Struct Placeholder 

pub struct Placeholder;

A zero-sized type used purely as a marker or placeholder when implementing Higher-Kinded Type (HKT) traits for concrete types.

This struct is essential for the “witness” pattern, where a concrete type (like Option<T>) cannot directly implement an HKT trait due to Rust’s type system limitations. Instead, a Witness type (e.g., OptionWitness) implements the HKT trait, using Placeholder to represent the generic parameters that are being abstracted over.

Examples

use deep_causality_haft::{NoConstraint, HKT, Placeholder};

// A witness for Option<T>
pub struct OptionWitness;

impl HKT for OptionWitness {
    type Constraint = NoConstraint;
    type Type<T> = Option<T>;
}

let my_option: <OptionWitness as HKT>::Type<i32> = Some(10);
assert_eq!(my_option, Some(10));