Worked on draft church-encoding-in-javascript
Signed-off-by: Collin J. Doering <collin.doering@rekahsoft.ca>
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@ -6,11 +6,188 @@ description: Encoding various types using only variables and functions
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tags: general
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---
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TODO
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In this post I will briefly explain the [untyped lambda calculus][], then go on to explain
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[church encoding][]. Following that, we will look at an implementation of church encoding using
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javascript; in this article we will focus on encoding booleans as well as natural numbers, as
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they are the simplest to understand. We will then provide functions to operate on our
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representation as well as functions to marshal to and from our representation to javascripts.
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Along the way we will also cover the topic of [currying][], as it will be used extensively in
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the javascript implementation. In the future I hope to go over pairs, lists, and perhaps some
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other types which are more complicated but for the time being booleans and natural numbers
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should provide a good enough introduction to [church encoding][].
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TLDR: Using only functions of one argument and variables (and assuming infinite space), one can
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compute any computable function. Here we will implement booleans and natural numbers using only
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functions and variable bindings to functions; we also implement functions to operate on those
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types (Eg. and, or, xor, not, add, subtract, multiply, etc…).
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<!--more-->
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TODO
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As I was learning church encoding and lambda calculus I implemented church encodings of various
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types along with functions that operate on those types. I did this in a few of my favorite
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languages; namely in [Haskell](https://www.haskell.org/), [Racket](https://www.haskell.org/),
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and [Lazy Racket](http://docs.racket-lang.org/lazy/index.html), though the implementations were
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to varying degrees of completion. Now many Haskell and Racket programmers are more likely to be
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aware of the lambda calculus and church encoding. However, I think most javascript programmers
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likely have never heard of the concept and would benefit by taking a moment to bend their
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minds.
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Before we get started into the 'meat' of this article, I need to explain curried functions as
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they are used extensively in the javascript implementation of church encoded booleans and
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natural numbers. If you already know what a curried function is, feel free to skip this
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section.
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Suppose you are given the challenge of writing a function that takes two arguments and returns
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its first. This is easy in every language, and javascript is no exception. Now consider how
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you would do it if you had the following restrictions:
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1. You can only use lambda functions and variables
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2. Lambda functions can only take one argument. It cannot be an object or array with some
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expected structure. For example, accepting an array and expecting the arguments to be at
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index 0 and 1 respectively is considered a violation.
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At first glance this may seem like a impossible task, but no need to fret. The solution: write
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a function that accepts the first argument but then returns a function which will accept the
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second argument and throw it away.
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``` {.javascript .code-term .numberLines }
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var lconst = function (x) {
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var ret = function {y) {
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return x;
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};
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return ret;
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};
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```
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On line 2-5 we use a variable `ret` to temporarily store a function to accept the 'second'
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argument; we then return this function. The function itself closes over the argument to the
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outer function and returns it. I could have written `return function (y) { return x; };`
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instead of lines 1-3 but prefer the added readability of newlines; but to do this and avoid
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javascripts automatic semicolon insertion we have to temporarily store the function in a
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variable. This is because semicolons are inserted automatically after return statements in
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javascript. For more details on javascripts automatic semicolon insertion see section 7.9.1 of the
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[ECMAScript 2015 language specification](http://www.ecma-international.org/publications/standards/Ecma-262.htm).
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Now lets see our `lconst` function in action:
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``` {.javascript .code-term}
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lconst(1)(2); // 1
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lconst("Alonzo")("Church"); // "Alonzo"
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var alwaysZero = lconst(0); // undefined
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alwaysZero(1); // 0
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alwaysZero("Alonzo"); // 0
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```
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TODO: Finish explanation about curried functions
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Now that we have gone over curried functions, we need to take a gander at the lambda calculus
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before we get started with church encoding and its implementation in javascript. Here I will
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describe it in terms of javascript, but I highly recommend taking the time to read the wiki
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page on the subject as it does a better job describing it then I can. Particularly, my
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description of the reduction rules are quite rough and lack rigor, but a full description of
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the lambda calculus is beyond the scope of this article.
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The lambda calculus' syntax can be given by three simple rules. A lambda term is any
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one of the following:
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Variable
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~ A single letter symbol (Eg. `x`)
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Lambda Abstraction
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~ if `t` is a lambda term and `x` is a variable, then `λx.t` is a lambda term
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Application
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~ if `t` and `s` are lambda terms, then `(t s)` is a lambda term
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Now that we can specify a lambda term we now have to talk about how they can be reduced. There
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are three types of reduction.
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α-conversion (alpha)
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~ Also called alpha-renaming, allows bound variable names to be changed. For example,
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`λx.λy.x` can be alpha converted to `λa.λb.a` or `λy.λx.y` among others. By changing the
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bound variables, the original meaning of the lambda expression is retained. That is, both
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`λx.λy.x` and `λa.λb.a` both can be thought of as curried functions of two arguments that
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return their first argument.
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β-reduction (beta)
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~ TODO
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η-conversion (eta)
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~ TODO
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When a two lambda terms can be reduced to the same expression by α-conversion we say they are
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α-equivalent. β-equivalence and η-equivalence can be defined similarly.
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TODO: Talk about lambda term reduction α-conversion (alpha), β-reduction (beta), η-conversion (eta)
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Some lambda terms are used commonly, and have standard names. Here are some examples
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- The `id` function (identity), that is a function that returns its argument. In the lambda
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calculus its given as `λx.x` whose corresponding javascript is as follows.
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``` {.javascript .code-term }
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function id (x) {
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return x;
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};
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```
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- The `const` function, which is a curried function of two arguments that returns its first
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argument. It is given in the lambda calculus as `λx.λy.x` and can be written in javascript as
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follows. Note however, that the function given in javascript can not have the name `const` as
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its a reserved word in javascript; instead we use `lconst`.
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TODO reference lconst js definition above
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- Similar to the `const` function but with no standard name, what I'll call `constid` is a
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curried function of two arguments that returns its second argument (unlike `const` which
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returns the first one). It is given by `λx.λy.y` in the lambda calculus and can be written in
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javascript as follows.
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``` {.javascript .code-term}
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var constid = function (x) {
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var ret = function (y) {
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return y;
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};
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return ret;
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};
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```
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Notice that the `constid` function can be obtained by applying `const` to `id` in the lambda
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calculus (`const id`). Similarly in javascript, the `lconst` function can be applied to the
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`id` to obtain `constid`.
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``` {.javascript .code-term}
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var constid = lconst(id);
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```
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Now by this point you are likely scratching your head a little. How in the world can just
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functions and variable bindings alone allow us to represent booleans, and nature numbers; let alone
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have the ability to express any turing computable function.
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((λx.λy.x) id)
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≡ (λy.id)
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≡ (λy.λa.a)
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≡ constid
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``` {.javascript .code-term .numberLines}
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/**
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@ -397,3 +574,7 @@ for (i = 0; i < 10; i += 1) {
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Find a way to include source files without having to copy-paste them into the article.
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See: http://stackoverflow.com/questions/21584396/pandoc-include-files-filter-in-haskell
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-->
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[untyped lambda calculus]: https://en.wikipedia.org/wiki/Lambda_calculus
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[church encoding]: https://en.wikipedia.org/wiki/Church_encoding
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[currying]: https://en.wikipedia.org/wiki/Currying
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