% File src/library/utils/man/relist.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2022 R Core Team
% Distributed under GPL 2 or later

\name{relist}
\alias{relist}
\alias{relist.default}
\alias{relist.list}
\alias{relist.factor}
\alias{relist.matrix}
\alias{as.relistable}
\alias{is.relistable}
\alias{unlist.relistable}

\title{Allow Re-Listing an unlist()ed Object}
\description{
  \code{relist()} is an S3 generic function with a few methods in order
  to allow easy inversion of \code{\link{unlist}(obj)} when that is used
  with an object \code{obj} of (S3) class \code{"relistable"}.
}
\usage{
relist(flesh, skeleton)
\method{relist}{default}(flesh, skeleton = attr(flesh, "skeleton"))
\method{relist}{factor}(flesh, skeleton = attr(flesh, "skeleton"))
\method{relist}{list}(flesh, skeleton = attr(flesh, "skeleton"))
\method{relist}{matrix}(flesh, skeleton = attr(flesh, "skeleton"))

as.relistable(x)
is.relistable(x)

\method{unlist}{relistable}(x, recursive = TRUE, use.names = TRUE)
}

\arguments{
  \item{flesh}{a vector to be relisted}
  \item{skeleton}{a list, the structure of which determines the structure
    of the result}
  \item{x}{an \R object, typically a list (or vector).}
  \item{recursive}{logical.  Should unlisting be applied to list
    components of \code{x}?}
  \item{use.names}{logical.  Should names be preserved?}
}
\details{
  Some functions need many parameters, which are most easily represented in
  complex structures, e.g., nested lists.  Unfortunately, many
  mathematical functions in \R, including \code{\link{optim}} and
  \code{\link{nlm}} can only operate on functions whose domain is
  a vector.  \R has \code{\link{unlist}()} to convert nested list
  objects into a vector representation.  \code{relist()}, its methods and
  the functionality mentioned here provide the inverse operation to convert
  vectors back to the convenient structural representation.
  This allows structured functions (such as \code{optim()}) to have simple
  mathematical interfaces.

  For example, a likelihood function for a multivariate normal model needs a
  variance-covariance matrix and a mean vector.  It would be most convenient to
  represent it as a list containing a vector and a matrix.  A typical parameter
  might look like
\preformatted{      list(mean = c(0, 1), vcov = cbind(c(1, 1), c(1, 0))).}
  However, \code{\link{optim}} cannot operate on functions that take
  lists as input; it only likes numeric vectors.  The solution is
  conversion. Given a function \code{mvdnorm(x, mean, vcov, log = FALSE)}
  which computes the required probability density, then
\preformatted{        ipar <- list(mean = c(0, 1), vcov = c bind(c(1, 1), c(1, 0)))
        initial.param <- as.relistable(ipar)

        ll <- function(param.vector)
        {
           param <- relist(param.vector, skeleton = ipar)
           -sum(mvdnorm(x, mean = param$mean, vcov = param$vcov,
                        log = TRUE))
        }

        optim(unlist(initial.param), ll)
}
  \code{relist} takes two parameters: skeleton and flesh.  Skeleton is a sample
  object that has the right \code{shape} but the wrong content.  \code{flesh}
  is a vector with the right content but the wrong shape.  Invoking
\preformatted{    relist(flesh, skeleton)}
  will put the content of flesh on the skeleton.  You don't need to specify
  skeleton explicitly if the skeleton is stored as an attribute inside flesh.
  In particular, if flesh was created from some object obj with
  \code{unlist(as.relistable(obj))}
  then the skeleton attribute is automatically set.  (Note that this
  does not apply to the example here, as \code{\link{optim}} is creating
  a new vector to pass to \code{ll} and not its \code{par} argument.)

  As long as \code{skeleton} has the right shape, it should be an inverse
  of \code{\link{unlist}}.  These equalities hold:
\preformatted{   relist(unlist(x), x) == x
   unlist(relist(y, skeleton)) == y

   x <- as.relistable(x)
   relist(unlist(x)) == x
}
  Note however that the relisted object might not be
  \emph{\link{identical}} to the skeleton because of implicit coercions
  performed during the unlisting step.  All elements of the relisted
  objects have the same type as the unlisted object.  \code{NULL} values
  are coerced to empty vectors of that type.
}% details
\value{
  an object of (S3) class \code{"relistable"} (and \code{"\link{list}"}).
}
\author{
  R Core, based on a code proposal by Andrew Clausen.
}
\seealso{
  \code{\link{unlist}}%, ..... MORE ?
}
\examples{
 ipar <- list(mean = c(0, 1), vcov = cbind(c(1, 1), c(1, 0)))
 initial.param <- as.relistable(ipar)
 ul <- unlist(initial.param)
 relist(ul)
 stopifnot(identical(relist(ul), initial.param))
}
\keyword{list}
\keyword{manip}
