Struct Codensity 

pub struct Codensity<F: HKT + 'static, A: 'static> { /* private fields */ }

Codensity Monad — the CPS transform of a type constructor F.

Codensity<F, A> ≅ ∀R. (A → F R) → F R

This is the right Kan extension of F along itself (Ran F F A), specialised into a concrete type. The key property: pure, fmap, and chain require no bounds on F — only to_monad needs F: Applicative + Chain.

Use cases

• Monad transformer improvement: wrapping a free monad in Codensity can improve asymptotic performance of left-associated binds.
• CPS conversion: build computations in CPS, then interpret into the target monad via to_monad.

Note: Due to Rust’s GAT limitations (type Of<T> cannot add T: 'static), CodensityF does not implement HKT or Monad. Use inherent methods.

Implementations

impl<F: HKT + 'static, A: 'static> Codensity<F, A>

pub fn pure(a: A) -> Self

Wrap a pure value. No bounds on F required.

pub fn fmap<B: 'static>(self, f: impl Fn(A) -> B + 'static) -> Codensity<F, B>

Map a function over the result. No bounds on F required.

pub fn chain<B: 'static>( self, f: impl Fn(A) -> Codensity<F, B> + 'static, ) -> Codensity<F, B>

Monadic bind. No bounds on F required.

pub fn to_monad(self) -> F::Of<A>

where F: Applicative + Chain,

Collapse the computation into F::Of<A>.

This is the standard way to extract a result from Codensity. Requires F: Applicative + Chain (i.e., F must be a monad).