1. Lisp, Then and Now

2. History
John McCarthy invented Lisp in 1958 as both a programming language and as a formalism for doing recursive function theory
The first two implementations were written by a graduate student, Steve “Slug” Russell
McCarthy is famous for inventing Lisp
Russell is famous for inventing Spacewar!, the first widely-known video game

3. Why is Lisp important? I
Lisp is a language that “got it right the first time”
Consequently, there have been no changes to the core language in over 50 years!
Later variants—Common Lisp, Scheme, Racket, Clojure—have been additions and minor name changes
Lisp is homoiconic—there is no distinction between the way data is represented, and the way programs are represented
Programs are data, and data are programs
Lisp is superb for metaprogramming—programs that write programs (or even rewrite themselves)
Lisp is functional—functions are just another kind of data, to be stored, passed around, and manipulated
Functional programming is much better for concurrent and distributed programs

4. Why is Lisp important? II
Lisp is an instantiation of the lambda calculus
Lisp is a kind of math implemented as a programming language, just as Prolog is a kind of logic implemented as a programming language
You don’t need to understand lambda calculus to use Lisp
But knowing Lisp really helps you understand lambda calculus!
Other languages get compiled into an Abstract Syntax Tree (AST), but Lisp’s syntax is the AST
What you see is what you get!

5. Lisp syntax An S-expression (symbolic expression) is an atom or a list
An atom is a variable or a number
A list is zero or more S-expressions enclosed in parentheses
… and that’s it!

6. Lisp semantics
Lisp has functions and special forms, with the syntax (function-or-form arg1 arg2 … argN)
For a function: The arguments are evaluated and their values passed to the function
There are a large number of predefined functions
For a form: The arguments are passed unevaluated to the form
The form decides when/whether to evaluate them
There are a large number of predefined forms
defn defines a new function
if and cond select among alternatives
quote returns its argument unevaluated (i.e. it is treated as data)
The names of forms and functions vary somewhat between different versions of Lisp (head, front, first, car), but the behavior changes very little

7. Lisp example
A non-empty list has a head (first element) and a tail (the remainder of the list)
Here’s how to find the last value in a list: (defun last (list) (if (empty (tail list)) (head list) (last (tail list)) ) )

8. The End
“We were after the C++ programmers. We managed to drag a lot of them about halfway to Lisp.” Guy Steele, co-author of the Java spec