Hello R language experts,
I get an error I don't understand in an assigment function
of the form
deref(z[i]) <- v
Error in deref(z[1:5]) <- -deref(z[1:5]) :
cannot change value of locked binding for '*tmp*'
*after* the assignment did what it should. I can workaround
using a standard function achieving the desired side-effects
refset(z[i], value=v)
Any explanation or idea how to make the standard notation working?
Context and replication code follows below.
Best
Jens
# I am trying to modernize package 'ref'
# which stems from the time of S+ and early R.
# Old package 'ref' (version 0.99) allows passing arguments by reference
# but has lost the ability to change parts of a vector
# without copying the complete vector since R version 1.8.
# I think we can restore performance if
# a new reference is not longer "an object name plus an environment"
# but is "an expression such as 'z' or 'z[i]' plus an environment"
# such that the ref-functions just become convenience wrappers to
# 'eval', 'substitute' and friends
ref <- function(expr, envir = parent.frame()){
ret <- list(expr=substitute(expr), envir=envir)
class(ret) <- "ref"
ret
}
# get data at referenced expression
deref <- refget <- function(x, ...){
# ensure that $ does the usual job once we define $.ref
oldClass(x) <- NULL
eval(x$expr, envir=x$envir)
}
# modify data at referenced expression
"deref<-" <- refset <- function(x, value){
oldClass(x) <- NULL
newexpr <- substitute(a <- b, list(a=x$expr, b=value))
eval(newexpr, envir=x$envir)
newexpr
}
# subscript referenced expression (without evaluating it)
"[.ref" <- function(x, ...){
cl <- oldClass(x)
oldClass(x) <- NULL
newexpr <- sys.call()
newexpr[[1]] <- call("[")[[1]]
newexpr[[2]] <- x$expr
x$expr <- newexpr
oldClass(x) <- cl
x
}
# some example data
n <- 1e1
x <- 1:n
# create a reference to it
z <- ref(x)
# using refget(z[i]) and refset(z[i], v)
# efficiently assignes to a referenced subset as desired
v <- refget(z[1:5])
refset(z[1:5], -v)
# so my code works
x[1:10]
# but doing it R-style
# with deref(z[i]) and deref(z[i]) <- v
# gives this error:
# Error in deref(z[1:5]) <- -deref(z[1:5]) :
# cannot change value of locked binding for '*tmp*'
v <- deref(z[1:5])
deref(z[1:5]) <- -v
# anyhow the referenced vector has been changed !?
x[1:10]
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