JavaScript in a Single Arrow

Lambda (𝛌) Calculus,

• Mathematical formalism
• Invented by this hero (Alonzo Church) starting 1932:
• Universal model of computation (Turing-complete) (NBD)

What is Lamda Calculus ?

	** Lambda function (𝛌) **	** Arrow function (=>) **
used for	thinking	programming
inputs	1	0+
outputs	1	0+
side effects?	no way!	maybe

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If we treat => like 𝛌, what can we do with it?

	** Lambda function (𝛌) **	** Arrow function (=>)**
making one ("abstraction")	λx.x	x => x
faking multiple args	λx.λy.x+y	(x, y) => x + y x => y => x + y
using one ("application")	(λx.λy.x+y) 5 1 5+1 6	(x => y => x + y)(5)(1) 5+1 6

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Numbers

What can we count if all we have are functions?

Yes, you can define numbers (and indeed, arbitrary data types) inside the lambda calculus. Here's the idea. -adding numbers in LC

The simplest numbers to work with are the natural numbers: 0, 1, 2, 3, and so on.

Peano Axioms describes this as follows:

• 0 is a natural number
• if n a natural number, then Sn is a natural number.

S denotes the successor of n or n+1.

First few Peano natural numbers

0, S0, SS0, SSS0, and so on - it's a unary representation.

We can represent function applications in lamda calculus, hence, we can easily represent Sn, however the problem is that we don't know how to represent 0 and S themselves.

TLDR: We can count function applications (calls)!

Let's write x for the 0 we're given, and f for the S we're given. Then we can represent the first few numbers as follows:

• zero 'λfx.x' => f => x => x
• one 'λfx.fx' => f => x => f(x)
• two 'λfx.f(fx)' => f => x => f(f(x))

Higher Numbers

Successor function : given a number,get the next number

var nextn = n => f => x => f(n(f)(x));
var four = nextn(three);
var five = nextn(nextn(three));
var six = nextn(nextn(nextn(three)));
var seven = nextn(nextn(nextn(nextn(three))));

Arithmetic

How can we add two numbers n and m, when numbers are call-counters?

Call the function n times, then call it m more times!

var add = n => m => f => x => m(f)(n(f)(x));

What about multiplying n and m?

var mul = n => m => f => x => m(n(f))(x);

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Booleans! Conditionals!

<boolean> ? <then do this> : <else do this>

var ifThenElse = bool => thn => els => bool(thn)(els);
var troo = thn => els => thn; 
var falz = thn => els => els;

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Logic

• not bool => thn => els => bool(els)(thn)
• or A => B => A(A)(B)
• and A => B => A(B)(A)

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Datastructures

• list
• pairs
• fibbo

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Acomplishments

• Data
  • (natural) numbers
  • booleans
• Arithmetic
• Logic & Control flow

Representing data this way is called...

Church Encoding

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Repl.it

<iframe height="400px" width="100%" src="https://repl.it/@aregee/lamdajs?lite=true" scrolling="no" frameborder="no" allowtransparency="true" allowfullscreen="true" sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"></iframe>

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There's so much more we could do!

• Subtract, Divide
• Successor, Predecessor
• Predicates (e.g. isZero, isEven, ...)
• (In)equality
• Strings (as lists of characters represented by their char codes)
• Lists (as nested pairs) & list manipulations (e.g. map, reduce, filter)
• ...y'know, all of computation

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Learning Resources

The Universe in a Single Arrow

Lambda Talk

JavScript MDN

Asynchronous programming

Promises in JavaScript

More JavaScript

Functional Programming With JavaScript

Structure and Interpretation of Computer Programs

The birth and death of JavaScript

JavaScript at Innovaccer