Functional Programming You Already Know - Kevlin Henney - Codemotion Rome 2015

ROME 27-28 march 2015
functional programming
you already know
@KevlinHenney

functional
programming
higher-order functions
recursion
statelessness
first-class functions
immutability
pure functions
unification
declarative
pattern matching
non-strict evaluation
idempotence
lists
mathematics
lambdas
currying
monads

Excel is the world's
most popular
functional language.
Simon Peyton Jones

To iterate is human,
to recurse divine.
L Peter Deutsch

int factorial(int n)
{
int result = 1;
while(n > 1)
result *= n--;
return result;
}

{
if(n > 1)
return n * factorial(n - 1);
else
return 1;
}

{
return
n > 1
? n * factorial(n - 1)
: 1;
}

n! =
1
(n – 1)!  n
if n = 0,
if n > 0.{

reduce(
int.__mul__,
range(1, n+1), 1)

reduce(
lambda l, r: l*r,
range(1, n+1), 1)

#include <stdio.h>
/* printd: print n in decimal */
void printd(int n)
{
if (n < 0) {
putchar('-');
n = -n;
}
if (n / 10)
printd(n / 10);
putchar(n % 10 + '0');
}

/* grep: search for regexp in file */
int grep(char *regexp, FILE *f, char *name)
{
int n, nmatch;
char buf[BUFSIZ];
nmatch = 0;
while (fgets(buf, sizeof buf, f) != NULL) {
n = strlen(buf);
if (n > 0 && buf[n-1] == 'n')
buf[n-1] = '0';
if (match(regexp, buf)) {
nmatch++;
if (name != NULL)
printf("%s:", name);
printf("%sn", buf);
}
}
return nmatch;
}
/* matchhere: search for regexp at beginning of text */
int matchhere(char *regexp, char *text)
{
if (regexp[0] == '0')
return 1;
if (regexp[1] == '*')
return matchstar(regexp[0], regexp+2, text);
if (regexp[0] == '$' && regexp[1] == '0')
return *text == '0';
if (*text!='0' && (regexp[0]=='.' || regexp[0]==*text))
return matchhere(regexp+1, text+1);
return 0;
}
/* match: search for regexp anywhere in text */
int match(char *regexp, char *text)
{
if (regexp[0] == '^')
return matchhere(regexp+1, text);
do { /* must look even if string is empty */
if (matchhere(regexp, text))
return 1;
} while (*text++ != '0');
return 0;
}
/* matchstar: search for c*regexp at beginning of text */
int matchstar(int c, char *regexp, char *text)
{
do { /* a * matches zero or more instances */
if (matchhere(regexp, text))
return 1;
} while (*text != '0' && (*text++ == c || c == '.'));
return 0;
}

int atexit(void (*func)(void));

void qsort(
void *base,
size_t nmemb, size_t size,
int (*compar)(
const void *, const void *));

void (*signal(
int sig, void (*func)(int)))(int);

Use procedure arguments to provide
flexibility in an interface.
This technique can greatly simplify
an interface, eliminating a jumble of
parameters that amount to a small
programming language.
Butler W Lampson
"Hints for Computer System Design"

public class HeatingSystem
{
public void turnOn() 
public void turnOff() 

}
public class Timer
{
public Timer(TimeOfDay toExpire, Runnable toDo) 
public void run() 
public void cancel() 

}

Timer on =
new Timer(
timeToTurnOn,
new Runnable()
{
public void run()
{
heatingSystem.turnOn();
}
});
Timer off =
new Timer(
timeToTurnOff,
new Runnable()
{
public void run()
{
heatingSystem.turnOff();
}
});

class Timer
{
public:
Timer(TimeOfDay toExpire, function<void()> toDo);
void Run();
void Cancel();
...
};

Timer on(
timeOn,
bind(&HeatingSystem::TurnOn, &heatingSystem));
Timer off(
timeOff,
bind(&HeatingSystem::TurnOff, &heatingSystem));

public class Timer
{
public Timer(TimeOfDay toExpire, Action toDo) ...
public void Run() ...
public void Cancel() ...
...
}

Timer on = new Timer(timeOn, heatingSystem.TurnOn);
Timer off = new Timer(timeOff, heatingSystem.TurnOff);

Timer on = new Timer(timeOn, heatingSystem::turnOn);
Timer off = new Timer(timeOff, heatingSystem::turnOff);

