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\title{Programming Languages 1\\
Course Specification}
\author{Gary T. Leavens \\
Department of Computer Science, Iowa State University \\
Ames, IA, 50011-1040 USA \\
leavens@cs.iastate.edu
}
\maketitle

\begin{abstract}
Computer Science 541
studies modern programming languages, with an emphasis on language design
and semantics
(especially for object-oriented languages).
This document specifies the course's general and specific objectives.
\end{abstract}

\section{Introduction}

The study of programming languages is primarily concerned with the
following questions:
\begin{itemize}
\item
What are good ways to program?
That is, what are expressive ways of specifying computational processes?

\item
How can a (good) strategy for programming be effectively expressed?

\item
What are the costs of various ways of programming and expressing programs?
And how can those costs be lowered?
\end{itemize}
Com S 541 addresses all these questions to some extent.
In addressing the third question, however, we focus on the costs of programming
in terms of human time and effort,
not on machine efficiency (time and space costs) or on issues of compilation.

\subsection{Course Description}

The catalog description of the course is as follows:
\begin{quotation}
Survey of the goals and problems of language design.
Formal and informal studies of a wide array of programming
language features including type systems, naming, state,
and control.  Creative use of functional, object-oriented,
declarative, concurrent and other programming paradigms.
(3 credits).
\end{quotation}

Com S 541 is distinguished from
Com S 342 (Principles of Programming Languages)
is that Com S 342 concentrates on essential semantic concepts,
studied with the use of interpreters (coded in a functional style).
Com S 342 avoids mathematical formalisms, while in Com S 541,
we will not shy away from them.
In this version of Com S 541, we concentrate on mathematical semantics.
In Com S 541 we aim to study modern functional and object-oriented languages,
and assume that the graduate students are capable of dealing with
the realistic versions of such languages.
In Com S 541, we use mathematical tools to draw design lessons from
our study of semantics, as opposed to simply understanding the features of
modern languages.

Com S 541 is distinguished from
Com S 641 (Semantics of Programming Languages)
in that Com S 641 discusses particular formal semantic description techniques
in depth, whereas a broader and less mathematically deep use
of semantic description techniques is made in Com S 541;
furthermore, an attempt is made in Com S 541
to show how to use the concepts behind these techniques,
not so much their details, in language design.

\subsection{Acknowledgements}

The course described here was originally developed with the help
of Kelvin Nilsen.
Khrishna Kishore Dhara has been helpful in designing several
offerings of this course.
The 1997 and 1998 offerings were designed with the help of the students
in the course.

Final exams for similar courses at other universities were provided by
Uday Redy (University of Illinois), John Mitchell (Stanford),
Dan Friedman and J. Michael Ashley (Indiana),
and Dave Gifford and Franklyn Turbak (MIT);
these helped provide perspective on what is important for such a course.
My early ideas for this course were formed as a teaching assistant
in similar graduate courses led by Dave Gifford and John Guttag of MIT.
Other ideas have been shaped by Barbara Liskov, Lawrence Flon,
Per Brinch-Hansen, and students in previous editions of Com S 541.
Thanks to Dale Miller and Bamshad Mobasher for material on $\lambda$Prolog.
Thanks to Dave Schmidt for help in answering questions about his books.

\section{Prerequisites}

The formal prerequisite in the Iowa State catalog
is successful completion of either Com S 342
or Com S 440 (Principles of Compiling);
that is, successful completion of an undergraduate course
in programming languages or compiler construction.

The skills taught in Com S 440 relevant to Com S 541 include the ability to:
\begin{itemize}
\item
read and write context free grammars,

\item
explain and implement the data structures used in a compiler,
and their purpose,

\item
estimate the run-time costs of various language features,
such as parameter passing mechanisms,

\item
modularize, document, and manage a large and evolving piece of software.
\end{itemize}


The topics taught in Com S 342
are perhaps more directly relevant to Com S 541 than the material
in Com S 440, but at many schools some of these topics are covered in a
course on compiler construction.
The skills of Com S 342 relevant to Com S 541 include the ability to:
\begin{itemize}
\item
Modify interpreters to change or enhance their behavior so as to
   implement various features of programming languages such as: control
   flow, variables, recursion, scoping, syntactic sugars,
   arrays, parameter passing mechanisms, objects, and inheritance.

\item
Write programs using such features,
   and explain (using appropriate terminology) and answer questions about
   the user-visible behavior of such programs.
 
\item
Explain (using appropriate terminology) and answer questions about
   the data structures and algorithms used in interpreters to
   implement such features.

\item
Compare alternatives in the design and implementation of such features.

\item
Solve problems using the functional paradigm.
\end{itemize}

If you do not have this background,
especially if you are interested in research in programming languages,
you should take Com S 342 or Com S 440 (preferably both if you want to do
research in this area).
Mere reading of texts on these subjects is not enough.

Please speak with me if you have any doubts about how your
background matches these prerequisites.

\section{General Objectives}

The general objectives for Com S 541 are divided into two parts:
a set of essential of objectives and a set of enrichment objectives.
The enrichment objectives are found on the course web page.

In all cases, we allow the use of reference material to complete the tasks
implied by the objectives.

