R: 'Tailcall' and 'Exec'

Tailcall {base}

R Documentation

`Tailcall` and `Exec`

Description

Tailcall and Exec allow writing more stack-space-efficient recursive functions in R.

Usage

Tailcall(FUN, ...)
Exec(expr, envir)

Arguments

`FUN`	a function or a non-empty character string naming the function to be called.
`...`	all the arguments to be passed.
`expr`	a call expression.
`envir`	environment for evaluating `expr`; default is the environment from which `Exec` is called.

Details

Tailcall evaluates a call to FUN with arguments ... in the current environment, and Exec evaluates the call expr in environment envir. If a Tailcall or Exec expression appears in tail position in an R function, and if there are no on.exit expressions set, then the evaluation context of the new calls replaces the currently executing call context with a new one. If the requirements for context re-use are not met, then evaluation proceeds in the standard way adding another context to the stack.

Using Tailcall it is possible to define tail-recursive functions that do not grow the evaluation stack. Exec can be used to simplify the call stack for functions that create and then evaluate an expression.

Because of lazy evaluation of arguments in R it may be necessary to force evaluation of some arguments to avoid accumulating deferred evaluations.

This tail call optimization has the advantage of not growing the call stack and permitting arbitrarily deep tail recursions. It does also mean that stack traces produced by traceback or sys.calls will only show the call specified by Tailcall or Exec, not the previous call whose stack entry has been replaced.

Note

Tailcall and Exec are experimental and may be changed or dropped in future released versions of R.

Examples

## tail-recursive log10-factorial
lfact <- function(n) {
    lfact_iter <- function(val, n) {
        if (n <= 0)
            val
        else {
            val <- val + log10(n) # forces val
            Tailcall(lfact_iter, val, n - 1)
        }
    }
    lfact_iter(0, n)
}
10 ^ lfact(3)
lfact(100000)

## simplified variant of do.call using Exec:
docall <- function (what, args, quote = FALSE) {
    if (!is.list(args)) 
        stop("second argument must be a list")
    if (quote) 
        args <- lapply(args, enquote)
    Exec(as.call(c(list(substitute(what)), args)), parent.frame())
}
## the call stack does not contain the call to docall:
docall(function() sys.calls(), list()) |> 
    Find(function(x) identical(x[[1]], quote(docall)), x = _)
## contrast to do.call:
do.call(function(x) sys.calls(), list()) |> 
    Find(function(x) identical(x[[1]], quote(do.call)), x = _)

[Package base version 4.4.0 Index]

Tailcall and Exec