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220 lines
5.8 KiB
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220 lines
5.8 KiB
Markdown
---
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category: Algorithms & Data Structures
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name: Lambda Calculus
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contributors:
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- ["Max Sun", "http://github.com/maxsun"]
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- ["Yan Hui Hang", "http://github.com/yanhh0"]
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---
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# Lambda Calculus
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Lambda calculus (λ-calculus), originally created by
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[Alonzo Church](https://en.wikipedia.org/wiki/Alonzo_Church),
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is the world's smallest programming language.
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Despite not having numbers, strings, booleans, or any non-function datatype,
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lambda calculus can be used to represent any Turing Machine!
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Lambda calculus is composed of 3 elements: **variables**, **functions**, and
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**applications**.
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| Name | Syntax | Example | Explanation |
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|-------------|------------------------------------|-----------|-----------------------------------------------|
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| Variable | `<name>` | `x` | a variable named "x" |
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| Function | `λ<parameters>.<body>` | `λx.x` | a function with parameter "x" and body "x" |
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| Application | `<function><variable or function>` | `(λx.x)a` | calling the function "λx.x" with argument "a" |
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The most basic function is the identity function: `λx.x` which is equivalent to
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`f(x) = x`. The first "x" is the function's argument, and the second is the
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body of the function.
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## Free vs. Bound Variables:
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- In the function `λx.x`, "x" is called a bound variable because it is both in
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the body of the function and a parameter.
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- In `λx.y`, "y" is called a free variable because it is never declared before hand.
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## Evaluation:
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Evaluation is done via
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[β-Reduction](https://en.wikipedia.org/wiki/Lambda_calculus#Beta_reduction),
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which is essentially lexically-scoped substitution.
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When evaluating the
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expression `(λx.x)a`, we replace all occurrences of "x" in the function's body
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with "a".
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- `(λx.x)a` evaluates to: `a`
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- `(λx.y)a` evaluates to: `y`
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You can even create higher-order functions:
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- `(λx.(λy.x))a` evaluates to: `λy.a`
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Although lambda calculus traditionally supports only single parameter
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functions, we can create multi-parameter functions using a technique called
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[currying](https://en.wikipedia.org/wiki/Currying).
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- `(λx.λy.λz.xyz)` is equivalent to `f(x, y, z) = ((x y) z)`
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Sometimes `λxy.<body>` is used interchangeably with: `λx.λy.<body>`
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----
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It's important to recognize that traditional **lambda calculus doesn't have
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numbers, characters, or any non-function datatype!**
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## Boolean Logic:
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There is no "True" or "False" in lambda calculus. There isn't even a 1 or 0.
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Instead:
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`T` is represented by: `λx.λy.x`
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`F` is represented by: `λx.λy.y`
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First, we can define an "if" function `λbtf` that
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returns `t` if `b` is True and `f` if `b` is False
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`IF` is equivalent to: `λb.λt.λf.b t f`
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Using `IF`, we can define the basic boolean logic operators:
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`a AND b` is equivalent to: `λab.IF a b F`
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`a OR b` is equivalent to: `λab.IF a T b`
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`NOT a` is equivalent to: `λa.IF a F T`
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*Note: `IF a b c` is essentially saying: `IF((a b) c)`*
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## Numbers:
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Although there are no numbers in lambda calculus, we can encode numbers using
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[Church numerals](https://en.wikipedia.org/wiki/Church_encoding).
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For any number n: <code>n = λf.f<sup>n</sup></code> so:
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`0 = λf.λx.x`
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`1 = λf.λx.f x`
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`2 = λf.λx.f(f x)`
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`3 = λf.λx.f(f(f x))`
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To increment a Church numeral,
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we use the successor function `S(n) = n + 1` which is:
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`S = λn.λf.λx.f((n f) x)`
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Using successor, we can define add:
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`ADD = λab.(a S)b`
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**Challenge:** try defining your own multiplication function!
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## Get even smaller: SKI, SK and Iota
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### SKI Combinator Calculus
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Let S, K, I be the following functions:
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`I x = x`
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`K x y = x`
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`S x y z = x z (y z)`
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We can convert an expression in the lambda calculus to an expression
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in the SKI combinator calculus:
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1. `λx.x = I`
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2. `λx.c = Kc` provided that `x` does not occur free in `c`
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3. `λx.(y z) = S (λx.y) (λx.z)`
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Take the church number 2 for example:
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`2 = λf.λx.f(f x)`
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For the inner part `λx.f(f x)`:
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```
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λx.f(f x)
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= S (λx.f) (λx.(f x)) (case 3)
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= S (K f) (S (λx.f) (λx.x)) (case 2, 3)
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= S (K f) (S (K f) I) (case 2, 1)
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```
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So:
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```
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2
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= λf.λx.f(f x)
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= λf.(S (K f) (S (K f) I))
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= λf.((S (K f)) (S (K f) I))
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= S (λf.(S (K f))) (λf.(S (K f) I)) (case 3)
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```
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For the first argument `λf.(S (K f))`:
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```
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λf.(S (K f))
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= S (λf.S) (λf.(K f)) (case 3)
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= S (K S) (S (λf.K) (λf.f)) (case 2, 3)
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= S (K S) (S (K K) I) (case 2, 3)
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```
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For the second argument `λf.(S (K f) I)`:
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```
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λf.(S (K f) I)
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= λf.((S (K f)) I)
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= S (λf.(S (K f))) (λf.I) (case 3)
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= S (S (λf.S) (λf.(K f))) (K I) (case 2, 3)
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= S (S (K S) (S (λf.K) (λf.f))) (K I) (case 1, 3)
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= S (S (K S) (S (K K) I)) (K I) (case 1, 2)
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```
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Merging them up:
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```
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2
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= S (λf.(S (K f))) (λf.(S (K f) I))
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= S (S (K S) (S (K K) I)) (S (S (K S) (S (K K) I)) (K I))
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```
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Expanding this, we would end up with the same expression for the
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church number 2 again.
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### SK Combinator Calculus
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The SKI combinator calculus can still be reduced further. We can
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remove the I combinator by noting that `I = SKK`. We can substitute
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all `I`'s with `SKK`.
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### Iota Combinator
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The SK combinator calculus is still not minimal. Defining:
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```
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ι = λf.((f S) K)
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```
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We have:
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```
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I = ιι
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K = ι(ιI) = ι(ι(ιι))
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S = ι(K) = ι(ι(ι(ιι)))
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```
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## For more advanced reading:
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1. [A Tutorial Introduction to the Lambda Calculus](http://www.inf.fu-berlin.de/lehre/WS03/alpi/lambda.pdf)
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2. [Cornell CS 312 Recitation 26: The Lambda Calculus](http://www.cs.cornell.edu/courses/cs3110/2008fa/recitations/rec26.html)
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3. [Wikipedia - Lambda Calculus](https://en.wikipedia.org/wiki/Lambda_calculus)
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4. [Wikipedia - SKI combinator calculus](https://en.wikipedia.org/wiki/SKI_combinator_calculus)
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5. [Wikipedia - Iota and Jot](https://en.wikipedia.org/wiki/Iota_and_Jot)
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