Programming Languages Fundamentals

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Programming Languages Fundamentals

• Cao Hoaøng Truï
• Khoa Coâng Ngheä Thoâng Tin
• Ñaïi Hoïc Baùch Khoa TP. HCM

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Contents

• Evolution and classification
• Formal syntax and semantics
• Compilation and interpretation

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Machine Language

CPU
Memory
0101001001101011
1101111001001001
I/O
0001101010101010
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Machine Language

Instruction
Operation Code
Operands
10110011010010010011010110110001
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Assembly Language

A B C if A 0 then body MOV r0, B move
B into register r0 ADD r0, C add MOV A, r0
store BNE L1 branch if result not equal
0 body L1
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Language Levels

Natural Language
High-Level
Low-Level
Machine Language
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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

• Evolution and classification
• Formal syntax and semantics
• Compilation and interpretation

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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 expression
• I identifier
• C constant
• O operator
• Syntactic form
• E I C E O E (E)

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Example Expressions

• a (2 b)
• E O E

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Example Expressions

• a - b - c
• E O E

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Abstract Syntax

• Advantage simple
• Disadvantages
• No terminal symbols defined
• Ambiguous

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Context-Free Grammars

• Start symbol
• Non-terminals
• Terminals
• Productions A ? a1 a2 an
• (Noam Chomsky, 1959)

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Example Unsigned Integers

• 6 2 5 7 3
• ltdigitgt ltunsigned_integergt

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Example Unsigned Integers

• Start symbol ltunsigned_integergt
• Non-terminals ltunsigned_integergt, ltdigitgt
• Terminals 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• Productions
• ltunsigned_integergt ? ltdigitgt ltdigitgt
  ltunsigned_integergt

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Backus-Naur Formalism

• ltunsigned_integergt ltdigitgt ltdigitgt
  ltunsigned_integergt
• (John Backus, 1960)

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Example Expressions

• ltfactorgt
• 12 3 4
• lttermgt

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Example Expressions

• Start symbol ltexpressiongt
• Non-terminals ltexpressiongt, lttermgt, ltfactorgt,
• ltunsigned_integergt, ltterm_opgt,
• ltfactor_opgt
• Terminals 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, , -, ,
  /

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Example Expressions

• Productions
• ltexpressiongt ? lttermgt ltexpressiongt
  ltterm_opgt lttermgt
• lttermgt ? ltfactorgt lttermgt ltfactor_opgt ltfactorgt
• ltfactorgt ? ltunsigned_integergt (ltexpressiongt)
• ltterm_opgt ? -
• ltfactor_opgt ? /

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Syntax Diagrams
expression
term
expression
term_op
term
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Derivations

• ltexpressiongt
• ? ltexpressiongt ltterm_opgt lttermgt
• ? lttermgt ltfactorgt
• ? lttermgt ltfactor_opgt ltfactorgt
  ltunsigned_integergt
• ? ltfactorgt ltunsigned_integergt 4
• ? ltunsigned_integergt 3 4
• ? 12 3 4

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Parse Trees
ltexpressiongt
ltexpressiongt
ltterm_opgt
lttermgt

lttermgt
ltfactorgt
lttermgt
ltfactorgt
ltfactor_opgt
ltfactorgt

ltunsigned_integergt
ltunsigned_integergt
ltunsigned_integergt
3
4
12
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Parse Trees
ltexpressiongt
ltexpressiongt
ltterm_opgt
lttermgt

lttermgt
ltfactorgt
lttermgt
ltfactorgt
ltfactor_opgt
ltfactorgt

ltunsigned_integergt
ltunsigned_integergt
ltunsigned_integergt
3
4
12
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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

• Axiom

Px?E x E P
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Axiomatic Semantics
x ? 2 x x 1 x ? 3
E x 1 P x gt 3 Px?E x 1 gt 3 x gt 2
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Axiomatic Semantics

• Rule

if (P S1 Q) ? (Q S2 R) then P S1
S2 R
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Contents

• Evolution and classification
• Formal syntax and semantics
• Compilation and interpretation

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Compilation and Interpretation
Compiler
Source program
Target program
Target program
Input
Output
Interpreter
Source program
Output
Input
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Compilation and Interpretation

• Interpreter better flexibility and diagnostics
• Compiler better performance

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Phases of Compilation
Character stream
Scanner (lexical analysis)
Token stream
Parser (syntactic analysis)
Parse tree
Semantic analysis
Intermediate code
Machine-independent code optimisation
Optimised intermediate code
Target code generation
Target code
Machine-specific code optimization
Optimised target code
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Phases of Compilation
c a b 7
Scanner (lexical analysis)
id1 id2 id3 7
Parser (syntactic analysis)
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Phases of Compilation
id1 id2 id3 7
Parser (syntactic analysis)
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Phases of Compilation
Semantic analysis
CNV (7, , t1) (id3, t1, t2) (id2, t2, t3) ASS
(t3, , id1)
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Phases of Compilation
CNV (7, , t1) (id3, t1, t2) (id2, t2, t3) ASS
(t3, , id1)
Machine-independent code optimisation
(id3, 7.0, t1) (id2, t1, id1)
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Phases of Compilation
(id3, 7.0, t1) (id2, t1, id1)
Target code generation
MOV reg, id3 MUL reg, 7.0 ADD reg, id2 MOV id1,
reg
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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.