1. INTRODUCTION
"When the only tool you have is a hammer, everything looks like a nail" (Abraham Maslow)

2. Administrivia This is me: Cyndi Rader
You can reach me:
Or find me here: BB 280D
Class notes here:
Office Hours: 8-9:15 T/Th
and by appointment

3. Why study programming languages?
LUA MEL Others??
To enhance your ability to learn new languages
To allow you to choose an appropriate tool for a given task
To gain an appreciation for the challenges involved in implementing a language
Let’s see, should I use a scripting language, do I need speed & reliability, …
What does that compiler message mean?
Why did they design the language
that way???

4. Why study programming languages?
To expand your ability to express your ideas (effectively) using a given language
To see some very different styles of programming (e.g., Ruby, Haskell)
To avoid being a language zealot
Because it's fun!

5. Course Format
Sage on stage vs Guide on side...

6. Course Evaluation 45% homework, projects 5% class participation
50% exams and quizzes

7. Programming Domains Pick your tool – often depends on your domain…
Scientific applications - Fortran
Large number of floating point computations
Efficiency (compete with assembly)
Business applications - COBOL
Produce reports, use decimal numbers and characters
Artificial intelligence - LISP
Symbols rather than numbers manipulated
Linked structures rather than arrays
Systems programming - C
Need efficiency because of continuous use
Need low-level features to interface with hardware
Web Software
Eclectic collection of languages: markup (e.g., XHTML), scripting (e.g., PHP), general-purpose (e.g., Java)
Pick your tool – often depends on your domain…

8. Language Evaluation Criteria
What criteria would you use to evaluate/choose a language?

9. Language Evaluation Criteria
Writability: how easy is it to write a program?
Readability: how easy is it to read a program?
Reliability: does it include features that help to produce more reliable software?
Cost: what’s the ultimate cost?

10. Writeable and Readable
Simplicity
Support for abstraction
Control statements
Data types
Syntax
Orthogonality
Expressivity
What aspects of a language might make it easier to read than write (or vice versa)?

11. Orthogonality
A relatively small set of primitive constructs can be combined in a relatively small number of ways
Every possible combination is legal
Changing one thing has no effect on another
As stated by Michael Scott:
Orthogonality means that features can be used in any combination, the combinations all make sense, and the meaning of a given feature is consistent regardless of other features with which it is combined.
261 example: arrays

12. Expressivity
Find something to share with the class – turn in for attendance points
programming.html
ranked-by-expressiveness/
when-referring-to-programming-languages
programming-language-literature.html

13. Evaluation Criteria: Reliability
Type checking
Testing for type errors
Exception handling
Intercept run-time errors and take corrective measures
Aliasing
Presence of two or more distinct referencing methods for the same memory location
Readability and writability
A language that does not support “natural” ways of expressing an algorithm will necessarily use “unnatural” approaches, and hence reduced reliability
What do you think of this (from
“I know that some readers will be looking at this code and thinking how unsafe it is. But what the hell – it fails fast, and in all the years I’ve had map in my toolkit I’ve never once messed it up.”

14. Evaluation Criteria: Cost
Training programmers to use language
Writing programs (closeness to particular applications)
Compiling programs
Executing programs
Language implementation system: availability of free compilers (Ada vs Java)
Reliability: poor reliability leads to high costs
Maintaining programs
How does this relate to programming languages?

15. Evaluation Criteria: Others
Portability
The ease with which programs can be moved from one implementation to another
Generality
The applicability to a wide range of applications
Well-definedness
The completeness and precision of the language’s official definition
Copyright © 2006 Addison-Wesley. All rights reserved.

16. Language Design Trade-Offs
Reliability vs. cost of execution
Conflicting criteria
Example: Java demands all references to array elements be checked for proper indexing but that leads to increased execution costs
Readability vs. writability
Another conflicting criteria
Example: APL provides many powerful operators (and a large number of new symbols), allowing complex computations to be written in a compact program but at the cost of poor readability
Writability (flexibility) vs. reliability
Example: C++ pointers are powerful and very flexible but not reliably used. Not included in Java.

17. Evaluation Criteria: the Players
Language implementors: concerned with difficulty of implementing constructs and features
Language users: worried about writability first, readability later
Language designers: likely to emphasize elegance and ability to attract widespread use
(on your own:
Copyright © 2006 Addison-Wesley. All rights reserved.

