Beta reducer and shorthand representer for the λ-calculus

## Project description

This tool helps you β-reduce terms in the λ-calculus by providing you with a REPL:

λ> (λc.c(λx.λy.x))((λx.λy.λf.fxy)(λf.λx.f(fx))(λf.λx.f(f(fx))))
INPUT (λc.c(λx.λy.x))((λx.λy.λf.fxy)(λf.λx.f(fx))(λf.λx.f(f(fx))))
β ==> (λx.λy.λf.fxy)(λf.λx.f(fx))(λf.λx.f(f(fx)))(λx.λy.x)
β ==> (λy.λf.f(λf.λx.f(fx))y)(λf.λx.f(f(fx)))(λx.λy.x)
β ==> (λf.f(λf.λx.f(fx))(λf.λx.f(f(fx))))(λx.λy.x)
β ==> (λx.λy.x)(λf.λx.f(fx))(λf.λx.f(f(fx)))
β ==> (λy.λf.λx.f(fx))(λf.λx.f(f(fx)))
β ==> λf.λx.f(fx)

Potential shorthand representations:
-> As Church numeral 2


## Input

Notice the pretty unicode λ in the input: this REPL uses Readline to input a λ when you type backslash (\).

## Shorthands

You can start any ‘Monty Python’ routine and people finish it for you. Everyone knows it like shorthand.

—Robin Williams

Notice above the input was converted to its corresponding shorthand Church numeral at the end. This interpreter has the notion of shorthand notations:

1. Shorthands are written in braces ({}) and are case insensitive.
2. Church numerals, as well as many other common shorthands, are predefined for you.
3. Braces can be omitted on church numerals, as digits are not allowed to be used as variables.
4. Use = operator to define a shorthand.
λ> {ABC}=λf.λx.x
λ> {ABC}
INPUT λf.λx.x

Potential shorthand representations:
-> As Church numeral 0
-> As {FALSE}
-> As {ABC}
λ> {IF}{FALSE}1{ABC}
INPUT (λp.λa.λb.pab)(λx.λy.y)(λf.λx.fx)(λf.λx.x)
β ==> (λa.λb.(λx.λy.y)ab)(λf.λx.fx)(λf.λx.x)
β ==> (λb.(λx.λy.y)(λf.λx.fx)b)(λf.λx.x)
β ==> (λx.λy.y)(λf.λx.fx)(λf.λx.x)
β ==> (λy.y)(λf.λx.x)
β ==> λf.λx.x

Potential shorthand representations:
-> As Church numeral 0
-> As {FALSE}
-> As {ABC}


### Builtin Shorthands

Besides a countably infinite number of Church numerals, the following shorthands come builtin to the interpreter:

{succ}=λn.λf.λx.f(nfx)
{true}=λx.λy.x
{false}=λx.λy.y
{and}=λp.λq.pqp
{or}=λp.λq.ppq
{not}=λp.p{false}{true}
{if}=λp.λa.λb.pab
{cons}=λx.λy.λf.fxy
{car}=λc.c{true}
{cdr}=λc.c{false}
{nil}=λx.{true}
{pred}=λn.λf.λx.n(λg.λh.h(gf))(λu.x)(λu.u)
{sub}=λm.λn.n{pred}m
{zero?}=λn.n(λx.{false}){true}
{nil?}=λp.p(λx.λy.{false})
{lte?}=λm.λn.{zero?}({sub}mn)


## Bugs

None. Mutts have fleas, not bugs.

—Mutt Manual Page

• {pred} (and, consequently, {sub}) appear to have some sort of issue.
• When variables conflict as a result of application, they will display as the same variable, but the interpreter will still treat them as separate variables. What needs to be done is some sort of detection mechanism when this happens, and an automatic α-rename of the variable.

If you solve either of these, a PR is much appreciated!

## Project details

This version 0.1

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