1. Introductory Functional
Programming
16.01.2013
Valera Rozuvan

2. We all know what programming is
#include <stdio.h>
int main(void) {
printf("Hello
world!n");
return 0;
}

3. Functional?
A function is a rule
which operates on an
input and produces an
output.
… programming ?

4. Wait... isn't this
#include <stdio.h>
int main(void) {
printf("Hello world!n");
return 0;
}
already functional programming? It is after all a
program that defines a function main(), which
is then executed ...

5. Wrong!

6. In computer science, functional
programming is a programming
paradigm that treats computation
as the evaluation of mathematical
functions and avoids state and
mutable data.
… just having functions isn't enough ...

7. procedural
FORTRAN, C, Pascal, BASIC
imperative
object-oriented
C++, Java, C#, VB.NET, Python
programming
logic
Gödel, PROLOG
declarative
functional
Lisp, Clojure, Erlang, Haskell

8. Imperative vs. Declarative
●
Imperative programming is a programming
paradigm that describes computation in
terms of statements that change a program
state. Imperative programs define
sequences of commands for the computer
to perform.
●
Declarative programming is a programming
paradigm that expresses the logic of a
computation without describing its control
flow.

9. Huh?

10. Example: factorial
Imperative: Declarative:
def factorial(x) def factorial(x)
res = 1 if x < 2
while (x > 1) return 1
res = res*x else
x = x­1 return x*factorial(x­1)
end end
return res end
end
print(factorial(10))
x = 10
print(factorial(x))

11. Functional programming languages
are declarative

12. … in the beginning there was mathematics ...

13. Lambda calculus
●
The λ-calculus calculus was introduced by
mathematician Alonzo Church in the 1930s.
●
The λ-calculus treats functions "anonymously",
without giving them explicit names.
●
In λ-calculus, functions are taken to be 'first
class values', so functions may be used as the
inputs and returned as outputs from other
functions.
●
λ-calculus – a formal system for function
definition, application, and recursion.

15. Wow...
… hard.

16. Lambda calculus For Dummies

17. Applying FP to the *real* world?
●
I am a JavaScript person, so I will write JS!
you
have
been
warned

18. Can you do FP in JS?
●
If you define functional language as the
language that supports first class functions
and lambdas, then yes, JavaScript *is* a
functional language.
●
JavaScript supports passing around functions
as variables.
●
JavaScript supports anonymous functions.
Short answer – yes! If you
start to argue – no.

19. A short demo
●
If you have any good taste at all, one ugly
detail must be starting to bother you - the
endlessly repeated for loop going over an
array.
function printArray(array) {
for (var i = 0; i < array.length; i++)
print(array[i]);
}

20. ●
But what if we want to do something other
than print? Because 'doing something' is really
a function, and functions are also values, we
can pass our action as a function value.
function forEach(array, action) {
for (var i = 0; i < array.length; i++)
action(array[i]);
}
forEach(
["Wampeter", "Foma", "Granfalloon"],
print
);

21. ●
And by making use of an anonymous function,
something just like a for loop can be written
with less useless details.
function sum(numbers) {
var total = 0;
forEach(numbers, function (number) {
total += number;
});
return total;
}
show(sum[1, 10, 100]);

22. ●
On the whole, using more abstract (or 'higher
level') constructs results in more information
and less noise. Compare the following:
var paragraphs = archive[today].split("n");
for (var i = 0; i < paragraphs.length; i++)
processParagraph(paragraphs[i]);
versus
forEach(
archive[today].split("n"),
ProcessParagraph
);

23. Use functional programming
for the greater good!

24. questions