Functional
Programming
in
5 Minutes
Which is impossible
So let's rather try...
Learn to Curry
in
5 Minutes
JSDC.tw 2013 Lightning Talk
Gias Kay Lee
Currying.
Perhaps you've been told by someone before that this is one of those "advanced JavaScript techniques" you'll need to learn to become a true JavaScript ninja.
But the truth is...
http://commons.wikimedia.org/wiki/File:Phanaeng_kai.jpg
Currying,
is a basic functional programming technique.
And in order to become a true JavaScript ninja,
what you really need to master is the art of
Functional Programming.
Lisp
Scheme
Functional
Smalltalk
Self
Object-oriented
has half of its root in it.
Because JavaScript
Let's start with something
familiar
λ Function
AKA Anonymous Function
originated from a mathematical system called
λ Calculus
λ Calculus
is about simplifying mathematical expressions into
λ Expressions
to unveil the true nature of underlying computations
To achieve this simplification
Functions
cube(x) = x * x * x
are expressed without names
(x) → x * x * x
...and can only have
1 argument
To handle a
multi-argument function
it needs to first go through the process of...
Currying (n.)
A technique of transforming a multi-argument function in such a way that it can be called as a chain of functions, each with a single argument.
And the name comes from...
http://commons.wikimedia.org/wiki/File:Curry_Ist.jpg
Haskell Curry
(1900 – 1982)
also immortalized as the name of
the popular pure functional language
Haskell
http://en.wikipedia.org/wiki/File:Haskell-Logo.svg
So
(x, y) → x + y
can be curried into
x → (y → x + y)
Or, in classical notation
f(x, y) = x + y
can be curried into
g(x) = y → f(x, y)
Still very confusing?
Let's speak JavaScript!
a multi-argument function
function(x, y) {
return x + y;
}
when curried, will become
a chain of functions, each
with a single argument
function(x) {
return function(y) {
return x + y;
};
}
if we give them names
function f(x, y) {
return x + y;
}
then...
function g(x) {
return function(y) {
return x + y;
};
}
f(1, 2) === g(1)(2);
now, since functions are
first-class citizens...
we can also do it
like this
function g(h) {
return function(y) {
return h(h(y));
};
}
81 === g(function(x) {
return x * x;
})(3);
function g(h) {
return function(y) {
return h(h(y));
};
}
if we only supply
one of the arguments
to the curried function...
var n = g(function(x) {
return x * x;
});
we'll be able to reuse it
like this
337 === n(3) + n(4);
This is called
Partial Application
and is not the same with currying
actually, for functions with
an arity greater than 2
function f(x, y, z) {
return x + y + z;
}
the correct way to
partially apply is through
bind()
var f1 = f.bind(null, 1);
call to a partially applied
function returns the result,
not another function down
the currying chain
6 === f1(2, 3);
Now I guess we're
running low on time
so...
One last bit
that the reverse process of currying
uncurrying
is closely related with
a more familiar concept
apply.
Questions?
Nah, we're running outta time. Seriously.
This slide can be found on
slid.es/gsklee
Functional Programming in 5 Minutes
By G. Kay Lee