\subsection{Essential Objectives}
In general terms the essential objectives for Com S 541 are
that you be able to:
\begin{itemize}
\item
Evaluate programming language features and designs based on their
use in building domain-specific languages or in doing metaprogramming.
\item
Solve problems using the object-oriented, functional, 
and declarative paradigms.
\item
Describe the strengths and limitations of the object-oriented,
functional, and logic paradigms
for solving different kinds of problems (or in different application domains),
especially in relation to each other.
\item
Explain and answer questions about specific languages
that illustrate different paradigms, including questions about relevant
concepts and major features.
\item
Design, define, and evaluate parts of programming languages or similar systems
and justify your design decisions.
Justifications can be by:
\begin{itemize}
\item referring to known programming language concepts,
\item referring to the semantics of the features,
\item making analogies to features in specific languages that illustrate the
different paradigms and their success or limitations, or
\item making some more direct argument or proof.
\end{itemize}
\item
Explain the tradeoffs and issues involved in the design of various language
features.
\item
Find and critically write about research in programming languages.
Evaluations of research should be based on an understanding of:
\begin{itemize}
\item the goals and problems of language design,
\item the successes and limitations of previous approaches, and
\item current research directions.
\end{itemize}
\end{itemize}

Programming languages are the main technology that makes it possible
for humans to instruct computers; however current programming languages
are too hard to use.  They take years of study to master, and
there are not enough people with the necessary training.
It also seems that there won't be enough people with this training
anytime soon.
As language designers, we should respond to this need by creating tools
to create languages that help others without specialized training in
programming to do their work.
Hence the study of domain-specific languages and metaprogramming.

Knowing how to solve problems using the different paradigms
is important for several reasons.
You can find solutions to problems more surely if you have many different
ways to approach problems.
In the twenty-first century you will not necessarily be programming
in FORTRAN or C;
if you can program in a language such as
Smalltalk,
C++, or Ada, or other new languages you will be much in demand.
As parallel programming becomes more important,
the use of
functional
(and declarative) languages may increase.
The large telephone company
Ericsson uses the functional language
Erlang
to write all of its software.
Functional programming is also a key technology for supporting
domain-specific languages.
See also
``Why Functional Programming Matters''
by John Hughes.

Even if you do not become a programmer, the ideas of the functional
paradigm (function abstraction, infinite data structures, continuations,
referential transparency) have important applications in all areas of
computer science and in many other contexts such as mathematics
and engineering.
Similar comments hold true of the
object-oriented
paradigm.
For example, the idea of data abstraction
is certainly a key concept in software engineering and even in contemporary
mathematics (category theory).

Understanding the strengths and weaknesses of the various paradigms
is important in applying them to solve problems.
Problems in the real world are not labelled with the paradigm that
``should'' be used to solve them, so the choice of paradigm will
be important.
In programming language and software engineering research,
understanding the strengths and weaknesses of the existing paradigms
is important for designing better ways to program.

Language design is fundamental to mathematics and science
because a crucial step in solving a problem
is designing an adequate notation for stating the problem
(the specification) and expressing the solution.
Because computers are general purpose tools,
computer scientists, unlike mathematicians and traditional scientists,
tend to look at widely different problems.
Problems from different application domains often come without
a familiar or ready-made notation;
thus as computer scientists we often find it convenient to
develop a special-purpose notation.
These special-purpose notations, when generalized,
are specification languages or programming languages.
In developing and defining such a language
it is helpful to draw on the results
of programming language research.
These results help you generate plausible designs,
avoid errors, evaluate alternative designs,
and precisely define the details of the design.
Such justification of a design is a necessary step in
debugging your design and in ultimately convincing
yourself and others that your (final) design is good.

Notations that are similar to programming languages
are found in every area of computer science.
Besides specification languages,
other similar notation systems include:
user-interfaces, program libraries,
formal models of computation,
database query languages, operating system command languages and system call
interfaces, mathematical logics, computer instruction sets,
expert system shells, network protocols, and many others.

In addition, language design is challenging.
Since it is one step removed from programming (you design notations
that are used by programmers to write many different programs),
the opportunities for good or ill are multiplied.
Because of that, it is great fun!

Understanding the tradeoffs and issues involved in language design,
and the semantics of various features is
useful in language design itself.
But it also helps one more easily understand and learn
new languages and new language features.
It also helps one choose a language for a programming project,
and to write compilers or interpreters.

Understanding the concepts and semantics of programming languages
is also necessary to make full use of them in new languages.
For example, if you want to program in an object-oriented language
you need to understand inheritance and message passing.
A better understanding of such features, may help you
to better program, reason about, and debug your programs.
You will become better able to discuss advanced programming ideas with others.

Formal methods (specification and verification)
are becoming increasingly important at many companies,
and a deep understanding of the semantics of programming languages
is also a great help in using formal methods.

Since computer science is a rapidly-changing field,
it is important to be able to find and evaluate relevant papers
from the research literature, even if you do not usually do research in
that area.
As a first step, you need to know where to find various kinds of
information.
It is important to distinguish peer-reviewed research literature,
most of which is only found in the library,
from drafts available on the web, and from trade publications.

Understanding the ``state of the art'' in programming
language research is important for the following reasons.
First, in the business world, it helps predict promising technology
for programming.
Knowing the goals and problems of language design helps
you categorize problems that may arise;
this gives you a start towards looking for existing solutions.
Knowing about previous approaches to solving such problems
will help you avoid mistakes
and can point out fruitful approaches to solving design problems.
Knowing the current research directions in language design also
helps you when you are designing a non-research language,
by helping you avoid features that are not well-understood.
Conversely, if you are a programming languages researcher,
this knowledge tells you places to spend effort.

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