In an earlier post I wrote about a way to implement cons,
car and cdr (also known as pair, first, and second) using
only function definitions and if statements in javascript. That was
pretty mindblowing for me, but it turns out there is an a way to do
without even using branching logic.
``` javascript pair function pair(x, y) { return function(s) { return s(x, y); }; };
The pair function take to arguments X and Y, and returns another
function which takes a function argument S. It is essentially a
promise to apply S to X and Y.
``` javascript first and second
function first(p) {
return p(function(x, y) { return x; });
}
function second(p) {
return p(function(x, y) { return y; });
}
Implementing first and second becomes pretty straightforward; it’s
just a matter of taking the output from a call to pair and applying
it to a function taking two arguments X and Y and returns either X
(first) or Y (second).
The CoffeeScript implementation is incredibly terse and elegant, reading almost like mathematical notation.
coffeescript The CoffeeScript version
pair = (x, y) -> (s) -> s x, y
first = (p) -> p (x, y) -> x
second = (p) -> p (x, y) -> y