R/env-binding.R

In hadley/rlang: Functions for Base Types and Core R and 'Tidyverse' Features

#' 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.
#'
#'
#' @section 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.
#'
#' @param .env An environment.
#' @param ... <[dynamic][dyn-dots]> Named objects (`env_bind()`),
#'   expressions `env_bind_lazy()`, or functions (`env_bind_active()`).
#'   Use [zap()] to remove bindings.
#' @return The input object `.env`, with its associated environment
#'   modified in place, invisibly.
#' @seealso [env_poke()] for binding a single element.
#' @export
#' @examples
#' # 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 <- function(.env, ...) {
  check_environment(.env)
  invisible(.Call(
    ffi_env_bind,
    env = .env,
    values = list3(...),
    needs_old = TRUE,
    bind_type = "value",
    eval_env = NULL
  ))
}

# Doesn't return list of old bindings for efficiency
env_bind0 <- function(.env, values) {
  invisible(.Call(
    ffi_env_bind,
    env = .env,
    values = values,
    needs_old = FALSE,
    bind_type = "value",
    eval_env = NULL
  ))
}

#' @rdname env_bind
#' @param .eval_env The environment where the expressions will be
#'   evaluated when the symbols are forced.
#' @export
#' @examples
#'
#' # 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
env_bind_lazy <- function(.env, ..., .eval_env = caller_env()) {
  check_environment(.env)
  invisible(.Call(
    ffi_env_bind,
    env = .env,
    values = exprs(...),
    needs_old = TRUE,
    bind_type = "lazy",
    eval_env = .eval_env
  ))
}
#' @rdname env_bind
#' @export
#' @examples
#'
#' # 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
env_bind_active <- function(.env, ...) {
  check_environment(.env)
  invisible(.Call(
    ffi_env_bind,
    env = .env,
    values = list3(...),
    needs_old = TRUE,
    bind_type = "active",
    eval_env = caller_env()
  ))
}
#' @rdname env_bind
#' @param lhs The variable name to which `rhs` will be lazily assigned.
#' @param rhs An expression lazily evaluated and assigned to `lhs`.
#' @export
`%<~%` <- function(lhs, rhs) {
  env <- caller_env()
  inject(
    base::delayedAssign(
      as_string(substitute(lhs)),
      !!substitute(rhs),
      eval.env = env,
      assign.env = env
    )
  )
}


#' Temporarily change bindings of an environment
#'
#' @description
#'
#' * `local_bindings()` temporarily changes bindings in `.env` (which
#'   is by default the caller environment). The bindings are reset to
#'   their original values when the current frame (or an arbitrary one
#'   if you specify `.frame`) goes out of scope.
#'
#' * `with_bindings()` evaluates `expr` with temporary bindings. When
#'   `with_bindings()` returns, bindings are reset to their original
#'   values. It is a simple wrapper around `local_bindings()`.
#'
#' @inheritParams env_bind
#' @param ... Pairs of names and values. These dots support splicing
#'   (with value semantics) and name unquoting.
#' @param .frame The frame environment that determines the scope of
#'   the temporary bindings. When that frame is popped from the call
#'   stack, bindings are switched back to their original values.
#' @return `local_bindings()` returns the values of old bindings
#'   invisibly; `with_bindings()` returns the value of `expr`.
#' @export
#' @examples
#' foo <- "foo"
#' bar <- "bar"
#'
#' # `foo` will be temporarily rebinded while executing `expr`
#' with_bindings(paste(foo, bar), foo = "rebinded")
#' paste(foo, bar)
local_bindings <- function(..., .env = .frame, .frame = caller_env()) {
  check_environment(.env)
  check_environment(.frame)

  old <- env_bind(.env, ...)
  defer(env_bind0(.env, old), envir = .frame)

  invisible(old)
}
#' @rdname local_bindings
#' @param .expr An expression to evaluate with temporary bindings.
#' @export
with_bindings <- function(.expr, ..., .env = caller_env()) {
  check_environment(.env)
  local_bindings(..., .env = .env)
  .expr
}