Timer on =
new Timer(timeOn, () => heatingSystem.TurnOn());
Timer off =
new Timer(timeOff, () => heatingSystem.TurnOff());

Timer on =
new Timer(timeOn, () -> heatingSystem.turnOn());
Timer off =
new Timer(timeOff, () -> heatingSystem.turnOff());

Timer on(
timeOn, [&]() { heatingSystem.TurnOn(); });
Timer off(
timeOff, [&]() { heatingSystem.TurnOff(); });

William Cook, "On Understanding Data Abstraction, Revisited"

newStack =
  (let items = ref() 
{
isEmpty =   #items = 0,
depth =   #items,
push =  x  items := xˆitems,
top =   items0
})

var newStack = function() {
var items = []
return {
isEmpty: function() {
return items.length === 0
},
depth: function() {
return items.length
},
push: function(newTop) {
items = items.unshift(newTop)
},
top: function() {
return items[0]
}
}
}

One of the most powerful mechanisms
for program structuring [...] is the block
and procedure concept.
A procedure which is capable of giving
rise to block instances which survive its
call will be known as a class; and the
instances will be known as objects of
that class.
A call of a class generates a new object
of that class.
Ole-Johan Dahl and C A R Hoare
"Hierarchical Program Structures"

Concatenative programming is so called
because it uses function composition instead of
function application—a non-concatenative
language is thus called applicative.
Jon Purdy
http://evincarofautumn.blogspot.in/2012/02/
why-concatenative-programming-matters.html

This is the basic reason Unix pipes are so
powerful: they form a rudimentary string-based
concatenative programming language.
Jon Purdy
http://evincarofautumn.blogspot.in/2012/02/
why-concatenative-programming-matters.html

Some people, when confronted with a
problem, think, "I know, I'll use threads,"
and then two they hav erpoblesms.
Ned Batchelder
https://twitter.com/#!/nedbat/status/194873829825327104

public class Date implements ...
{
...
public int getYear() ...
public int getMonth() ...
public int getDayInMonth() ...
public void setYear(int newYear) ...
public void setMonth(int newMonth) ...
public void setDayInMonth(int newDayInMonth) ...
...
}

public class Date implements ...
{
...
public int getWeekInYear() ...
public int getDayInYear() ...
public int getDayInWeek() ...
public void setYear(int newYear) ...
public void setMonth(int newMonth) ...
public void setWeekInYear(int newWeek) ...
public void setDayInYear(int newDayInYear) ...
public void setDayInMonth(int newDayInMonth) ...
public void setDayInWeek(int newDayInWeek) ...
...
}

public final class Date implements ...
{
...
public int getWeekInYear() ...
public int getDayInYear() ...
public int getDayInWeek() ...
...
}

When it is not
necessary to
change, it is
necessary not
to change.
Lucius Cary

public final class Date implements ...
{
...
public int year() ...
public int month() ...
public int weekInYear() ...
public int dayInYear() ...
public int dayInMonth() ...
public int dayInWeek() ...
...
}

Mutable
Immutable
Unshared Shared
Unshared mutable
data needs no
synchronisation
Unshared immutable
data needs no
synchronisation
Shared mutable
data needs
synchronisation
Shared immutable
data needs no
synchronisation

Instead of using threads and shared memory
as our programming model, we can use
processes and message passing. Process here
just means a protected independent state
with executing code, not necessarily an
operating system process.
Russel Winder
"Message Passing Leads to Better Scalability in Parallel Systems"

OOP to me means only messaging,
local retention and protection and
hiding of state-process, and
extreme late-binding of all things.
It can be done in Smalltalk and in
LISP. There are possibly other
systems in which this is possible,
but I'm not aware of them.
Alan Kay

Languages such as Erlang (and occam before
it) have shown that processes are a very
successful mechanism for programming
concurrent and parallel systems. Such
systems do not have all the synchronization
stresses that shared-memory, multithreaded
systems have.
Russel Winder
"Message Passing Leads to Better Scalability in Parallel Systems"

http://xkcd.com/224/
We lost the documentation on quantum mechanics.
You'll have to decode the regexes yourself.

Functional Programming You Already Know - Kevlin Henney - Codemotion Rome 2015

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Functional Programming You Already Know - Kevlin Henney - Codemotion Rome 2015