiomatic Semantics

• The effect of a statement is defined via its precondition and postcondition.

• A set of axioms and rules to define the effect of program constructs.

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Axiomatic Semantics

{P} S {Q}

precondition

statement

postcondition

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Axiomatic Semantics

{PxE} x := E {P}

• Axiom:

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Axiomatic Semantics

{x 2} x := x + 1 {x 3}

E = x + 1P = x > 3PxE = x + 1 > 3 = x > 2

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Axiomatic Semantics

if ({P} S1 {Q}) ({Q} S2 {R})

then {P} S1 ; S2 {R}

• Rule:

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Contents




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Compilation and Interpretation

CompilerSource program

Target program

Target programInput Output

InterpreterSource program

Output

Input

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Compilation and Interpretation

• Interpreter: better flexibility and diagnostics

• Compiler: better performance

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Phases of CompilationScanner (lexical analysis)

Parser (syntactic analysis)

Semantic analysis

Machine-independentcode optimisation

Target code generation

Machine-specificcode optimization

Character stream

Token stream

Parse tree

Intermediate code

Optimised intermediate code

Target code

Optimised target code

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Phases of Compilation

Scanner (lexical analysis)


c := a + b 7

id1 := id2 + id3 7

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id1 := id2 + id3 7

:=

id1

id2

id3

+

7

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Semantic analysis

CNV (7, , t1)

(id3, t1, t2)

+ (id2, t2, t3)

ASS (t3, , id1)

:=

id1

id2

id3

+

7

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CNV (7, , t1)

(id3, t1, t2)

+ (id2, t2, t3)

ASS (t3, , id1)

Machine-independentcode optimisation

(id3, 7.0, t1)

+ (id2, t1, id1)

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MOV reg, id3

MUL reg, 7.0

ADD reg, id2

MOV id1, reg

(id3, 7.0, t1)

+ (id2, t1, id1)Target code generation

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Exercises

• Define a formal syntax for a simple language supporting only the assignment statement and arithmetic expressions.

• Write the derivation and draw the parse tree of ‘c := (a + b) 7’ using the defined syntax.

programming languages fundamentals

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