Function adapton::engine::thunk [] [src]

pub fn thunk<Arg: Hash + Eq + Debug + Clone + 'static, Spurious: Clone + 'static, Res: Hash + Eq + Debug + Clone + 'static>(
    id: ArtIdChoice,
    prog_pt: ProgPt,
    fn_box: Rc<Box<Fn(Arg, Spurious) -> Res>>,
    arg: Arg,
    spurious: Spurious
) -> Art<Res>

Allocates a thunk, an Art<T> that consists of a suspended computation that produces a value of type T.

Use the macros thunk!, memo! and eager! to create and force thunks with less typing.

A full invocation of thunk consists of the following:

  • It has an id of type ArtIdChoice, either giving a Name, requesting structural identity, or requesting no caching whatsoever.

  • Its code resides at a program point, of type ProgPt. This uniquely identifies the static elements of the suspended computation (the code, but not the closing environment).

  • It has a function for this code, of type Rc<Box<Fn(Art,Spurious) -> Res >>. The Rc<_> is required for cloning this function, which we generally want to do. We divide the arguments into the ordinary arguments of type Arg, and additional "spurious" arguments. When we judge whether a thunk's closing environment is equal to another, we use the type Arg. When we judge whether the result of a thunk has changed or not, we use the type Res.

  • Spurious arguments are not compared for equality, and are ignored during the memo-matching process. They are, however, saved in the thunk and supplied to the suspended computation when it runs. Typically, these arguments consist of higher-order functions that parameterize the thunk; we make them spurious because (1) we cannot, and do not wish to compare them for equality and (2) the triple of type (ProgPt, ArtIdChoice, Arg) should determine these spurious arguments, if any. Because some arguments have no equality relation, the presence of these arguments is sometimes a necessary hack.