INTRODUCTION "When the only tool you have is a hammer, everything looks like a nail"...

INTRODUCTION

"When the only tool you have is a hammer, everything looks like a nail"

(Abraham Maslow)

Administrivia

•This is me: Cyndi Rader•You can reach me: crader@mines.edu•Or find me here: BB 280D•Class notes here:

http://eecs.mines.edu/Courses/csci400/•Office Hours: 8-9:15 T/Th

and by appointment

Why study programming languages? 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

LUA MEL Others??

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???

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!

Course Format

•Sage on stage vs Guide on side...

Course Evaluation

•45% homework, projects

•5% class participation

•50% exams and quizzes

Programming Domains• 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…

Language Evaluation Criteria

What criteria would you use to evaluate/choose a language?

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?

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)?

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

Expressivity

Find something to share with the class – turn in for attendance points• http://gafter.blogspot.com/2007/03/on-expressive-power-of-

programming.html• http://redmonk.com/dberkholz/2013/03/25/programming-

languages-ranked-by-expressiveness/• http://babel.ls.fi.upm.es/~jjmoreno/expre.html• http://stackoverflow.com/questions/638881/what-does-expressive-

mean-when-referring-to-programming-languages• http://en.wikipedia.org/wiki/Expressive_power• http://mt4.radified.com/2009/08/expressive-power-computer-

programming-language-literature.html

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 http://expressivejava.com/): “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.”

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

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How does this relate to programming languages?

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

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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▫ Another conflicting criteria▫ Example: C++ pointers are powerful and very flexible

but not reliably used. Not included in Java.

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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:

http://www.paulgraham.com/popular.html)

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Influences on Language Design•Computer Architecture•Programming Methodologies

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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

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John Von Neuman (1903-1957). 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.

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

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Language Categories• Imperative/Procedural

▫ 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

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What about CSS?

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

Implementation Methods

•Compilation•Pure Interpretation•Hybrid Implementation Systems

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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:intcount=20;

Grammar:Based on BNF

The Compilation Process

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reserved.

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

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

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)

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Pure Interpretation Process

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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)

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Hybrid Implementation Process

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reserved.

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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

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OrthogonalityAnother Topic Exploration

▫ http://stackoverflow.com/questions/1527393/what-is-orthogonality

▫ http://stackoverflow.com/questions/3272019/is-java-orthogonal▫ http://www.artima.com/intv/rubyP.html

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

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