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% File src/library/base/man/delayedAssign.Rd
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% File src/library/base/man/delayedAssign.Rd
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% Part of the R package, https://www.R-project.org
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% Part of the R package, https://www.R-project.org
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% Copyright 1995-2015 R Core Team
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% Copyright 1995-2022 R Core Team
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% Distributed under GPL 2 or later
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% Distributed under GPL 2 or later
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\name{delayedAssign}
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\name{delayedAssign}
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\title{Delay Evaluation and Promises}
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\alias{delayedAssign}
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\alias{delayedAssign}
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\alias{promise}
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\alias{promise}
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\alias{promises}
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\alias{promises}
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\title{Delay Evaluation}
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\description{
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\description{
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  \code{delayedAssign} creates a \emph{promise} to evaluate the given
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  \code{delayedAssign} creates a \emph{promise} to evaluate the given
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  expression if its value is requested.  This provides direct access
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  expression if its value is requested.  This provides direct access
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  to the \emph{lazy evaluation} mechanism used by \R for the evaluation
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  to the \emph{lazy evaluation} mechanism used by \R for the evaluation
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  of (interpreted) functions.
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  of (interpreted) functions.
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  the variable is first accessed.
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  the variable is first accessed.
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  When the promise is eventually forced, it is evaluated within the
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  When the promise is eventually forced, it is evaluated within the
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  environment specified by \code{eval.env} (whose contents may have changed in
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  environment specified by \code{eval.env} (whose contents may have changed in
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  the meantime).  After that, the value is fixed and the expression will
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  the meantime).  After that, the value is fixed and the expression will
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  not be evaluated again.
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  not be evaluated again, where the promise still keeps its expression.
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}
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}
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\seealso{
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\seealso{
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  \code{\link{substitute}}, to see the expression associated with a
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  \code{\link{substitute}}, to see the expression associated with a
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  promise, if \code{assign.env} is not the \code{\link{.GlobalEnv}}.
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  promise, if \code{assign.env} is not the \code{\link{.GlobalEnv}}.
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}
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}
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### Promises in an environment [for advanced users]:  ---------------------
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### Promises in an environment [for advanced users]:  ---------------------
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e <- (function(x, y = 1, z) environment())(cos, "y", {cat(" HO!\n"); pi+2})
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e <- (function(x, y = 1, z) environment())(cos, "y", {cat(" HO!\n"); pi+2})
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## How can we look at all promises in an env (w/o forcing them)?
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## How can we look at all promises in an env (w/o forcing them)?
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gete <- function(e_)
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gete <- function(e_) {
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   ne <- names(e_)
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   names(ne) <- ne
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   lapply(lapply(ls(e_), as.name),
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   lapply(lapply(ne, as.name),
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          function(n) eval(substitute(substitute(X, e_), list(X=n))))
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          function(n) eval(substitute(substitute(X, e_), list(X=n))))
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}
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(exps <- gete(e))
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(exps <- gete(e))
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sapply(exps, typeof)
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sapply(exps, typeof)
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(le <- as.list(e)) # evaluates ("force"s) the promises
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(le <- as.list(e)) # evaluates ("force"s) the promises
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stopifnot(identical(unname(le), lapply(exps, eval))) # and another "Ho!"
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stopifnot(identical(le, lapply(exps, eval))) # and another "Ho!"
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}
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}
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\keyword{programming}
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\keyword{programming}
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\keyword{data}
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\keyword{data}