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# stop() and warning() — signal conditions
safe_sqrt <- function(x) {
    if (!is.numeric(x)) stop("x must be numeric")
    if (any(x < 0))     warning("negative values will produce NaN")
    sqrt(x)
}
safe_sqrt(c(4, 9, 16))    # 2 3 4
safe_sqrt(c(4, -1))       # warning + NaN result

# tryCatch — catch errors
result <- tryCatch({
    log(-1)                 # produces NaN with a warning
    "success"
}, warning = function(w) {
    cat("Caught warning:", conditionMessage(w), "
")
    NA
}, error = function(e) {
    cat("Caught error:  ", conditionMessage(e), "
")
    NA
})
result   # NA (the warning handler returned NA)

# tryCatch around a real error
tryCatch(
    stop("something went wrong"),
    error = function(e) paste("Handled:", conditionMessage(e))
)

# withCallingHandlers — log and continue
withCallingHandlers({
    message("Step 1 complete")
    warning("Minor issue at step 2")
    42                   # final value
}, message = function(m) {
    cat("[MSG]", conditionMessage(m))
    invokeRestart("muffleMessage")
}, warning = function(w) {
    cat("[WARN]", conditionMessage(w), "
")
    invokeRestart("muffleWarning")
})

# stopifnot — assert preconditions
divide <- function(a, b) {
    stopifnot(is.numeric(a), is.numeric(b), b != 0)
    a / b
}
divide(10, 2)    # 5
# divide(10, 0)  # Error: b != 0 is not TRUE