Download PL Intro

INTRODUCTION
"When the only tool you have is a hammer,
everything looks like a nail"
(Abraham Maslow)
Course Themes/Topics
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Syntax & Feature decisions
Support for Reuse/Polymorphism
Error handling
Dynamic vs static, implicit vs explicit
Meta programming
Functional programming
Course Format
• Sage on stage vs Guide on side...
▫ Group discussions are not intended to return you to kindergarten.
Some questions are just at the factual level, but some require you
to go to stackoverflow etc. to compare, contrast, etc.
▫ Stackoverflow discussions can actually be richer than textbook
definitions, because they are written by professionals who have
spent a lot of time reflecting. Lifelong learning.
▫ Tests tend to focus less on code, more on language concepts.
Categories of Programming Languages
• https://en.wikipedia.org/wiki/List_of_program
ming_languages_by_type
With a partner:
• What categories include Java?
• What categories include Ruby?
• Read through all the categories. Which ones are
familiar? Which have you never heard of?
Language Evaluation Criteria
What criteria would you use to
evaluate/choose a language?
Brainstorm – don’t look ahead!
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?
Categories proposed by Sebesta
Writeable and Readable
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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
Java: no default params
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
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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?
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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.
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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.
Expressivity
Find something to share with the class – turn in for
attendance points
• http://babel.ls.fi.upm.es/~jjmoreno/expre.html
• http://redmonk.com/dberkholz/2013/03/25/programming-languagesranked-by-expressiveness/
• http://stackoverflow.com/questions/638881/what-does-expressive-meanwhen-referring-to-programming-languages
• http://en.wikipedia.org/wiki/Expressive_power
• http://mt4.radified.com/2009/08/expressive-power-computerprogramming-language-literature.html
• http://gafter.blogspot.com/2007/03/on-expressive-power-ofprogramming.html
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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)
Copyright © 2006 Addison-Wesley. All rights reserved.
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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
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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.
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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
• Current
▫ Functional programming, concurrency
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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, Haskell
• 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
More info for you to explore on your own
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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: Grammar:
Based on BNF
int
count
=
20
;
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