Names, definitions, and the global environment in Scheme (SICP)

” A critical aspect of a programming language is the means it provides for using names to refer to computational objects. We say that the name identifies a variable whose value is the object.

In the Scheme dialect of Lisp, we name things with define. Typing

(define size 2)

causes the interpreter to associate the value 2 with the name size. Once the name size has been associated with the number 2, we can refer to the value 2 by name:


(* 5 size)

Here are further examples of the use of define:

(define pi 3.14159)
(define radius 10)
(* pi (* radius radius))

(define circumference (* 2 pi radius))

define is our language’s simplest means of abstraction, for it allows us to use simple names to refer to the results of compound operations, such as the circumference computed above. In general, computational objects may have very complex structures, and it would be extremely inconvenient to have to remember and repeat their details each time we want to use them. Indeed, complex programs are constructed by building, step by step, computational objects of increasing complexity. The interpreter makes this step-by-step program construction particularly convenient because name-object associations can be created incrementally in successive interactions. This feature encourages the incremental development and testing of programs and is largely responsible for the fact that a Lisp program usually consists of a large number of relatively simple procedures.

It should be clear that the possibility of associating values with symbols and later retrieving them means that the interpreter must maintain some sort of memory that keeps track of the name-object pairs. This memory is called the environment (more precisely the global environment, since we will see later that a computation may involve a number of different environments).”

Structure and Interpretation of Computer Programs (SICP), ‘1.1.2 Naming and the Environment

Computer programs as developing models / Design to handle complexity (SICP)

“Every computer program is a model, hatched in the mind, of a real or mental process. These processes, arising from human experience and thought, are huge in number, intricate in detail, and at any time only partially understood. They are modeled to our permanent satisfaction rarely by our computer programs. Thus even though our programs are carefully handcrafted discrete collections of symbols, mosaics of interlocking functions, they continually evolve: we change them as our perception of the model deepens, enlarges, generalizes until the model ultimately attains a metastable place within still another model with which we struggle. The source of the exhilaration associated with computer programming is the continual unfolding within the mind and on the computer of mechanisms expressed as programs and the explosion of perception they generate. […]

For all its power, the computer is a harsh taskmaster. Its programs must be correct, and what we wish to say must be said accurately in every detail. As in every other symbolic activity, we become convinced of program truth through argument. Lisp itself can be assigned a semantics (another model, by the way), and if a program’s function can be specified, say, in the predicate calculus, the proof methods of logic can be used to make an acceptable correctness argument. Unfortunately, as programs get large and complicated, as they almost always do, the adequacy, consistency, and correctness of the specifications themselves become open to doubt, so that complete formal arguments of correctness seldom accompany large programs. Since large programs grow from small ones, it is crucial that we develop an arsenal of standard program structures of whose correctness we have become sure — we call them idioms — and learn to combine them into larger structures using organizational techniques of proven value. […] More than anything else, the uncovering and mastery of powerful organizational techniques accelerates our ability to create large, significant programs. Conversely, since writing large programs is very taxing, we are stimulated to invent new methods of reducing the mass of function and detail to be fitted into large programs.

— Alan J. Perlis, Structure and Interpretation of Computer Programs (2nd ed), ‘Foreword

Computational processes, sorcerers and systems (SICP)

“We are about to study the idea of a computational process. Computational processes are abstract beings that inhabit computers. As they evolve, processes manipulate other abstract things called data. The evolution of a process is directed by a pattern of rules called a program. People create programs to direct processes. In effect, we conjure the spirits of the computer with our spells.

A computational process is indeed much like a sorcerer’s idea of a spirit. It cannot be seen or touched. It is not composed of matter at all. However, it is very real. It can perform intellectual work. It can answer questions. It can affect the world by disbursing money at a bank or by controlling a robot arm in a factory.The programs we use to conjure processes are like a sorcerer’s spells. They are carefully composed from symbolic expressions in arcane and esoteric programming languages that prescribe the tasks we want our processes to perform.

A computational process, in a correctly working computer, executes programs precisely and accurately.Thus, like the sorcerer’s apprentice, novice programmers must learn to understand and to anticipate the consequences of their conjuring. Even small errors (usually called bugs or glitches) in programs can have complex and unanticipated consequences.

Fortunately, learning to program is considerably less dangerous than learning sorcery, because the spirits we deal with are conveniently contained in a secure way. Real-world programming, however, requires care, expertise, and wisdom. A small bug in a computer-aided design program, for example, can lead to the catastrophic collapse of an airplane or a dam or the self-destruction of an industrial robot.

Master software engineers have the ability to organize programs so that they can be reasonably sure that the resulting processes will perform the tasks intended. They can visualize the behavior of their systems in advance. They know how to structure programs so that unanticipated problems do not lead to catastrophic consequences, and when problems do arise, they can debug their programs. Well-designed computational systems, like well-designed automobiles or nuclear reactors, are designed in a modular manner, so that the parts can be constructed, replaced, and debugged separately.”

Structure and Interpretation of Computer Programs, ‘Chapter 1: Building Abstractions with Procedures

Functional programming in Python and Lisp

I’ve been trying to learn more about functional programming – specifically, looking at functional programming patterns in Python and Lisp/Scheme code. While reading about Python’s “functional” features (first-class functions, generators, list comprehensions, etc) I ran across many articles/books on functional programming in Haskell, Lisp, Scheme etc.

Years ago I had picked up a few books on Common Lisp and Scheme, but was unable to grok it due to some fundamental conceptual pieces I was missing at the time. But over the past few weeks as I have started exploring Lisp-like languages again, my mind has been blown.

The single biggest problem that I’ve had with programming languages I’ve used (C/Perl/Java/Python) is rigid and complicated syntax being forced upon me, the programmer. Constantly, this syntax is getting in the way of me clearly expressing what I want the computer to do, and is instead forcing me to contort my ideas into a form that fits into the languages’ strict and inflexible syntax rules.

The two things that have intrigued me most about Lisp are:

1) the ability to write beautiful, readable code with a simple/minimal yet highly expressive syntax (s-expressions and function application / evaluation)

2) the ability to define your own syntax using macros. If a language feature is getting in the way, you simply change it by writing macros that define customized syntax that clearly expresses concepts in a particular context / problem domain …

I am excited to explore these features more in the coming months – specifically by working with tools for Lisp/Python interaction (sharing code/data between programs written in each) … I’ll be posting writeups periodically to describe what I’ve learned.