rlang (version 0.0.0.9000)

env_assign_promise: Assign a promise to an environment.

Description

These functions let you create a promise in an environment. Such promises behave just like lazily evaluated arguments. They are evaluated whenever they are touched by code, but not when they are passed as arguments.

Usage

env_assign_promise(env = caller_env(), nm, expr, eval_env = caller_env())
env_assign_promise_(env = caller_env(), nm, expr, eval_env = caller_env())

Arguments

env

An environment or an object with a S3 method for env(). If missing, the environment of the current evaluation frame is returned.

nm

The name of the binding.

expr

An expression to capture for env_assign_promise(), or a captured expression (either quoted or a formula) for the standard evaluation version env_assign_promise_(). This expression is used to create a promise in env.

eval_env

The environment where the promise will be evaluated when the promise gets forced. If expr is a formula, its environment is used instead. If not a formula and eval_env is not supplied, the promise is evaluated in the environment where env_assign_promise() (or the underscore version) was called.

See Also

env_assign()

Examples

env <- new_env()
env_assign_promise(env, "name", cat("forced!\n"))
env$name

# Use the standard evaluation version with quoted expressions:
f <- ~message("forced!")
env_assign_promise_(env, "name2", f)
env$name2