# Lisp Index Page

This is the lisp index page, containing general staff for lisp.

There's a page for corresponding APIs of common lisp, racket, clojure, emacs lisp.

## 1 Why Lisp?

As Peter Norvig put in Paradigms of Artificial Intelligence Programming

• you can make DSL for your specific topic
• support whatever programming style by just defining some macros
• the basic and universal data structure: list
• developing lisp is fast
• dynamic, define function during running
• REPL

There is a myth that Lisp is "special-purpose" languages, while languages like Pascal and C are "general purpose". Actually, just the reverse is true.

## 2 Lisp-1 vs. Lisp-2

Lisp-1 has seperate namespace for functions and variables, while Lisp-2 only have one universal namespace.

Lisp-1 includes Elisp, common lisp. All scheme are lisp-2.

## 3 fold & unfold

In academic functional programming literature, folds are often called catamorphisms, unfolds are often called anamorphisms, and the combinations of the two are often called hylomorphisms. They're interesting because any for-each loop can be represented as a catamorphism. To convert from a loop to a foldl, package up all mutable variables in the loop into a data structure (records work well for this, but you can also use an algebraic data type or a list). The initial state becomes the accumulator; the loop body becomes a function with the loop variables as its first argument and the iteration variable as its second; and the list becomes, well, the list. The result of the fold function is the new state of all the mutable variables.

Similarly, every for-loop (without early exits) can be represented as a hylomorphism. The initialization, termination, and step conditions of a for-loop define an anamorphism that builds up a list of values for the iteration variable to take. Then, you can treat that as a for-each loop and use a catamorphism to break it down into whatever state you wish to modify.

## 4 Car & Cdr

The names CAR and CDR derive from the history of Lisp. The original Lisp implementation ran on an IBM 704 computer which divided words into two parts, called the “address” part and the “decrement”; CAR was an instruction to extract the contents of the address part of a register, and CDR an instruction to extract the contents of the decrement. By contrast, “cons cells” are named for the function ‘cons’ that creates them, which in turn was named for its purpose, the construction of cells.

## 5 The right thing vs. Worse is better 1

These are two software design philosophies. The key different is:

• The right thing: interface should be simple
• Worse is better: implementation should be simple

The worse-is-better philosophy means that implementation simplicity has highest priority, which means Unix and C are easy to port on such machines.

Unix and C are the ultimate computer viruses.

The code will be portable because it is written on top of a virus.

The good news is that in 1995 we will have a good operating system and programming language; the bad news is that they will be Unix and C++.