#' @title Defining protein-protein interactions (PPI) over a list of genes,

#' @description This function uses STRING-api. The outcome of STRING analysis

#'   will be stored in comma-separated values files.

#' @export

#' @param data A gene list.

#' @param FileName A string vector showing the name to be used to save the

#'   resulted table. If null, no file will be exported

#' @param species The taxonomy name/id. Default is "9606" for Homo sapiens.

#' @importFrom httr content

#' @importFrom utils read.table write.table

#' @return Either CSV files stored in the user's file system and its

#' corresponding `data.frame` object in R or and R object containing that

#' information.

PPI <- function(data, FileName = NULL, species = "9606") {

  repos <- retrieveURL(data, species, "tsv")

  # Process API request content

  repo_content <- content(repos)

  results <- data.frame(repo_content[, , 1])

  if (!is.null(FileName)) {

    write.csv(results, file = paste0("PPI-", FileName, ".csv"))

  }

  return(results)

}