Rust by Example

use std::marker::PhantomData; // Generic tuplestruct with same types in both // generic <A> and definition (A,) struct Tuple<A>(A,); // This tuple is a phantom type. B is a hidden // parameter. Storage is allocated for generic type A // yet not for B. Therefore, B cannot be used in computations. #[derive(PartialEq)] // Allow equality test for this type struct PhantomTuple<A, B>(A,PhantomData<B>); // Similarly, a phantom type struct which is generic over A // with hidden parameter B #[derive(PartialEq)] // Allow equality test for this type struct PhantomStruct<A, B> { first: A, phantom: PhantomData<B> } fn main() { // instantiate Tuple let _tuple: Tuple<char> = Tuple('R'); // We can create similar types without carrying around extra info // PhantomTuple specialized to <char, f32> let _tuple1: PhantomTuple<char, f32> = PhantomTuple('Q', PhantomData); // PhantomTuple specialized to <char, f64> let _tuple2: PhantomTuple<char, f64> = PhantomTuple('Q', PhantomData); // Error: type mismatch so these cannot be compared //println!("_tuple1 == _tuple2 yields: {}", // _tuple1 == _tuple2); // Specialized to <char, f32> let _struct1: PhantomStruct<char, f32> = PhantomStruct { first: 'Q', phantom: PhantomData, }; // Specialized to <char, f64> let _struct2: PhantomStruct<char, f64> = PhantomStruct { first: 'Q', phantom: PhantomData, }; // Error: type mismatch so these cannot be compared //println!("_struct1 == _struct2 yields: {}", // _struct2 == _struct2); }