tryExcept: Condition Handling and Recovery

In ArcadeAntics/essentials: Essential Functions not Included in Base R

Condition Handling and Recovery

Description

tryExcept provides a mechanism for handling unusual conditions, including errors and warnings.

Usage

tryExcept(expr, ..., finally)

Arguments

expr	expression to be evaluated.
...	handlers established for the duration of the evaluation of expr.
finally	expression to be evaluated before returning or exiting.

Details

If finally is missing or a simple expression, tryExcept and tryCatch will behave the same.

However, if finally is a compound expression, usually of the form { expr1 ; expr2 }, tryExcept will split the compound expression into its elements and put each within their own on.exit, whereas tryCatch would put the whole compound expression into one on.exit. This means that all elements of finally in tryExcept will be run even if one throws an error, but in tryCatch, finally will run up until the first error.

Examples

# the following example won't work using 'utils::example' because it signals
# errors, so copy and paste this code into the R Console


## Not run: 
tryCatch({
    stop("error in 'expr'")
}, finally = {
    stop("tryCatch will not reach the second expression in 'finally'")
    stop("but tryExcept will")
})


essentials::tryExcept({
    stop("error in 'expr'")
}, finally = {
    stop("tryCatch will not reach the second expression in 'finally'")
    stop("but tryExcept will")
})


essentials::tryExcept({
    stop("error in 'expr'")
}, finally = {
    cat("this environment = "); print(environment())
    stop("err1")
    stop("err2")
    print(5 + 6)
    stop("err3")


    # just checking that the arguments aren't evaluated in the wrong frame
    # (why would they be though??)
    expr
    finally


    # testing that code with a source reference will be evaluated properly
    print(function(x) {
        x  # testing comments only appearing in source reference
    })


    stop("err4")
    print(6 + 7)


})


essentials::tryExcept({
    cat("this one should behave the same as tryCatch",
        "because 'finally' is not a compound expression",
        "(is not of class \"{\")",
        sep = "\n")
    stop("error in 'expr'")
}, finally =


    # checking again that 'finally' is evaluated in the correct environment
    cat("this environment =", utils::capture.output(environment()), "\n"))


essentials::tryExcept({


    cat("Here is a situation in which you might actually use this.",
        "Suppose you're changing a bunch of settings and options that you want",
        "to reset when 'tryExcept' finishes. For example:",
        "* changing the working directory",
        "* changing options stored in `options()`",
        "* changing graphical parameters stored in `graphics::par()`",
        "* shutting down a graphics device with `dev.off()`",
        "* changing the current graphics device with `dev.set()`",
        "* changing the state of `grDevices::devAskNewPage()`",
        "* deleting a temporary file created with `tempfile()`",
        "    or downloaded with `utils::download.file()`",
        "* closing a connection",
        "* changing the random number generator state",
        "    with `RNGkind()` or `set.seed()`",
        "* any other type of cleaning process",
        sep = "\n")
    owd <- getwd()
    oopt <- options(max.print = 10, digits = 17)
    odev <- grDevices::dev.cur()
    if (names(odev) != "null device")
        oldask <- grDevices::devAskNewPage(ask = FALSE)
    FILE <- tempfile()
    setwd(dirname(FILE))
    con <- file(FILE, "w")
    if (has.Random.seed <- exists(".Random.seed", envir = globalenv(), inherits = FALSE))
        oldSeed <- get(".Random.seed", envir = globalenv(), inherits = FALSE)
    else oldRNG <- RNGkind()
    RNGkind("default", "default", "default")
    set.seed(1)


    cat("\nYou've setup your settings and options above",
        "so now we do something with it", sep = "\n")


    cat("\nOnly 10 of these will print\n")
    print(1:100)


    cat("\nWe're plotting an image which we will remove afterwards\n")
    plot(1:10)
    cdev <- grDevices::dev.cur()
    Sys.sleep(2)


}, finally = {


    cat("\nNow clean up the everything, but unlike 'tryCatch', run all",
        "cleaning steps even if one signals an error", sep = "\n")


    grDevices::dev.off(cdev)
    close(con)
    file.remove(FILE)


    if (has.Random.seed)
        assign(".Random.seed", oldSeed, envir = globalenv(), inherits = FALSE)
    else RNGkind(oldRNG[1L], oldRNG[2L], oldRNG[3L])


    if (names(odev) != "null device") {
        grDevices::dev.set(oldask)
        grDevices::devAskNewPage(oldask)
    }


    setwd(owd)
    options(oopt)
})

## End(Not run)

ArcadeAntics/essentials documentation built on Nov. 7, 2024, 4:33 p.m.