lambda_calculus
lambda_calculus is a simple implementation of the untyped lambda calculus in Rust.
The library tries to find a compromise between the spirit of the lambda calculus and Rust's
best practices; the lambda Term
s implemented by the library are produced by functions (in order
to allow arbitrary application), but they are not Copy
able and the methods they provide allow
memory-friendly disassembly and referencing their internals.
Documentation
Features
- a parser for lambda expressions
- 7 β-reduction strategies with optional display of reduction steps
- standard terms (combinators)
- Church numerals and arithmetic operations
- Church booleans
- Church pairs
- Church lists
The terms are implemented using De Bruijn indices, but are displayed using the classic lambda notation and can be parsed both ways.
The data and operators follow the Church encoding. Library functions utilizing the fixed-point combinator use its call-by-value variant and are built for compatibility with as many β-reduction strategies as possible. The bodies of functions are normalized for maximum performance.
Installation
Include the library by adding the following to your Cargo.toml:
[dependencies]
lambda_calculus = "^1.0"
And the following to your code:
#[macro_use]
extern crate lambda_calculus;
Compilation features:
backslash_lambda
: changes the display of lambdas fromλ
to\
no_church
: doesn't build thechurch
module; useful if you want to implement it on your own or introduce a different data encoding
To apply a feature setup the dependency in your Cargo.toml like this:
[dependencies.lambda_calculus]
version = "^1.0"
features = ["no_church"]
Usage
Comparing classic and De Bruijn index notation
code:
use lambda_calculus::arithmetic::{succ, pred};
fn main() {
println!("SUCC := {0} = {0:?}", succ());
println!("PRED := {0} = {0:?}", pred());
}
stdout:
SUCC := λa.λb.λc.b (a b c) = λλλ2(321)
PRED := λa.λb.λc.a (λd.λe.e (d b)) (λd.c) (λd.d) = λλλ3(λλ1(24))(λ2)(λ1)
Parsing lambda expressions
code:
use lambda_calculus::parser::*;
fn main() {
assert_eq!(parse(&"λa.λb.λc.b (a b c)", Classic), parse(&"λλλ2(321)", DeBruijn));
}
Showing β-reduction steps
code:
use lambda_calculus::reduction::*;
use lambda_calculus::arithmetic::pred;
fn main() {
let mut expr = app!(pred(), 1.into());
expr.beta(NOR, 0, true);
}
stdout:
β-reducing (λa.λb.λc.a (λd.λe.e (d b)) (λd.c) (λd.d)) (λa.λb.a b) [normal order]:
1. (λa.λb.λc.a (λd.λe.e (d b)) (λd.c) (λd.d)) (λa.λb.a b)
=> λa.λb.(λc.λd.c d) (λc.λd.d (c a)) (λc.b) (λc.c)
2. (λc.λd.c d) (λc.λd.d (c a))
=> λc.(λd.λe.e (d a)) c
3. (λc.(λd.λe.e (d a)) c) (λc.b)
=> (λc.λd.d (c a)) (λc.b)
4. (λc.λd.d (c a)) (λc.b)
=> λc.c ((λd.b) a)
5. (λc.c ((λd.b) a)) (λc.c)
=> (λc.c) ((λc.b) a)
6. (λc.c) ((λc.b) a)
=> (λc.b) a
7. (λc.b) a
=> b
result after 7 reductions: λa.λb.b
Comparing the number of steps of different reduction strategies
code:
use lambda_calculus::reduction::*;
use lambda_calculus::arithmetic::fac;
fn main() {
let expr = app!(fac(), 4.into());
compare(&expr, &[NOR, APP, HNO, HAP], false); // compare normalizing strategies
}
stdout:
comparing β-reduction strategies for (λa.a (λb.λc.λd.b (λe.c (d e)) (λe.λf.e (d e f))) (λb.λc.b) (λb.λc.b c) (λb.λc.b c)) (λa.λb.a (a (a (a b)))):
normal: 87
applicative: 65
hybrid normal: 87
hybrid applicative: 40
TODO
- a few more tests
- more independent tests (less integration)
- further optimizations