Introduction Functional Programming - Tech Hangout #11 - 2013.01.16

Introductory Functional
Programming

16.01.2013
Valera Rozuvan

We all know what programming is

#include <stdio.h>

int main(void) {

printf("Hello
world!n");
return 0;

}

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

… programming ?

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 ...

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 ...

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

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.

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))

Functional programming languages
are declarative

… in the beginning there was mathematics ...

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.

Introduction Functional Programming - Tech Hangout #11 - 2013.01.16

Applying FP to the *real* world?
●
I am a JavaScript person, so I will write JS!

you
have
been
warned

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.

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]);
}

●
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
);

●
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]);

●
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
);

Use functional programming
for the greater good!

Introduction Functional Programming - Tech Hangout #11 - 2013.01.16

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Introduction Functional Programming - Tech Hangout #11 - 2013.01.16