CS 655: Programming Languages
Scheming Schemers, Mocking Mockingbirds, and
Objecting Objectively
Lecture 1:Wherefore CS655?
The use of <???> cripples the mind;
its teaching should, therefore, be
regarded as a criminal offence.
Edsger W.Dijkstra, 1975
CS655: Programming Languages
David Evans
University of Virginia
http://www.cs.virginia.edu/~evans
Computer Science
Menu
• What is Computer Science?
• Goals for the Course
• Programming Languages
– Design Tradeoffs
Send registration email by Sunday
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What is
Computer Science?
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Let AB and CD be the two given numbers not
relatively prime. It is required to find the greatest
common measure of AB and CD.
If now CD measures AB, since it also measures
itself, then CD is a common measure of CD and
AB. And it is manifest that it is also the greatest, for
no greater number than CD measures CD. But, if
CD does not measure AB, then, when the less of
the numbers AB and CD being continually
subtracted from the greater, some number is left
which measures the one before it.
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For a unit is not left, otherwise AB and CD would be relatively prime, which is contrary to
the hypothesis. Therefore some number is left which measures the one before it. Now
let CD, measuring BE, leave EA less than itself, let EA, measuring DF, leave FC less
than itself, and let CF measure AE.
Since then, CF measures AE, and AE measures DF, therefore CF also measures DF.
But it measures itself, therefore it also measures the whole CD. But CD measures BE,
therefore CF also measures BE. And it also measures EA, therefore it measures the
whole BA. But it also measures CD, therefore CF measures AB and CD. Therefore CF
is a common measure of AB and CD.
I say next that it is also the greatest. If CF is not the greatest common measure of AB
and CD, then some number G, which is greater than CF, measures the numbers AB and
CD.
Now, since G measures CD, and CD measures BE, therefore G also measures BE. But
it also measures the whole BA, therefore it measures the remainder AE. But AE
measures DF, therefore G also measures DF. And it measures the whole DC, therefore
it also measures the remainder CF, that is, the greater measures the less, which is
impossible. Therefore no number which is greater than CF measures the numbers AB
and CD. Therefore CF is the greatest common measure of AB and CD.
Euclid’s Elements, Book VII, Proposition 2 (300BC)
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By the word operation, we mean any process which
alters the mutual relation of two or more things, be this
relation of what kind it may. This is the most general
definition, and would include all subjects in the universe.
Again, it might act upon other things besides number,
were objects found whose mutual fundamental relations
could be expressed by those of the abstract science of
operations, and which should be also susceptible of
adaptations to the action of the operating notation and
mechanism of the engine... Supposing, for instance,
that the fundamental relations of pitched sounds in the
science of harmony and of musical composition were
susceptible of such expression and adaptations, the
engine might compose elaborate and scientific pieces of
music of any degree of complexity or extent.
Ada, Countess of Lovelace, around 1830
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What is the difference
between Euclid and Ada?
“It depends on what your definition of ‘is’ is.”
Bill Gates
(speaking at Microsoft’s anti-trust trial)
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Geometry vs. Computer Science
• Geometry (mathematics) is about
declarative knowledge: “what is”
If now CD measures AB, since it also measures itself,
then CD is a common measure of CD and AB
• Computer Science is about imperative
knowledge: “how to”
Computer Science has nothing to do
with beige (or translucent blue) boxes
called “computers” and is not a science.
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Next: My Goals for the Course
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Goal #1
Make this the first
real Computer
Science most of
you have taken.
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Imperative Knowledge
• The core of Computer Science is
describing and reasoning about
computations
• This class is about:
– Tools for describing computations
(programming languages)
– Tools for reasoning about those tools
(mostly formal semantics)
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Goal #2
Not waste your
time.
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Graduate Students
• Should make research their highest
priority (after sleeping, eating, and
maintaining sanity and happiness)
• Should not spend time on courses
except when it:
– Makes them better researchers
– Is of intrinsic interest and value
• Should be mature enough to decide
what it is worthwhile to spend time on
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Hence:
• Everything in this class is optional
– Except Problem Set 0: Registration Survey
• You should not take this course unless
you believe most of the material we will
cover is either:
– Relevant to your research
– Intrinsically cool and interesting
(Note: all of it is both)
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Evaluation
• I still need to grade you, of course...
• To get an A in this course, you need to
convince me of your deep
understanding of the most important
course material.
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Ways to get an A in CS 655
1. Do well on the assignments
or
2. Do well on an oral final exam
or
3. Produce a quality research paper on a
relevant topic
or
4. Demonstrate your understanding through
outstanding class contributions
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Questions
Next: Goals 3-8
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Goal #3
When you need to invent
a language (and most of
you will at some point in
your career) you will
design it, not just make it
up.
