Lisp Basics and Idioms

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Lisp Basics and Idioms
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Lisp Is Old

• 1958 John McCarthy writes an algebraic list
  processing language for AI work
• McCarthys grad student wrote an interpreter for
  it
• Strongly tied to AI research during the 70s and
  80s
• Fell from prominence during the AI Winter

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Lisp Is New

• Paul Grahams essays Beating the Averages,
  Lisp in Web-Based Applications
• Eric Raymond "LISP is worth learning for the
  profound enlightenment experience you will have
  when you finally get it. That experience will
  make you a better programmer for the rest of your
  days, even if you never actually use LISP itself
  a lot.
• Recent development of open source versions

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Lisp is a Family of Languages

• Common Lisp ANSI Standard written in the 80s
  caused languages to coalesce, then
  implementations to flourish
• Scheme A conceptually cleaner variant with a
  smaller specification
• Proprietary LispWorks, Allegro
• Open Source SBCL, CLisp
• Others compile to C, run on JVM, etc

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Lisp is Functional, But Not Strictly

• Functional is the most natural to write
• Can make sequential blocks, either explicitly or
  in constructs
• Can make and change values if needed
• Can build and incorporate new paradigms as
  necessary, i.e. CLOS
• Lisp is strongly typed, dynamic typed

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Lisp Has Lots of Parentheses

• "Lisp has all the visual appeal of oatmeal with
  fingernail clippings mixed in." -Larry Wall
• Used to group expressions
• Makes syntax simple and consistent
• Most forms are (function args)

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But the Parentheses Arent a Big Deal

• "Parentheses? What parentheses? I haven't
  noticed any parentheses since my first month of
  Lisp programming. I like to ask people who
  complain about parentheses in Lisp if they are
  bothered by all the spaces between words in a
  newspaper" - Ken Tilton

• Editors indent automatically
• Emacs commands to balance and close parens
• Paredit (Emacs library) lets you manipulate sexps
  directly

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Lisp Uses Symbols

• Like variables but better
• Like pointers but less dangerous
• Assign a name to a value
• Values can be lots of things numbers, strings,
  functions, lists, other data structures

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Lisp Has First Class Functions

• Easy to define
• (defun hello-world () (format t hello, world))
• Can be passed as parameters
• Can be returned as values from other functions
• Anonymous functions too!

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Lisp Has Flexible Parameters

• Parameters can be optional, with defaults
• (defun foo (a optional b) (list a b))
• (defun foo2 (a optional (b 10)) (list a b))
• Parameter lists can be variable length
• (defun (rest numbers) )
• Keyword parameters
• (defun foo3 (key a b) )

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Lisp Uses Pairs

• Lists are chains of pairs
• Can make other trees, etc as well

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Lisp Uses Lists

• "It is better to have 100 functions operate on
  one data structure than to have 10 functions
  operate on 10 data structures." - Alan J. Perlis
• Tons of functions for manipulating lists
• Useful for recursive definitions
• Lists arent perfect so...

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Lisp Has More Than Just Lists

• Vectors fixed size sequences
• Arrays can be multidimensional, resizable
• Sequence functions on collections COUNT, FIND,
  POSITION, REMOVE, SUBSTITUTE, etc
• This is one place where syntax would help
• (aref a 5) instead of a5

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Lisp Does Lots More

• File and File I/O
• Advanced object system using generic functions
  and message passing
• Text formatting
• Fancy iteration constructs
• Conditions and restarts
• Libraries for other things

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Lisp Has Lots of Free Online Resources

• Common Lisp First Contact
• A Gentle Introduction to Symbolic Computation
• Structure and Interpretation of Computer Programs
• Practical Common Lisp
• OnLisp