R/subsetting.r

In ElianHugh/enumr: Static Enumerations

#' Subsetting enums
#'
#' @description
#'
#' Access enum members through subsetting with numeric or character indices.
#'
#' * Enums are converted to a list type through the
#' [`as.list`][base::as.list()] method, before being subset by an index.
#' * Partial matching via `[`, `[[`, or `$` is not supported, and will return
#' NULL. This is intended to incentivise literate programming, as partial
#' matching can lead to errors.
#' * Enums are not intended to be used as interactive data structures.
#' Assigning to enum members is not supported, and will
#' throw an error if attempted. If you require an enum to be modifiable,
#' consider using a [`list`][base::list()] instead.
#'
#' @param x enum to subset
#' @param arg value to subset enum with, generally the indices of the enum
#' @param value assigning values to enums is not allowed
#' @name subsetting
#' @examples
#' a <- enum(a,b,c)
#'
#' a[1]
#'
#' a[[1]]
#'
#' a$a
NULL

#' @export
#' @rdname subsetting
`[.enum` <- function(x, arg) {
    ret <- as.list.enum(x)[arg]
    if (is.null(ret[[1]])) {
        error_undefined_member(arg)
    }
    ret
}

#' @export
#' @rdname subsetting
`[[.enum` <- function(x, arg) {
    as.list.enum(x)[[arg]]
}

#' @export
#' @rdname subsetting
`$.enum` <- function(x, arg) {
    ret <- as.list.enum(x)[[arg]]
    if (is.null(ret)) {
        error_undefined_member(arg)
    }
    ret
}

#' @export
#' @rdname subsetting
`$<-.enum` <- function(x, arg, value) {
    error_illegal_assignment()
}

#' @export
#' @rdname subsetting
`[<-.enum` <- `$<-.enum`

#' @export
#' @rdname subsetting
`[[<-.enum` <- `$<-.enum`