Crate trait_eval

Compile-Time Evaluation

We all know Rust's trait system is Turing complete, so tell me, why aren't we exploiting this???

Who needs const-fn when we've got a crate like this?!

Example

Here's an eminently readable example where we play FizzBuzz at compile-time!

trait FizzBuzzType {
    fn show() -> String; // Don't worry about this -- it's just so we can print the result
}

struct Fizz;

impl FizzBuzzType for Fizz {
    fn show() -> String {
        "Fizz".to_string()
    }
}

struct Buzz;

impl FizzBuzzType for Buzz {
    fn show() -> String {
        "Buzz".to_string()
    }
}

struct FizzBuzz;

impl FizzBuzzType for FizzBuzz {
    fn show() -> String {
        "FizzBuzz".to_string()
    }
}

impl<T: Nat> FizzBuzzType for T
where
    T: Eval,
    <T as Eval>::Output: Display,
{
    fn show() -> String {
        format!("{}", T::eval())
    }
}

trait FizzBuzzEval: Nat {
    type Result: FizzBuzzType;
}

impl<T: Nat,
    Mod3: Nat,
    Mod5: Nat,
    ShouldFizz: Bool,
    ShouldBuzz: Bool,
    ShouldFizzBuzz: Bool,
    DidBuzz: FizzBuzzType,
    DidFizz: FizzBuzzType,
    DidFizzBuzz: FizzBuzzType> FizzBuzzEval for T
where
    T: Mod<Three, Result = Mod3> + Mod<Five, Result = Mod5>,
    Mod3: Equals<Zero, Result = ShouldFizz>,
    Mod5: Equals<Zero, Result = ShouldBuzz>,
    ShouldFizz: AndAlso<ShouldBuzz, Result = ShouldFizzBuzz>,
    (Fizz, T): If<ShouldFizz, Result = DidFizz>,
    (Buzz, DidFizz): If<ShouldBuzz, Result = DidBuzz>,
    (FizzBuzz, DidBuzz): If<ShouldFizzBuzz, Result = DidFizzBuzz>,
{
    type Result = DidFizzBuzz;
}

assert_eq!(<One as FizzBuzzEval>::Result::show(), "1");
assert_eq!(<Two as FizzBuzzEval>::Result::show(), "2");
assert_eq!(<Three as FizzBuzzEval>::Result::show(), "Fizz");
assert_eq!(<Four as FizzBuzzEval>::Result::show(), "4");
assert_eq!(<Five as FizzBuzzEval>::Result::show(), "Buzz");
assert_eq!(<Six as FizzBuzzEval>::Result::show(), "Fizz");
assert_eq!(<Seven as FizzBuzzEval>::Result::show(), "7");
assert_eq!(<Eight as FizzBuzzEval>::Result::show(), "8");
assert_eq!(<Nine as FizzBuzzEval>::Result::show(), "Fizz");
assert_eq!(<Ten as FizzBuzzEval>::Result::show(), "Buzz");

type Fifteen = <Three as Times<Five>>::Result;
assert_eq!(<Fifteen as FizzBuzzEval>::Result::show(), "FizzBuzz"); // !!!

Structs

False	False (false)
Succ	Peano-style increment operator
True	True (true)
Zero	Constant zero (0)

Traits

AndAlso	Logical And
Bool	The bool type (true, false)
Equals	Equality testing
Eval	trait_eval to Rust conversion
Fact	Factorial!
Fib	Built-in Fibonacci sequence
If	Conditional execution!
LessThan	Integer comparison
Minus	Saturating Subtraction
Mod	Remainders
Nat	The type of natural numbers (0..)
Not	Logical Not
OrElse	Logical Or
Plus	Addition
Pred	Saturating Decrement
Times	Multiplication

Type Definitions

Eight	Constant three (8)
Five	Constant three (5)
Four	Constant three (4)
Nine	Constant three (9)
One	Constant one (1)
Seven	Constant three (7)
Six	Constant three (6)
Ten	Constant three (10)
Three	Constant three (3)
Two	Constant two (2)