ensurer: Ensure values at runtime

#' ensurer - ensure values at runtime
#'
#' ensurer is a small utility package for R which provides a simple
#' and light-weight mechanism for ensuring certain aspects of values at runtime.
#'
#' R does not provide any mechanism for type-safety and since it is not
#' a compiled language, the risk of having unexpected results is there at
#' runtime. R functions often accept different types for the same input and/or
#' have different return types for different sitations.
#'
#' As an example, a query to a database or the scraping of a website might
#' not return valid data, where "validity" can refer to a number of conditions.
#' It might a positive or certain number of records; that all cases are complete;
#' that some column is weekly increasing; or simply that the result is a
#' \code{data.frame}.
#'
#' If one does not deal with these ambiguities and risks appropriately,
#' some resulting errors may be hard to track down. It is desirable to
#' get an error as soon as a value does not have the correct type or
#' does not satisfy certain criteria.
#'
#' "Ensuring values" is here meant as a "contract", or a set of conditions,
#' such that if a value does not comply an error is raised instantly.
#' An ensuring contract (a function) is created with \code{\link{ensures_that}}
#' (ideal for multiple use or readability with complex contracts).
#'
#' It is also possible to ensure properties on the fly using
#' \code{\link{ensure_that}} (ideal for simple, one-time contracts).
#'
#' Using the \code{magrittr} pipe \code{\%>\%} greatly improves semantics of the
#' functionality provided by this package, but it is not necessary.
#'
#' This package is not meant as a substitute for unit testing, and great
#' packages for this already exist, e.g. \code{testthat} by Hadley Wickham.
#' The ensurer package is meant as a simple and ideal tool for scripts or
#' programs where runtime conditions may break the functionality, and
#' where errors should be raised as soon and clear as possible.
#'
#' For a more thorough introduction, see \code{vignette(ensurer)}.
#'
#' @docType package
#' @name ensurer
#' @author Stefan Holst Milton Bache <stefan@@stefanbache.dk>
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