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What’s the difference?
Designed
Just Made Up
Examples
Esperanto, CLU English, C++
Properties
Predictable
Unpredictable
Ways to do
something
One
Many
Users
~1M; .5 - 15
~1B; ~1M
Moral? A bigger army beats good design every time
But, good designs can influence people with armies
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Goal #4
When you design your
language, you will have
sufficient knowledge of other
languages not to repeat their
mistakes, and solid enough
theory background to describe
your language well.
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Goal #5
You will become better at
programming and building
systems. You will think more
clearly and maybe even write
better.
A language that doesn't affect the way you think
about programming, is not worth knowing.
Alan Perlis
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Goal #6
Some of you will do projects
that lead to conference
papers and influence your
theses. All of you will be
able to understand nearly all
PLDI papers and many
POPL papers.
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Goal #7
Enable at least half of
you to understand at
5
least half of R RS
Section 7.2 (2½ pages)
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Goal #8
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Disclaimer
• Sorry, no refunds if not all goals
are satisfied.
• Tell me your goals on the
survey.
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Questions
Next: What is a Programming
Language?
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What is a language?
Webster:
A systematic means of
communicating ideas or feelings
by the use of conventionalized
signs, sounds, gestures, or marks
having understood meanings.
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Nerdy Linguist’s Definition
A description of pairs (S, M),
where S stands for sound, or any
kind of surface forms, and M
stands for meaning. A theory of
language must specify the
properties of S and M, and how
they are related.
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Programming Language
A language (according to the Nerdy
Linguist’s definition) intended to be read
and written by humans and processed
by machines.
Stricter definition add:
that has some notion of control flow.
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Some other definitions
• Bruce MacLellan’s definition:
A language that is intended for the expression
of computer programs and that is capable of
expressing any computer program.
• Ravi Sethi’s definition:
Notations used for specifying, organizing, and
reasoning about computations.
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What makes a PL important?
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Lots of People Learn It
100
Java
HTML
1000
Eiffel
Pascal
Ada
Lisp
FORTRAN
COBOL
Tcl
C++
C
Basic
10000
No books in print:
Algol60, CLU, Simula
100000
Amazon.com sales rank for best-selling book related to language
(30 November 1999)
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You Can Get a Job
1000
HTML
Basic
Java
C++
100
Pascal
FORTRAN
No job listings:
Algol60, CLU, Simula
Tcl
Ada
COBOL
10
1
Monster.com job listings in Virginia matching language
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Changes How Programmers Think
Influences other Languages
•
•
•
•
•
FORTRAN - use a high level language
Algol60 - program in a structured way
Simula - use classes
CLU - use data abstraction
FP - program without state
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Languages we will focus on
• Scheme
– Simplest, cleanest, most elegant language I know
– Extremely powerful tool for describing computations
• CLU
– Because I like it, and its my course
– Because it has the best support for data abstraction
• Java
– Because it is popular and modern
• Others, possibly including:
– Ada, Algol60, Algol68, C, C++, Eiffel, Euclid, FL, ML,
Linda, Pascal, Sather, Simula, Tcl
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Language Design
Art and Engineering
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Language Design as Engineering
Safety
Performance
Java, CLU
FORTRAN, BLISS, C
Ease of Use
BASIC, CLU
Ease/size of Implementation
Forth, C
Orthogonality
Safety, Simplicity
Algol68
Pascal
Simplicity
Expressiveness
Scheme
Common LISP
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Example: Is assignment an expression?
• Wulf (BLISS): Of course, everything is
a := if .c >4 then .e else f := 7
• Richie (C): Yes, why not
while (x = 0) x++;
but not: x = if (a > 3) 4 else 5;
• Wirth (Pascal): No, only math-like things
are expressions
Who’s right?
What were their design tradeoffs?
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C’s main design requirement
Memory Space
24K on PDP-11 total. Needed to fit OS,
compiler, and leave a little room for user
programs. The “good” languages and
operating systems of the day (Algol60,
Multics) didn’t fit.
Note: C++’s main design requirement: be as
compatible as possible with C.
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Language Design as Art
• Aesthetics matter
• Readability is usually more important
than Writability
– CLU vs. APL
• Intuitive meaning more important than
formal semantics
– But formal semantics necessary to make
sure we agree on intuitive meaning
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Interaction of Art & Engineering
Example: C
void test (int x) {
while (x = 1) {
printf (“I’m an imbecile!”);
x = x + 1;
}
}
Art: use = for assignment (not :=)
Engineering: make assignments expressions,
no Boolean type, weak types, etc.
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Charge
• Think about my definition of Computer
Science
– Send me a better one if you can
• Think about how well the languages you
use are designed
• Send me your registration survey by
Sunday
• Next time: Scheme
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