Promise class formally and abstractly represents the
potential result of a deferred computation.
Lazy programming is useful in a number of contexts, including interaction with external/remote systems like databases, where we want the computation to occur within the external system, despite appearances to the contrary. Typically, the user constructs one or more promises referring to pre-existing objects. Operations on those objects produce new promises that encode the additional computations. Eventually, usually after some sort of restriction and/or aggregation, the promise is “fulfilled” to yield a materialized, eager object, such as an R vector.
Promise and its partial implementation
provide a foundation for implementations that mostly helps with
creating and fulfilling promises, while the implementation is
responsible for deferring particular computations, which is
Promise(expr, context, ...): A generic constructor that
expr to construct a
the specific type of which corresponds to the language of
context argument should be a
Context object, in which
expr will be evaluated when
the promise is fulfilled. The
... are passed to methods.
fulfill(x): Fulfills the promise by evaluating the deferred
computation and returning a materialized object.
The basic coercion functions in R, like
as.data.frame, have methods for
Promise that simply call
fulfill on the promise, and then perform the coercion. Coercion
is preferred to calling
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