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? 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???

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 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…

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 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 Copyright © 2006 Addison-Wesley. All rights reserved.

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.

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) Copyright © 2006 Addison-Wesley. All rights reserved.

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

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 (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. Copyright © 2006 Addison-Wesley. All rights reserved.

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

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?

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

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

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.

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)

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

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)

Hybrid Implementation Process Copyright © 2006 Addison-Wesley. All rights reserved.

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.

Orthogonality Another 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 Copyright © 2006 Addison-Wesley. All rights reserved.