Functional Programming with LISP

Functional Programming
with

LISt Processing

© 2010 Anil Kumar Pugalia <email@sarika-pugs.com>
All Rights Reserved.

Introduction

© 2010 Anil Kumar Pugalia <email@sarika-pugs.com>

What to Expect?
W's of LISP
Language Specifics
Fun with Recursion

© 2010 Anil Kumar Pugalia <email@sarika-pugs.com> 3

What is LISP?
Functional Programming Language
Conceived by John McCarthy in 1956
Name comes from its initial powerful List Processing features
Natural computation mechanism: Recursion
Standardization as Common LISP
ANSI released standards in 1996
Thought of as for Artificial Intelligence
But could pretty much do anything
Examples range from OS, editors, compilers, games, GUIs, and
you think of it

Current Available Forms
ANSI Common Lisp: clisp
Compiler, Interpreter, Debugger
GNU Common Lisp: gcl
Compiler, Interpreter
CMU Common Lisp: cmucl
By Carnegie Mellon University
Default available as .deb packages
Use “alien –to-rpm" to convert them to rpm
Allegro CL: Commercial Common Lisp implementation

Why LISP?
“The programmable programming language"
What's good for the language's designer
Is good for the language's users
Wish for new features for easier programming?
As you can just add the feature yourself
Code the way our brain thinks: Recursive
Most natural way of programming
50 lines of Code

Language Specifics
Data Structures
Basic Operations
Control Structures
Basic I/O


Data Structures
S-Expression: Atom or List
Atom: String of characters ('Values or Variables)
Peace, 95432, -rtx, etc
List: Collection of S-Expressions enclosed by ()
(The 100 times done)
(Guava (43 (2.718 5) 56) Apple)
What is a Null List: ()?
Common Pitfall: Lists need to start with '

Basic Operations
Lisp Program = Sequence of Functions
Applied to their Arguments
Returning Lisp Objects
Function Types
Predicate
Tests conditions with its arguments
Returns Nil (FALSE) or anything else (TRUE)
Command
Performs operation with its arguments
Returns an S-Expression

Basic Operations ...
Format: (function arg1 arg2 … argn)
Let's try
Commands: car, cdr, cons, quote
Predicates: atom, null
NB Lisp is case-insensitive
More: first, last, rest, append, consp, ...


Mathematical Operations
Predicates: zerop, plusp, evenp, integerp, floatp
Arithmetic: +, -, *, /, rem, 1+, 1-
Comparisons: =, /=, <, >, <=, >=
Rounding: floor, ceiling, truncate, round
More Functions
max, min, exp, expt, log, abs, signum, sqrt, isqrt


Control Structures
Constants & Variables: Atoms w/ & w/o quote (')
Assignment: setq, psetq, set, setf
Conditionals: equal, cond, if
Logical: and, or, not
Functions
(defun func-name (par1 … parn) (commands))
Unnamed: (lambda (par1 … parn) (commands))


Let's try some functions
Extract the second element from a list
Insert an element at second position in a list
Change nth element in a list
Find length of a list (iteratively)
Find variance of a list of elements


Iteration is Human
Recursion is God


Recursion
For any recursion, we need 2 things
Recursive Relation
Termination Condition
Functional Procedural

Recursive Relation From Mathematics. Tricky Extreme
Fairly Simple Conditions

Termination Needs Thought Fairly Trivial
Condition

Let's try some examples to understand

Tracing & Analysis
Tracing Recursive Function Calls
Enable tracing: (trace recursive-func)
Invoke the function to be traced: (recursive-func …)
Performance Analysis
(time (func …))
Samples with our recursive functions


Tail Recursion
Bottom most call's return = Topmost call's return
Let's observe the trace on list reversal
May not be always possible
But if possible, it is a smart compiler advantage
Cuts-off processing as soon as lowest level returns


Example: Set Operations
Sets: List of AToms (LATs)
Operations: member, union, intersection, adjoin
Let's write some examples
which support sets being elements of set


Basic Input / Output
Basic Input
(read [input-stream] [eof-error] [eof-value]) → s-expr
Basic Output
(print s-expr [output-stream]) → s-expr
(format destination control-string [args])


Loading Files
Loading Lisp file for interpretation
(load "file.lisp")
Compiling a Lisp file
(compile "file.lisp") or (compile "file")
Loading a compiled Lisp file
(load (compile "file.lisp"))


References
Common Lisp: http://www.lisp.org
CLISP: http://clisp.cons.org
Practical Common Lisp by Peter Seibel
Also @ http://www.gigamonkeys.com/book/
LISP Tutorial: http://www.mars.cs.unp.ac.za/lisp/


What all have we learnt?
W's of LISP
Language Specifics Demonstration
Data Structures
Basic Operations
Control Structures
Basic I/O
Fun with Recursion
Power, Tail Recursion, Tracing & Analysis

Any Queries?


Functional Programming with LISP

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