Struct Thunk 

pub struct Thunk<'a, A>(/* private fields */);

A deferred computation that produces a value of type A.

Thunk is NOT memoized - each call to Thunk::evaluate re-executes the computation. This type exists to build computation chains without allocation overhead.

Unlike Trampoline, Thunk does NOT require 'static and CAN implement HKT traits like Functor, Semimonad, etc.

Higher-Kinded Type Representation

The higher-kinded representation of this type constructor is ThunkBrand, which is fully polymorphic over the result type.

Trade-offs vs Trampoline

Aspect	Thunk<'a, A>	Trampoline<A>
HKT compatible	✅ Yes	❌ No (requires 'static)
Stack-safe	⚠️ Partial (tail_rec_m only)	✅ Yes (unlimited)
Lifetime	'a (can borrow)	'static only
Use case	Glue code, composition	Deep recursion, pipelines

Algebraic Properties

Thunk is a proper Monad:

• pure(a).evaluate() == a (left identity).
• thunk.bind(pure) == thunk (right identity).
• thunk.bind(f).bind(g) == thunk.bind(|a| f(a).bind(g)) (associativity).

Limitations

Cannot implement Traversable: Thunk wraps Box<dyn FnOnce() -> A>, which cannot be cloned because FnOnce is consumed when called. The Traversable trait requires Clone bounds on the result type (to build the output structure), making it fundamentally incompatible with Thunk’s design. This is an intentional trade-off: Thunk prioritizes zero-overhead deferred execution and lifetime flexibility over structural cloning.

Implemented typeclasses:

• ✅ Functor, Foldable, Semimonad/Monad, Semiapplicative/Applicative
• ❌ Traversable (requires Clone)

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.

Fields

• 0: The closure that performs the computation.

Implementations

impl<'a, A: 'a> Thunk<'a, A>

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.

pub fn new(f: impl FnOnce() -> A + 'a) -> Self

Creates a new Thunk from a thunk.

Type Signature

forall A. (() -> A) -> Thunk A

Parameters

• f: The thunk to wrap.

Returns

A new Thunk instance.

Examples

use fp_library::types::*;

let thunk = Thunk::new(|| 42);
assert_eq!(thunk.evaluate(), 42);

pub fn pure(a: A) -> Self

where A: 'a,

Returns a pure value (already computed).

Type Signature

forall A. A -> Thunk A

Parameters

• a: The value to wrap.

Returns

A new Thunk instance containing the value.

Examples

use fp_library::{
	brands::*,
	classes::*,
	functions::*,
};

let thunk = pure::<ThunkBrand, _>(42);
assert_eq!(thunk.evaluate(), 42);

pub fn defer(f: impl FnOnce() -> Thunk<'a, A> + 'a) -> Self

Defers a computation that returns a Thunk.

Type Signature

forall A. (() -> Thunk A) -> Thunk A

Parameters

• f: The thunk that returns a Thunk.

Returns

A new Thunk instance.

Examples

use fp_library::{
	brands::*,
	functions::*,
	types::*,
};

let thunk = Thunk::defer(|| pure::<ThunkBrand, _>(42));
assert_eq!(thunk.evaluate(), 42);

pub fn bind<B: 'a>(self, f: impl FnOnce(A) -> Thunk<'a, B> + 'a) -> Thunk<'a, B>

Monadic bind: chains computations.

Note: Each bind adds to the call stack. For deep recursion, use Trampoline instead.

Type Signature

forall A B. (Thunk A, A -> Thunk B) -> Thunk B

Type Parameters

• B: The type of the result of the new computation.

Parameters

• self: The thunk instance.
• f: The function to apply to the result of the computation.

Returns

A new Thunk instance representing the chained computation.

Examples

use fp_library::{
	brands::*,
	functions::*,
};

let thunk = pure::<ThunkBrand, _>(21).bind(|x| pure::<ThunkBrand, _>(x * 2));
assert_eq!(thunk.evaluate(), 42);

pub fn map<B: 'a>(self, f: impl FnOnce(A) -> B + 'a) -> Thunk<'a, B>

Functor map: transforms the result.

Type Signature

forall A B. (Thunk A, A -> B) -> Thunk B

Type Parameters

• B: The type of the result of the transformation.

Parameters

• self: The thunk instance.
• f: The function to apply to the result of the computation.