#' Remove bindings from an environment
#'
#' `env_unbind()` is the complement of [env_bind()]. Like `env_has()`,
#' it ignores the parent environments of `env` by default. Set
#' `inherit` to `TRUE` to track down bindings in parent environments.
#'
#' @inheritParams get_env
#' @param nms A character vector of binding names to remove.
#' @param inherit Whether to look for bindings in the parent
#'   environments.
#' @return The input object `env` with its associated environment
#'   modified in place, invisibly.
#' @export
#' @examples
#' env <- env(foo = 1, bar = 2)
#' env_has(env, c("foo", "bar"))
#'
#' # Remove bindings with `env_unbind()`
#' env_unbind(env, c("foo", "bar"))
#' env_has(env, c("foo", "bar"))
#'
#' # With inherit = TRUE, it removes bindings in parent environments
#' # as well:
#' parent <- env(empty_env(), foo = 1, bar = 2)
#' env <- env(parent, foo = "b")
#'
#' env_unbind(env, "foo", inherit = TRUE)
#' env_has(env, c("foo", "bar"))
#' env_has(env, c("foo", "bar"), inherit = TRUE)
env_unbind <- function(env = caller_env(), nms, inherit = FALSE) {
  .Call(ffi_env_unbind, env, nms, inherit)
  invisible(env)
}

#' Does an environment have or see bindings?
#'
#' `env_has()` is a vectorised predicate that queries whether an
#' environment owns bindings personally (with `inherit` set to
#' `FALSE`, the default), or sees them in its own environment or in
#' any of its parents (with `inherit = TRUE`).
#'
#' @inheritParams env_unbind
#' @param nms A character vector of binding names for which to check
#'   existence.
#' @return A named logical vector as long as `nms`.
#' @export
#' @examples
#' parent <- child_env(NULL, foo = "foo")
#' env <- child_env(parent, bar = "bar")
#'
#' # env does not own `foo` but sees it in its parent environment:
#' env_has(env, "foo")
#' env_has(env, "foo", inherit = TRUE)
env_has <- function(env = caller_env(), nms, inherit = FALSE) {
  check_environment(env)
  .Call(ffi_env_has, env, nms, inherit)
}

#' Get an object in an environment
#'
#' `env_get()` extracts an object from an enviroment `env`. By
#' default, it does not look in the parent environments.
#' `env_get_list()` extracts multiple objects from an environment into
#' a named list.
#'
#' @inheritParams get_env
#' @inheritParams env_has
#' @param nm Name of binding, a string.
#' @param nms Names of bindings, a character vector.
#' @param default A default value in case there is no binding for `nm`
#'   in `env`.
#' @param last Last environment inspected when `inherit` is `TRUE`.
#'   Can be useful in conjunction with [base::topenv()].
#' @return An object if it exists. Otherwise, throws an error.
#'
#' @seealso [env_cache()] for a variant of `env_get()` designed to
#'   cache a value in an environment.
#' @export
#' @examples
#' parent <- child_env(NULL, foo = "foo")
#' env <- child_env(parent, bar = "bar")
#'
#' # This throws an error because `foo` is not directly defined in env:
#' # env_get(env, "foo")
#'
#' # However `foo` can be fetched in the parent environment:
#' env_get(env, "foo", inherit = TRUE)
#'
#' # You can also avoid an error by supplying a default value:
#' env_get(env, "foo", default = "FOO")
env_get <- function(env = caller_env(),
                    nm,
                    default,
                    inherit = FALSE,
                    last = empty_env()) {
  check_environment(env)
  check_environment(last)

  if (missing(default)) {
    default %<~% stop_env_get_missing(nm)
  }

  .Call(
    ffi_env_get,
    env = env,
    nm = nm,
    inherit = inherit,
    last = last,
    closure_env = environment()
  )
}
#' @rdname env_get
#' @export
env_get_list <- function(env = caller_env(),
                         nms,
                         default,
                         inherit = FALSE,
                         last = empty_env()) {
  check_environment(env)
  check_environment(last)
  .Call(
    ffi_env_get_list,
    env = env,
    nms = nms,
    inherit = inherit,
    last = last,
    closure_env = environment()
  )
}

stop_env_get_missing <- function(nm) {
  msg <- sprintf("Can't find %s in environment.", format_arg(nm))
  abort(msg, call = caller_env())
}

