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Programming Languages:
Application and Interpretation
Shriram Krishnamurthi
Brown University
Copyright c 2003, Shriram Krishnamurthi
This work is licensed under the
Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.
If you create a derivative work, please include the version information below in your attribution.
This book is available free-of-cost from the author’s Web site.
This version was generated on 2007-04-26.
ii
Preface
The book is the textbook for the programming languages course at Brown University, which is taken pri-
marily by third and fourth year undergraduates and beginning graduate (both MS and PhD) students. It
seems very accessible to smart second year students too, and indeed those are some of my most successful
students. The book has been used at over a dozen other universities as a primary or secondary text. The
book’s material is worth one undergraduate course worth of credit.
This book is the fruit of a vision for teaching programming languages by integrating the “two cultures”
that have evolved in its pedagogy. One culture is based on interpreters, while the other emphasizes a survey
of languages. Each approach has significant advantages but also huge drawbacks. The interpreter method
writes
programs
to learn concepts, and has its heart the fundamental belief that by teaching the computer to
execute a concept we more thoroughly learn it ourselves.
While this reasoning is internally consistent, it fails to recognize that understanding definitions does
not imply we understand consequences of those definitions. For instance, the difference between strict
and lazy evaluation, or between static and dynamic scope, is only a few lines of interpreter code, but the
consequences
of these choices is enormous. The survey of languages school is better suited to understand
these consequences.
The text therefore melds these two approaches. Concretely, students program with a new set of features
first, then try to distill those principles into an actual interpreter. This has the following benefits:
•
By seeing the feature in the context of a real language, students can build something interesting with
it first, so they understand that it isn’t an entirely theoretical construct, and will actually
care
to build
an interpreter for it. (Relatively few students are excited in interpreters for their own sake, and we
have an obligation to appeal to the remainder too.)
•
Students get at least fleeting exposure to multiple languages, which is an important educational at-
tribute that is being crushed by the wide adoption of industrially fashionable languages. (Better still,
by experimenting widely, they may come to appreciate that industrial fashions are just that, not the
last word in technological progress.)
•
Because they have already programmed with the feature, the explanations and discussions are much
more interesting than when all students have seen is an abstract model.
•
By first building a mental model for the feature through experience, students have a much better
chance of actually discovering how the interpreter is supposed to work.
iii
iv
PREFACE
In short, many more humans learn by induction than by deduction, so a pedagogy that supports it is much
more likely to succeed than one that suppresses it. The book currently reflects this design, though the survey
parts are done better in lecture than in the book.
Separate from this vision is a goal. My goal is to not only teach students new material, but to also change
the way they solve problems. I want to show students where languages come from, why we should regard
languages as the ultimate form of abstraction, how to recognize such an evolving abstraction, and how to
turn what they recognize into a language. The last section of the book, on domain-specific languages, is a
growing step in this direction.
Design Principles
•
Concepts like design, elegance and artistic sensibility are rarely manifest in computer science courses;
in the name of not being judgmental, we may be running the risk of depriving our students of judg-
ment itself. We should reverse this trend. Students must understand that artificial objects have their
own aesthetic; the student must learn to debate the tradeoffs that lead to an aesthetic. Programming
languages are some of the most thoroughly designed artifacts in computer science. Therefore, the
study of programming languages offers a microcosm to study design itself.
•
The best means we have to lead students to knowledge is through questions, not answers. The best
education prepares them to assess new data by confronting it with questions, processing the responses,
and iterating until they have formed a mental model of it. This book is therefore structured more like
a discussion than a presentation. It leads the reader down wrong paths (so don’t blindly copy code
from it!). It allows readers to get comfortable with mistaken assumptions before breaking them down
systematically.
•
The programming languages course is one of the few places in the curriculum where we can tease
out and correct our students’ misconceptions about this material. They are often misled on topics
such as efficiency and correctness. Therefore, material on compilation, type systems and memory
management should directly confront their biases. For instance, a presentation of garbage collection
that does not also discuss the trade-offs with manual memory management will fail to address the
prejudices students bear.
Background and Prerequisite
This book assumes that students are comfortable reasoning informally about loop invariants, have modest
mathematical maturity, and are familiar with the existence of the Halting Problem. At Brown, they have all
been exposed to Java but not necessarily to any other languages (such as Scheme).
Supplementary Material
There is some material I use in my course that isn’t (currently) in this book:
preparation in Scheme
For the first week, I offer supplementary sessions that teach students Scheme. The
material from these sessions is available from my course Web pages. In addition, I recommend the
v
use of a simple introduction to Scheme, such as the early sections of
The Little Schemer
or of
How to
Design Programs.
domain-specific languages
I discuss instances of real-world domain-specific languages, such as the access-
control language
XACML
. Students find the concepts easy to grasp, and can see why the language is
significant. In addition, it is one they may themselves encounter (or even decide to use) in their
programming tasks.
garbage collection
I have provided only limited notes on garbage collection because I feel no need to offer
my own alternative to Paul Wilson’s classic survey,
Uniprocessor Garbage Collection Techniques.
I
recommend choosing sections from this survey, depending on student maturity, as a supplement to
this text.
model checking
I supplement the discussion of types with a presentation on model checking, to show
students that it is possible to go past the fixed set of theorems of traditional type systems to systems
that permit developers to state theorems of interest. I have a pre-prepared talk on this topic, and would
be happy to share those slides.
Web programming
Before plunging into continuations, I discuss Web programming
API
s and demonstrate
how they mask important control operators. I have a pre-prepared talk on this topic, and would
be happy to share those slides. I also wrap up the section on continuations with a presentation on
programming in the PLT Scheme Web server, which natively supports continuations.
articles on design
I hand out a variety of articles on the topic of design. I’ve found Dan Ingalls’s dissection
of Smalltalk, Richard Gabriel’s on Lisp, and Paul Graham’s on both programming and design the
most useful. Graham has now collected his essays in the book
Hackers and Painters.
logic programming
The notes on logic programming are the least complete. Students are already familiar
with unification from type inference by the time I arrive at logic programming. Therefore, I focus on
the implementation of backtracking. I devote one lecture to the use of unification, the implications
of the occurs-check, depth-first versus breadth-first search, and tabling. In another lecture, I present
the implementation of backtracking through continuations. Concretely, I use the presentation in Dorai
Sitaram’s
Teach Yourself Scheme in Fixnum Days.
This presentation consolidates two prior topics,
continuations and macros.
Exercises
Numerous exercises are sprinkled throughout the book. Several more, in the form of homework assignments
and exams, are available from my course’s Web pages (where
year
is one of
2000, 2001, 2002, 2003,
2004
and
2005):
http://www.cs.brown.edu/courses/cs173/
year
/
In particular, in the book I do
not
implement garbage collectors and type checkers. These are instead
homework assignments, ones that students generally find extremely valuable (and very challenging!).

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