env_bind: Bind symbols to objects in an environment

Description

These functions create bindings in an environment. The bindings are supplied through ... as pairs of names and values or expressions. env_bind() is equivalent to evaluating a <- expression within the given environment. This function should take care of the majority of use cases but the other variants can be useful for specific problems.

env_bind() takes named values which are bound in .env. env_bind() is equivalent to base::assign().
env_bind_active() takes named functions and creates active bindings in .env. This is equivalent to base::makeActiveBinding(). An active binding executes a function each time it is evaluated. The arguments are passed to as_function() so you can supply formulas instead of functions.

Remember that functions are scoped in their own environment. These functions can thus refer to symbols from this enclosure that are not actually in scope in the dynamic environment where the active bindings are invoked. This allows creative solutions to difficult problems (see the implementations of dplyr::do() methods for an example).
env_bind_lazy() takes named expressions. This is equivalent to base::delayedAssign(). The arguments are captured with exprs() (and thus support call-splicing and unquoting) and assigned to symbols in .env. These expressions are not evaluated immediately but lazily. Once a symbol is evaluated, the corresponding expression is evaluated in turn and its value is bound to the symbol (the expressions are thus evaluated only once, if at all).
%<~% is a shortcut for env_bind_lazy(). It works like <- but the RHS is evaluated lazily.

Usage

env_bind(.env, ...)
env_bind_lazy(.env, ..., .eval_env = caller_env())
env_bind_active(.env, ...)
lhs %

Value

The input object .env, with its associated environment modified in place, invisibly.

Arguments

.env: An environment.
...: <dynamic> Named objects (env_bind()), expressions env_bind_lazy(), or functions (env_bind_active()). Use zap() to remove bindings.
.eval_env: The environment where the expressions will be evaluated when the symbols are forced.
lhs: The variable name to which rhs will be lazily assigned.
rhs: An expression lazily evaluated and assigned to lhs.

Side effects

Since environments have reference semantics (see relevant section in env() documentation), modifying the bindings of an environment produces effects in all other references to that environment. In other words, env_bind() and its variants have side effects.

Like other side-effecty functions like par() and options(), env_bind() and variants return the old values invisibly.

Examples

Run this code

# env_bind() is a programmatic way of assigning values to symbols
# with `<-`. We can add bindings in the current environment:
env_bind(current_env(), foo = "bar")
foo

# Or modify those bindings:
bar <- "bar"
env_bind(current_env(), bar = "BAR")
bar

# You can remove bindings by supplying zap sentinels:
env_bind(current_env(), foo = zap())
try(foo)

# Unquote-splice a named list of zaps
zaps <- rep_named(c("foo", "bar"), list(zap()))
env_bind(current_env(), !!!zaps)
try(bar)

# It is most useful to change other environments:
my_env <- env()
env_bind(my_env, foo = "foo")
my_env$foo

# A useful feature is to splice lists of named values:
vals <- list(a = 10, b = 20)
env_bind(my_env, !!!vals, c = 30)
my_env$b
my_env$c

# You can also unquote a variable referring to a symbol or a string
# as binding name:
var <- "baz"
env_bind(my_env, !!var := "BAZ")
my_env$baz


# The old values of the bindings are returned invisibly:
old <- env_bind(my_env, a = 1, b = 2, baz = "baz")
old

# You can restore the original environment state by supplying the
# old values back:
env_bind(my_env, !!!old)

# env_bind_lazy() assigns expressions lazily:
env <- env()
env_bind_lazy(env, name = { cat("forced!\n"); "value" })

# Referring to the binding will cause evaluation:
env$name

# But only once, subsequent references yield the final value:
env$name

# You can unquote expressions:
expr <- quote(message("forced!"))
env_bind_lazy(env, name = !!expr)
env$name


# By default the expressions are evaluated in the current
# environment. For instance we can create a local binding and refer
# to it, even though the variable is bound in a different
# environment:
who <- "mickey"
env_bind_lazy(env, name = paste(who, "mouse"))
env$name

# You can specify another evaluation environment with `.eval_env`:
eval_env <- env(who = "minnie")
env_bind_lazy(env, name = paste(who, "mouse"), .eval_env = eval_env)
env$name

# Or by unquoting a quosure:
quo <- local({
  who <- "fievel"
  quo(paste(who, "mouse"))
})
env_bind_lazy(env, name = !!quo)
env$name

# You can create active bindings with env_bind_active(). Active
# bindings execute a function each time they are evaluated:
fn <- function() {
  cat("I have been called\n")
  rnorm(1)
}

env <- env()
env_bind_active(env, symbol = fn)

# `fn` is executed each time `symbol` is evaluated or retrieved:
env$symbol
env$symbol
eval_bare(quote(symbol), env)
eval_bare(quote(symbol), env)

# All arguments are passed to as_function() so you can use the
# formula shortcut:
env_bind_active(env, foo = ~ runif(1))
env$foo
env$foo