#' Poke an object in an environment
#'
#' `env_poke()` will assign or reassign a binding in `env` if `create`
#' is `TRUE`. If `create` is `FALSE` and a binding does not already
#' exists, an error is issued.
#'
#'
#' @details
#'
#' If `inherit` is `TRUE`, the parents environments are checked for
#' an existing binding to reassign. If not found and `create` is
#' `TRUE`, a new binding is created in `env`. The default value for
#' `create` is a function of `inherit`: `FALSE` when inheriting,
#' `TRUE` otherwise.
#'
#' This default makes sense because the inheriting case is mostly
#' for overriding an existing binding. If not found, something
#' probably went wrong and it is safer to issue an error. Note that
#' this is different to the base R operator `<<-` which will create
#' a binding in the global environment instead of the current
#' environment when no existing binding is found in the parents.
#'
#'
#' @inheritParams env_get
#' @param value The value for a new binding.
#' @param create Whether to create a binding if it does not already
#'   exist in the environment.
#' @return The old value of `nm` or a [zap sentinel][zap] if the
#'   binding did not exist yet.
#'
#' @seealso [env_bind()] for binding multiple elements. [env_cache()]
#'   for a variant of `env_poke()` designed to cache values.
#' @export
env_poke <- function(env = caller_env(),
                     nm,
                     value,
                     inherit = FALSE,
                     create = !inherit) {
  check_environment(env)
  invisible(.Call(
    ffi_env_poke,
    env = env,
    nm = nm,
    values = value,
    inherit = inherit,
    create = create
  ))
}

#' Cache a value in an environment
#'
#' @description
#' `env_cache()` is a wrapper around [env_get()] and [env_poke()]
#' designed to retrieve a cached value from `env`.
#'
#' - If the `nm` binding exists, it returns its value.
#' - Otherwise, it stores the default value in `env` and returns that.
#'
#' @inheritParams env_get
#' @param default The default value to store in `env` if `nm` does not
#'   exist yet.
#' @return Either the value of `nm` or `default` if it did not exist
#'   yet.
#'
#' @examples
#' e <- env(a = "foo")
#'
#' # Returns existing binding
#' env_cache(e, "a", "default")
#'
#' # Creates a `b` binding and returns its default value
#' env_cache(e, "b", "default")
#'
#' # Now `b` is defined
#' e$b
#' @export
env_cache <- function(env, nm, default) {
  check_required(default)
  check_name(nm)

  if (env_has(env, nm)) {
    env_get(env, nm)
  } else {
    env_poke(env, nm, default)
    default
  }
}

#' Names and numbers of symbols bound in an environment
#'
#' `env_names()` returns object names from an enviroment `env` as a
#' character vector. All names are returned, even those starting with
#' a dot. `env_length()` returns the number of bindings.
#'
#' @section Names of symbols and objects:
#'
#' Technically, objects are bound to symbols rather than strings,
#' since the R interpreter evaluates symbols (see [is_expression()] for a
#' discussion of symbolic objects versus literal objects). However it
#' is often more convenient to work with strings. In rlang
#' terminology, the string corresponding to a symbol is called the
#' _name_ of the symbol (or by extension the name of an object bound
#' to a symbol).
#'
#' @section Encoding:
#'
#' There are deep encoding issues when you convert a string to symbol
#' and vice versa. Symbols are _always_ in the native encoding. If
#' that encoding (let's say latin1) cannot support some characters,
#' these characters are serialised to ASCII. That's why you sometimes
#' see strings looking like `<U+1234>`, especially if you're running
#' Windows (as R doesn't support UTF-8 as native encoding on that
#' platform).
#'
#' To alleviate some of the encoding pain, `env_names()` always
#' returns a UTF-8 character vector (which is fine even on Windows)
#' with ASCII unicode points translated back to UTF-8.
#'
#' @inheritParams get_env
#' @return A character vector of object names.
#' @export
#' @examples
#' env <- env(a = 1, b = 2)
#' env_names(env)
env_names <- function(env) {
  check_environment(env)
  nms <- names(env)
  .Call(ffi_unescape_character, nms)
}

