moodler: Use data from Moodle DB

#' Test mdl_ objects
#'
#' These objects contain summary information about particular module instance.
#' @param x Object to be tested
#' @name is.mdl_

#' @export
#' @rdname is.mdl_
is.mdl_quiz = function(x)
  inherits(x, "mdl_quiz")

#' @export
#' @rdname is.mdl_
is.mdl_forum = function(x)
  inherits(x, "mdl_forum")

#' @export
#' @rdname is.mdl_
is.mdl_quiz_data = function(x)
  inherits(x, "mdl_quiz_data")

#' @export
#' @rdname is.mdl_
is.mdl_forum_data = function(x)
  inherits(x, "mdl_forum_data")

#' Get module data
#'
#' Fetch data for a specific module.
#' @param x An object of class \code{"mdl_"}, e.g. \code{"mdl_quiz"}
#' @param ... Further arguments passed on to methods
#' @export

get_module_data = function(x, ...)
  UseMethod("get_module_data")

#' Extract key
#'
#' Extract key (only when distractor data are available).
#' @param x An object of class \code{"mdl_quiz_data"}
#' @param question.type List of distractors for to get key for; if \code{NULL} (the default) all keys will be extracted
#' @param complete Should question name, text etc. be provided? Returns a \code{data.frame}
#' @export

quiz_key = function(x, question.type = NULL, complete = FALSE)
  UseMethod("quiz_key")

#' Extract item-level and distractor data
#'
#' Extract a matrix or data.frame of question marks
#'
#' Extract item marks either as binary (correct/incorrect) or categorical data (for multiplechoice questions). If the test contains a mixture of binary and categorical items (such as truefalse and multichoice), than \code{marks = "binary"} will collapse multiple choice items into correct/incorrect (1/0). When using \code{marks = "categorical"}, mutliplechoice options will be preserved but a key needs to be obtained (see \code{\link{quiz_key}} for details). Weighted percentages can also be extracted by \code{marks = "weighted"} - in this case, each item's contribution will be weighted by its nominal weight, set in the quiz edit page in Moodle. Finally, you can obtain nominal weights as they appear on the edit quiz page, using \code{marks = "nominal"}.
#' @param x An object of class \code{"mdl_quiz_data"}
#' @param marks Char, defaults to "categorical"
#' @param question.type List of distractors for to get key for; if \code{NULL} (the default) all keys will be extracted
#' @param fill What to use to fill missing values, defaults to \code{NA}
#' @param mat If \code{TRUE} (default), the result will be a \code{matrix} otherwise a \code{data.frame}
#' @importFrom dplyr %>% select left_join
#' @importFrom tidyr spread
#' @export

quiz_items = function(x, marks = "categorical", question.type = NULL,
                      fill = NA, mat = TRUE)
  UseMethod("quiz_items")

#' Extract whole-quiz scores
#'
#' Extract whole-quiz scores for each attempt.
#'
#' See \code{\link{quiz_items}} on how the item marks can be specified.
#' @param x An object of class \code{"mdl_quiz_data"}
#' @param marks How items should be graded; see \code{\link{quiz_items}} for details
#' @export

quiz_scores = function(x, marks = "binary")
  UseMethod("quiz_scores")

#' Create a nodelist
#'
#' Create a nodelist.
#' @param x An object of class \code{"mdl_forum_data"}
#' @param ... Further arguments passed on to methods
#' @importFrom dplyr select group_by summarise %>%
#' @export

extract_nodes = function(x, ...)
  UseMethod("extract_nodes")

#' Create an edgelist
#'
#' Create an edgelist.
#' @param x An object of class \code{"mdl_forum_data"}
#' @param ... Further arguments passed on to methods
#' @importFrom dplyr select count
#' @export

extract_edges = function(x, ...)
  UseMethod("extract_edges")