18. Influences on Language Design
Computer Architecture
Programming Methodologies
Copyright © 2006 Addison-Wesley. All rights reserved.

19. Computer Architecture Influence
Well-known computer architecture: Von Neumann
Imperative languages, most dominant, because of von Neumann computers
Data and programs stored in memory
Memory is separate from CPU
Instructions and data are piped from memory to CPU
Basis for imperative languages
Variables model memory cells
Assignment statements model piping
Iteration is efficient
John Von Neuman ( ). Mathematician, influential in set theory, quantum mechanics, game theory, self-replicating cellular automata, pseudo-random numbers and more. On faculty of Princeton Institute for Advanced Studies with Einstein and Godel. VonNeumann architecture based on ENIAC.
Copyright © 2006 Addison-Wesley. All rights reserved.

20. Programming Methodologies Influences
1950s and early 1960s: Simple applications; worry about machine efficiency
Late 1960s: People efficiency became important; readability, better control structures
structured programming
top-down design and step-wise refinement
Late 1970s: Process-oriented to data-oriented
data abstraction
Middle 1980s: Object-oriented programming
Data abstraction + inheritance + polymorphism

21. Language Categories Imperative/Procedural Functional Logic
Central features are variables, assignment statements, and iteration
Examples: C, Pascal, scripting languages such as Perl
Functional
Main means of making computations is by applying functions to given parameters
Examples: LISP, Scheme
Logic
Rule-based (rules are specified in no particular order)
Example: Prolog, SQL
Object-oriented
Data abstraction, inheritance, late binding
Examples: Java, C++, C#
Markup
New; not a programming language per se, but used to specify the layout of information in Web documents
Examples: XHTML, XML
What about CSS?

22. More Language Categories
Stack Based
Stack is central feature
Examples: PostScript, Forth
Prototype
Use the object, not the class, as the basis for object definition and inheritance
Examples: Io, Lua, Self, JavaScript

23. Implementation Methods
Compilation
Pure Interpretation
Hybrid Implementation Systems

24. Think like a compiler
Scanner (lexical analyzer): identifies the tokens of a program statement
Parser (syntax analyzer): determines whether the statement is valid, based on the language definition/grammar
int count = 20;
Tokens:
int
count = 20 ;
Grammar:
Based on BNF

25. The Compilation Process
Compilation process has several phases:
lexical analysis: converts characters in the source program into lexical units
syntax analysis: transforms lexical units into parse trees which represent the syntactic structure of program
Semantics analysis: generate intermediate code
code generation: machine code is generated
Slow translation, fast execution
Copyright © 2006 Addison-Wesley. All rights reserved.

26. Additional Compilation Terminologies
Load module (executable image): the user and system code together
Linking and loading: the process of collecting system program and linking them to user program

27. Pure Interpretation No translation
Easier implementation of programs (run-time errors can easily and immediately displayed)
Slower execution (10 to 100 times slower than compiled programs)
Often requires more space
Becoming rare on high-level languages
Significant comeback with some Web scripting languages (e.g., JavaScript)

28. Pure Interpretation Process
Copyright © 2006 Addison-Wesley. All rights reserved.

29. Hybrid Implementation Systems
A compromise between compilers and pure interpreters
A high-level language program is translated to an intermediate language that allows easy interpretation
Faster than pure interpretation
Examples
Perl programs are partially compiled to detect errors before interpretation
Initial implementations of Java were hybrid; the intermediate form, byte code, provides portability to any machine that has a byte code interpreter and a run-time system (together, these are called Java Virtual Machine)

30. Hybrid Implementation Process
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31. Just-in-Time Implementation Systems
Initially translate programs to an intermediate language
Then compile intermediate language into machine code
Machine code version is kept for subsequent calls
JIT systems are widely used for Java programs
.NET languages are implemented with a JIT system
Copyright © 2006 Addison-Wesley. All rights reserved.

32. Orthogonality Another Topic Exploration

33. Summary
The study of programming languages is valuable for a number of reasons:
Increase our capacity to use different constructs
Enable us to choose languages more intelligently
Makes learning new languages easier
Most important criteria for evaluating programming languages include:
Readability, writability, reliability, cost
Major influences on language design have been machine architecture and software development methodologies
The major methods of implementing programming languages are: compilation, pure interpretation, and hybrid implementation
Copyright © 2006 Addison-Wesley. All rights reserved.