Returns

A new Thunk instance with the transformed result.

Examples

use fp_library::{
	brands::*,
	functions::*,
};

let thunk = pure::<ThunkBrand, _>(21).map(|x| x * 2);
assert_eq!(thunk.evaluate(), 42);

pub fn evaluate(self) -> A

Forces evaluation and returns the result.

Type Signature

forall A. Thunk A -> A

Returns

The result of the computation.

Examples

use fp_library::{
	brands::*,
	functions::*,
};

let thunk = pure::<ThunkBrand, _>(42);
assert_eq!(thunk.evaluate(), 42);

Trait Implementations

impl<'a, A: 'a> Deferrable<'a> for Thunk<'a, A>

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.

fn defer(f: impl FnOnce() -> Self + 'a) -> Self

where Self: Sized,

Creates a Thunk from a computation that produces it.

Type Signature

forall A. (() -> Thunk A) -> Thunk A

Parameters

• f: A thunk that produces the thunk.

Returns

The deferred thunk.

Examples

use fp_library::{
	brands::*,
	classes::Deferrable,
	functions::*,
	types::*,
};

let task: Thunk<i32> = Deferrable::defer(|| Thunk::pure(42));
assert_eq!(task.evaluate(), 42);

impl<'a, A, Config> From<Lazy<'a, A, Config>> for Thunk<'a, A>

where A: Clone + 'a, Config: LazyConfig,

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.
• Config: The memoization configuration.

fn from(lazy: Lazy<'a, A, Config>) -> Self

Type Signature

forall A Config. LazyConfig Config => Lazy A Config -> Thunk A Config

Parameters

• lazy: The lazy value to convert.

Returns

A thunk that evaluates the lazy value.

Examples

use fp_library::types::*;
let lazy = Lazy::<_, RcLazyConfig>::pure(42);
let thunk = Thunk::from(lazy);
assert_eq!(thunk.evaluate(), 42);

impl<'a, A> From<Thunk<'a, A>> for Lazy<'a, A, RcLazyConfig>

Type Parameters

• 'a: The lifetime of the reference.
• A: The type of the computed value.

fn from(eval: Thunk<'a, A>) -> Self

Type Signature

forall A. Thunk A -> Lazy A

Parameters

• eval: The thunk to convert.

Returns

A new Lazy instance that will evaluate the thunk on first access.

Examples

use fp_library::types::*;
let thunk = Thunk::new(|| 42);
let lazy = Lazy::from(thunk);
assert_eq!(*lazy.evaluate(), 42);

impl<'a, A: 'a, E: 'a> From<Thunk<'a, A>> for TryThunk<'a, A, E>

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the success value.
• E: The type of the error value.

fn from(eval: Thunk<'a, A>) -> Self

Type Signature

forall A E. Thunk A -> TryThunk A E

Parameters

• eval: The thunk to convert.

Returns

A new TryThunk instance that wraps the thunk.

Examples

use fp_library::types::*;
let thunk = Thunk::new(|| 42);
let try_thunk: TryThunk<i32, ()> = TryThunk::from(thunk);
assert_eq!(try_thunk.evaluate(), Ok(42));

impl<'a, A: Monoid + 'a> Monoid for Thunk<'a, A>

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.

fn empty() -> Self

Returns the identity Thunk.

Type Signature

forall A. Monoid A => () -> Thunk A

Returns

A Thunk producing the identity value of A.

Examples

use fp_library::{
	classes::*,
	types::*,
};

let t: Thunk<String> = Thunk::empty();
assert_eq!(t.evaluate(), "");

impl<'a, A: Semigroup + 'a> Semigroup for Thunk<'a, A>

Type Parameters

• 'a: The lifetime of the computation.
• A: The type of the value produced by the computation.

fn append(a: Self, b: Self) -> Self

Combines two Thunks by combining their results.

Type Signature

forall A. Semigroup A => (Thunk A, Thunk A) -> Thunk A

Parameters

• a: The first Thunk.
• b: The second Thunk.

Returns

A new Thunk containing the combined result.

Examples

use fp_library::{
	brands::*,
	classes::*,
	functions::*,
};

let t1 = pure::<ThunkBrand, _>("Hello".to_string());
let t2 = pure::<ThunkBrand, _>(" World".to_string());
let t3 = append::<_>(t1, t2);
assert_eq!(t3.evaluate(), "Hello World");