1.
Demystifying (λ)Functional Programming with
Clojure
-Rohit Vaidya

2.
Agenda
● Functional Programming
● Some Functional Jargon
● Understand Clojure
● Clojure Syntax
● Lein - Build tools for Clojure
● Meta Programming

3.
Functional Programming
● Functional Programming takes a
mathematical view of the world
● Nothing but elaboration of Lambda
Calculus

4.
λ- Calculus
● λ-Calculus is universal model of
Computation
● Can be used to simulate a taped turing
machine
● λ-Calculus treats functions anonymously
● Computable functions are fundamental to
math and CS.
● λ-Calculus provides semantics for
computation

5.
λ- Calculus
squaresum( x , y)→ x
2
+y
2
(x , y) → x
2
+ y
2
(( x , y)→ x
2
+y
2
)(4,3)
→4
2
+3
2
→25
(x →( y → x
2
+y
2
)(5))(2)
( y →5
2
+y
2
)(2)
5
2
+2
2
29

6.
Jargon
● Homoiconic Language - Program
structure is similar to its syntax
● Metaprogramming - Metaprogramming is
the art of writing of computer programs with
the ability to treat programs as their data
● Referential Transparency (Pure) -
Always returns the same result for a given
argument

7.
Functional Programming-Clojure
● First class functions
● Immutable Data Structures
● Recursive looping
● Facilitates concurrency

8.
Why Clojure?
● A Lisp – dynamic language
● Functional Programming
● Symbiotic with an established Platform
● Designed for Concurrency
● Embraces the JVM (Native to the JVM)
● Clojure is concise – Code as Data
– (+ 3 2) This is a function call
– '(+ 3 2) This is data

9.
Why Clojure?
● You can tranlate data into function call at
runtime
● Performs better than JavaScript, Ruby and
Python
● Macros- Extending the Language
– e.g: HTML templating is bloated
– In java you mix HTML with Java or Java
with HTML
– What if your language knows generating
HTML?

10.
Why Clojure?
– [:a {:href "http://github.com"} "GitHub"]
– This converts to
– <a href="http://github.com">GitHub</a>
– The above can be done using Hiccup
● Has a REPL
● Define functions on the fly

11.
Clojure Syntax
● ()[]{} Everything within () gets evaluated
● Almost no syntax rules
● Lisp Syntax (data = code).Code as Data!!!
● Lets write a simple function
– Anonymous Function
– Named function

12.
Atomic Data Types
● Nil means nothing. Same as Java null
● Booleans true false
● Doubles 1.234 BigDecimals 10.123M
● Ratios 22/7
● Strings “fred” Characters a b c
● Regex #”a*x”

13.
Atomic Data
● Use clojure.core/class function to identify
type of data

14.
Clojure Functions
● Create a Clojure Function
1.Bind Name add to the function
2.Anonymous function with arguments x y
1.fn creates a anonymous function
3.Function description
4.Function body
1
2
31
4

15.
Clojure Function
● Define function with macro form defn
1
2
3
1. Create a function add with x, y
arguments
2. Function description
3. Function Body

16.
Clojure Function Pure vs Impure
● Referential Transparency
1. Is pure. For certain x,y it will always return
same result
2. Is impure. Getting a hike is dependent on
side effect and not always deterministic
1 2

17.
Special Forms
● Primitives build in clojure to perform core
operations
● If do let fn loop recur etc are special forms
● (if true 1 2)
– Returns 1
● (let [x 3] println x)
– Prints 3
– Scope restricted to let statment

18.
Lists, Vectors, Sets and Maps
● Vectors
– Similar to Array
– 0 based collection
– Syntax
● [1 2 3]
● (def abc [1 2 3])
● (get abc 0)

19.
Lists, Vectors, Sets and Maps
● Lists
– Similar to Vectors
– Cannot use get against lists
– Syntax
● '(1 2 3)
● (nth '(1 2 3) 0)
● Lists to used if you want to add elements to
the beginning
– (conj '(1 2 3) 4) returns 4 1 2 3

20.
Lists, Vectors, Sets and Maps
● Maps
– Similars to dictionaries or hashes
– Two types in Clojure
● HashMaps
● SortedMaps
– Syntax:
● {:firstName “Rohit” :lastName “Vaidya”}
● (def hm {:a 1 :b 2})
● (get hm :a) returns 1

21.
Lists, Vectors, Sets and Maps
● HashSet
– Sets are collection on unique values
– Syntax
● (hash-set 1 2 3 3 2 1)
– Create a hash set
● (contains? (hash-set 1 2 2 1 1) 1)
– Check presense of a element
– Returns true

22.
Lists, Vectors, Sets and Maps
● HashSet
– Sets are collection on unique values
– Syntax
● (hash-set 1 2 3 3 2 1)
– Create Hash Set
● (contains? (hash-set 1 2 2 1 1) 1)
– Check presense of element
– Returns true

23.
Programming to Abstractions
● Sequence Abstraction, abstracts
– List,
– Vector
– Set and Map
● Clojure defines map and reduce in terms of
sequence abstractions and not any specific
data structure
● y1 = f(x1), y2 = f(x2), . . . yn = f(x n).

24.
Programming to Abstractions
● Visualizing mapping on a sequence abstraction

25.
Programming to Abstractions
1. Define a increment function
2. Function description
3. Argument
4. Function Body
5. Map applied to a sequence(Vector). Map is a higher order function
1
2
3
4
5

26.
Loops using Recursion
1. First function overload
with zero arugment with
arity 0
2. Second function overload
with 1 argument i.e arity 1
3. Recursive call to by
passing incremented
value of number
1
2
3

27.
Loops with Recursion
1
2
• Clojure equivalent of 1 in 2

28.
Metaprogramming – Alchemy
● Reader: Is a clojure Parser
– Converts text into clojure data structure
● Read String converts to list

29.
Metaprogramming - Alchemy
1. Read a string and convert to list (data)
2. Read a String and evaluate it. Clojure expected operator.
3. Constructed prefix (reverse polish) with infix expression
4. Evaluated the constrcuted prefix expression
1
2
3
4

30.
Metaprogramming - Alchemy
● Defmacro defines a
macro
● Lesser verbose,
metaprogramming
1. Define a macro
2. Argument for macro
3. Prefix to infix
4. Call to a macro
1
2
3
4

31.
High Order Functions
● When a language takes a fn as arugment
or returns fn as result
● A higher order function is
– A function that takes function arguments
– A function that retuns a function
● Some well known higher order functions
– Map Reduce Remove
Filter Iterate

32.
Lein – the build tool
● Lein commands
– lein new app clojure-noob
– lein run
– lein test
● Lein web app
– lein new luminus my-app
– cd my-app
– lein run

33.
References
● https://clojurebridge.github.io/community-docs/
● http://www.slideshare.net/smartrevolution/how-a-
● http://www.braveclojure.com/
● http://xahlee.info/clojure/clojure_index.html
● https://www.gnu.org/software/emacs/
● https://mitpress.mit.edu/sicp/
● Clojure by Rich Hickey
● 4Clojure

34.
SICP