#' @rdname env_names
#' @export
env_length <- function(env) {
  check_environment(env)
  length(env)
}

#' Lock or unlock environment bindings
#'
#' @description
#'
#' `r lifecycle::badge("experimental")`
#'
#' Locked environment bindings trigger an error when an attempt is
#' made to redefine the binding.
#'
#' @param env An environment.
#' @param nms Names of bindings. Defaults to all bindings in `env`.
#'
#' @return `env_binding_are_unlocked()` returns a logical vector as
#'   long as `nms` and named after it. `env_binding_lock()` and
#'   `env_binding_unlock()` return the old value of
#'   `env_binding_are_unlocked()` invisibly.
#'
#' @seealso [env_lock()] for locking an environment.
#'
#' @keywords internal
#' @export
#' @examples
#' # Bindings are unlocked by default:
#' env <- env(a = "A", b = "B")
#' env_binding_are_locked(env)
#'
#' # But can optionally be locked:
#' env_binding_lock(env, "a")
#' env_binding_are_locked(env)
#'
#' # If run, the following would now return an error because `a` is locked:
#' # env_bind(env, a = "foo")
#' # with_env(env, a <- "bar")
#'
#' # Let's unlock it. Note that the return value indicate which
#' # bindings were locked:
#' were_locked <- env_binding_unlock(env)
#' were_locked
#'
#' # Now that it is unlocked we can modify it again:
#' env_bind(env, a = "foo")
#' with_env(env, a <- "bar")
#' env$a
env_binding_lock <- function(env, nms = NULL) {
  nms <- env_binding_validate_names(env, nms)
  old <- env_binding_are_locked(env, nms)
  map(nms, lockBinding, env = env)
  invisible(old)
}
#' @rdname env_binding_lock
#' @export
env_binding_unlock <- function(env, nms = NULL) {
  nms <- env_binding_validate_names(env, nms)
  old <- env_binding_are_locked(env, nms)
  map(nms, unlockBinding, env = env)
  invisible(old)
}
#' @rdname env_binding_lock
#' @export
env_binding_are_locked <- function(env, nms = NULL) {
  nms <- env_binding_validate_names(env, nms)
  set_names(map_lgl(nms, bindingIsLocked, env = env), nms)
}

#' What kind of environment binding?
#'
#' `r lifecycle::badge("experimental")`
#'
#' @inheritParams env_binding_lock
#'
#' @keywords internal
#' @return A logical vector as long as `nms` and named after it.
#' @export
env_binding_are_active <- function(env, nms = NULL) {
  env_binding_are_type(env, nms, 2L)
}
#' @rdname env_binding_are_active
#' @export
env_binding_are_lazy <- function(env, nms = NULL) {
  env_binding_are_type(env, nms, 1L)
}
env_binding_are_type <- function(env,
                                 nms,
                                 type,
                                 error_call = caller_env()) {
  check_environment(env, call = error_call)

  nms <- env_binding_validate_names(env, nms, call = error_call)
  promise <- env_binding_types(env, nms)

  if (is_null(promise)) {
    promise <- rep(FALSE, length(nms))
  } else {
    promise <- promise == type
  }
  set_names(promise, nms)
}

env_binding_validate_names <- function(env, nms, call = caller_env()) {
  if (is_null(nms)) {
    nms <- env_names(env)
  } else {
    check_character(
      nms,
      what = "a character vector of names",
      call = call
    )
  }
  nms
}
env_binding_types <- function(env, nms = env_names(env)) {
  .Call(ffi_env_binding_types, env, nms)
}

env_binding_type_sum <- function(env, nms = NULL) {
  nms <- env_binding_validate_names(env, nms)

  active <- env_binding_are_active(env, nms)
  promise <- env_binding_are_lazy(env, nms)
  other <- !active & !promise

  types <- new_character(length(nms), nms)
  types[active] <- "active"
  types[promise] <- "lazy"
  types[other] <- map_chr(env_get_list(env, nms[other]), rlang_type_sum)

  types
}