1. CSE3302 Programming Languages (notes continued)
Dr. Carter Tiernan
CSE 3302
Programming Languages

2. Generality and Hierarchy: Algol-60
Close as possible to standard mathematical notation and readable
Usable for describing computing processes
Mechanically translatable to machine language
CSE 3302
Programming Languages

3. Programming LanguagesB C D
Algol Design
Hierarchical structure / Nesting
(unlike C & C++)
Declarative and Imperative constructs
Variable, procedure, and switch
Computational and control-flow
Compilation handled differently than Fortran
Stack for run-time structures
CSE 3302
Programming Languages

4. Algol: Name Structures
Names are NOT statically bound
Block structure supports scoping
Blocks are compound statements with declarations
Scope - contour diagrams
Context
Inner blocks implicitly inherit access to variables in its immediately surrounding block A B C D
CSE 3302
Programming Languages

5. Programming Languages
Algol: Blocks
Allows shared data to be easily organized with the subprograms that need it (but does allow indiscriminate access)
Scoping choices
Static: called in the environment of its definition (used exclusively in Algol)
Dynamic: called in the environment of its caller
CSE 3302
Programming Languages

6. Programming Languages
Dynamic Scoping
Called in environment of caller
Can write general procedures that make use of variables and procedures supplied by caller’s environment
Vulnerable to being called from an environment without correct supporting procedures
Not generally used in current languages
CSE 3302
Programming Languages

7. Programming Languages
Static Scoping
Called in the environment of its definition
Requires the user to pass parameters of local environment
Cannot behave differently based on call location only
Ensures that the static structure of the program more closely agrees with the dynamic behavior of the program.
CSE 3302
Programming Languages

8. Algol: Stacks for managementB C D
Disjoint blocks allow conservation of memory on the stack
Assumption: a variable is retained only so long as the program is executing in the scope of that variable or in a block that will return to the scope of that variable
Blocks follow LIFO execution; ergo, a stack handles the activation records in a LIFO fashion D C A
CSE 3302
Programming Languages

9. Programming Languages
Responsible Design
Understand the programmers’ problem
Provide a high quality solution
Which is not necessarily the solution the programmers (or users) asked for
Responsible Design Principle
Caveat: If the user wants it and you don’t supply it (in their fashion), they may not accept your alternate (even if better) solution
CSE 3302
Programming Languages

10. Algol: Data Structures
Primitives are scalars
Boolean, integer, and real
No double precision because it’s machine dependent
No complex due to tradeoffs of usefulness vs. complexity
Limited string type that required procedures coded in some other language in order to be output or used
CSE 3302
Programming Languages

11. Zero-One-Infinity Principle
Supports regularity - easier to learn, remember, and master
Eliminates arbitrary numerical limits
Applies to arrays in
Number of dimensions
Start and end values for indices
CSE 3302
Programming Languages

12. Programming Languages
Algol: Arrays
Generalized
Dynamic due to stack allocation
Data Types
Strong typing
Coercions and conversions
Type system as “safety feature”
CSE 3302
Programming Languages

13. Algol: Control structures
Primitives
Assignment :=
No input/output - handled with library routines
Structures
Regularize the FORTRAN structures
if condition then statement1 else statement2
for initialization keyword test do statement
CSE 3302
Programming Languages

14. Algol: Control and nesting
All control structures can handle arbitrary number of bracketed statements
Compound statements formed with begin and end
Algol structuring led to Dijkstra’s letter, “GOTO Considered Harmful”
CSE 3302
Programming Languages

15. Programming Languages
Algol: More control
Hierarchical structure
Structured programming
Recursion supported
Instantiation of locals
Every call creates a new activation record
Conditional expression
CSE 3302
Programming Languages

16. Algol: Parameter passing
Passing by value
Local copy of parameter
Requires keyword value
Pass by name
Default mode
Based on substitution - Copy Rule
Not actually substituted for formal params
CSE 3302
Programming Languages

17. Programming Languages
Algol: Pass by name
Compare to pass by reference
Name parameters bound to string
Jensen’s device
Use pass by name with index variable to cycle through array elements
Implementation - thunk
Compile parameter
Pass address of compiled parameter code
Inconsistent behavior
CSE 3302
Programming Languages

18. Language Design Issues
GO TO controversy
Conceptual models
System image
Feature interaction
CSE 3302
Programming Languages

19. Programming Languages
Algol: Control (more)
Out of block GO TOs
Levels of activation must be popped
For-loop
For-list-elements reevaluated every pass
Step, until
While
Enumerated list
All of the above
Baroque
CSE 3302
Programming Languages

20. Programming Languages
Algol: Control etal.
Switch declaration
Array of statement labels
Processes cases - not necessarily distinct
Best use recombines after cases
Difficult to correlate with dynamic structure
Baroque
CSE 3302
Programming Languages

21. Algol: Syntactic Structures
Machine independent / portable
Free-format
Programmers had to think about how it should look
Reflect the structure of the program
Independent of character sets
Reference language
Publication language
Hardware representation
CSE 3302
Programming Languages

22. Programming Languages
Algol: Lexics
Lexical conventions
Reserved words
Keywords (Algol approach)
Keywords in context
Dangling else
Style
CSE 3302
Programming Languages

23. Backus-Naur Form (BNF)
Precise programming language descriptions
English difficult to read
Examples incomplete
Perceptual clarity with precision
Backus - syntactic categories
Naur added improvements to notation
Metalanguage
CSE 3302
Programming Languages

24. Programming Languages
BNF
Terminals
Nonterminals
Defined in terms of nonterminals and terminals
Recursive definition allowed
Related to Chomsky language hierarchy
Context-free grammar
Regular (non-recursive) grammar
CSE 3302
Programming Languages

25. Programming Languages
Design Criteria
Efficiency - scientific issues
Minimize resources
Maximize safety
Economy - social issues
Maximize social benefit
Minimize cost
Elegance - aesthetic issues
Function follows form
Manifest interactions
CSE 3302
Programming Languages

26. Second Generation Languages
Elaborations and generalizations of first generation languages
Stronger typing
Hierarchical structure
Blocks
Control flow
Recursion
Parameter passing modes
Free-format syntax
Machine independent conventions
Reserved word/ Keywords
2nd
CSE 3302
Programming Languages