## Sophie Scholl: “I choose my own way to burn”

“The real damage is done by those millions who want to ‘survive.’ The honest men who just want to be left in peace. Those who don’t want their little lives disturbed by anything bigger than themselves. Those with no sides and no causes. Those who won’t take measure of their own strength, for fear of antagonizing their own weakness. Those who don’t like to make waves—or enemies. Those for whom freedom, honour, truth, and principles are only literature. Those who live small, mate small, die small. It’s the reductionist approach to life: if you keep it small, you’ll keep it under control. If you don’t make any noise, the bogeyman won’t find you. But it’s all an illusion, because they die too, those people who roll up their spirits into tiny little balls so as to be safe. Safe?! From what? Life is always on the edge of death; narrow streets lead to the same place as wide avenues, and a little candle burns itself out just like a flaming torch does. I choose my own way to burn.”

― Sophie Scholl, member of the anti-Nazi resistance group the White Rose executed by guillotine on February 22, 1943 at the age of 22.

## 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:

```size
2

(* 5 size)
10```

Here are further examples of the use of define:

```(define pi 3.14159)
(define radius 10)
(* pi (* radius radius))
314.159

(define circumference (* 2 pi radius))
circumference
62.8318```

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

## Simple expressions, combinations & evaluation in Scheme (SICP)

” […] Imagine that you are sitting at a computer terminal. You type an expression, and the interpreter responds by displaying the result of its evaluating that expression. One kind of primitive expression you might type is a number. […]

Expressions representing numbers may be combined with an expression representing a primitive procedure (such as + or *) to form a compound expression that represents the application of the procedure to those numbers. For example:

```(+ 137 349)
486

(- 1000 334)
666

(* 5 99)
495

[...]
```

Expressions such as these, formed by delimiting a list of expressions within parentheses in order to denote procedure application, are called combinations. The leftmost element in the list is called the operator, and the other elements are called operands.

The value of a combination is obtained by applying the procedure specified by the operator to the arguments that are the values of the operands.”

Structure and Interpretation of Computer Programs (SICP), ‘1.1  The Elements of Programming

## Why Scheme makes a good teaching language (SICP)

“In teaching our material we use a dialect of the programming language Lisp. We never formally teach the language, because we don’t have to. We just use it, and students pick it up in a few days. This is one great advantage of Lisp-like languages: They have very few ways of forming compound expressions, and almost no syntactic structure. All of the formal properties can be covered in an hour, like the rules of chess. After a short time we forget about syntactic details of the language (because there are none) and get on with the real issues — figuring out what we want to compute, how we will decompose problems into manageable parts, and how we will work on the parts. Another advantage of Lisp is that it supports (but does not enforce) more of the large-scale strategies for modular decomposition of programs than any other language we know. We can make procedural and data abstractions, we can use higher-order functions to capture common patterns of usage, we can model local state using assignment and data mutation, we can link parts of a program with streams and delayed evaluation, and we can easily implement embedded languages. All of this is embedded in an interactive environment with excellent support for incremental program design, construction, testing, and debugging. We thank all the generations of Lisp wizards, starting with John McCarthy, who have fashioned a fine tool of unprecedented power and elegance.

Scheme, the dialect of Lisp that we use, is an attempt to bring together the power and elegance of Lisp and Algol. From Lisp we take the metalinguistic power that derives from the simple syntax, the uniform representation of programs as data objects, and the garbage-collected heap-allocated data. From Algol we take lexical scoping and block structure, which are gifts from the pioneers of programming-language design who were on the Algol committee.”

Structure and Interpretation of Computer Programs, ‘1.1  The Elements of Programming

## The development of Lisp as a symbol-manipulation language (SICP)

“Despite its inception as a mathematical formalism, Lisp is a practical programming language. A Lisp interpreter is a machine that carries out processes described in the Lisp language. The first Lisp interpreter was implemented by McCarthy with the help of colleagues and students in the Artificial Intelligence Group of the MIT Research Laboratory of Electronics and in the MIT Computation Center. Lisp, whose name is an acronym for LISt Processing, was designed to provide symbol-manipulating capabilities for attacking programming problems such as the symbolic differentiation and integration of algebraic expressions. It included for this purpose new data objects known as atoms and lists, which most strikingly set it apart from all other languages of the period.

Lisp was not the product of a concerted design effort. Instead, it evolved informally in an experimental manner in response to users’ needs and to pragmatic implementation considerations. Lisp’s informal evolution has continued through the years, and the community of Lisp users has traditionally resisted attempts to promulgate any “official” definition of the language. This evolution, together with the flexibility and elegance of the initial conception, has enabled Lisp, which is the second oldest language in widespread use today (only  Fortran is older), to continually adapt to encompass the most modern ideas about program design. Thus, Lisp is by now a family of dialects, which, while sharing most of the original features, may differ from one another in significant ways.”

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

## 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.