Simple error handling with subclasses in R. Code is a direct port of Python code from https://codereview.stackexchange.com/q/225419/308

errors.r

os_error = function (message, call = NULL) {
    class = c('os_error', 'error', 'condition')
    structure(list(message = message, call = call), class = class)
}

value_error = function (message, call = NULL) {
    class = c('value_error', 'error', 'condition')
    structure(list(message = message, call = call), class = class)
}


open = function (filename, mode = 'rb') {
    tryCatch(
        base::file(filename, open = mode),
        error = function (.) stop(os_error(glue::glue('Cannot open file {filename}')))
    )
}

readline = function (file) {
    tryCatch(
        readLines(file, n = 1L),
        error = function (.) stop(value_error('Error reading from file'))
    )
}

int = function (str) {
    tryCatch(
        as.integer(str),
        warning = function (w) stop(value_error(conditionMessage(w)))
    )
}

i = tryCatch(
    {
        f = open('errors.r')
        on.exit(close(f))
        s = readline(f)
        int(s)
    },
    os_error = function (e) {
        # This code could be shortened with an appropriate S3 method.
        message(glue::glue('OS error: {conditionMessage(e)}'))
    },
    value_error = function (e) {
        message(glue::glue('Value error: {conditionMessage(e)}'))
    },
    error = function (e) {
        message(glue::glue('Unexpected error: {conditionMessage(e)}'))
        stop(e)
    }
)