added store and retrieve functions

NAMESPACE

@@ -23,7 +23,9 @@ export(match_n)
 export(match_s)
 export(one_or_more)
 export(reporter)
+export(retrieve)
 export(satisfy)
+export(store)
 export(succeed)
 export(zero_or_more)
 export(zero_or_one)

R/store.R

@@ -0,0 +1,44 @@
+#' The `.parcr` environment is used for storing and retrieving variables by the
+#' user.
+#' @returns \value{None}
+#' @noRd
+.parcr <- list2env(list(), parent = emptyenv())
+
+#' Store and retrieve objects
+#'
+#' Sometimes you want to use a parsed object to modify a later parser operation,
+#' as in the example below. The `store()` and `retrieve()` functions provide the
+#' tools to create such a parser.
+#'
+#' @param name a string used as the name of the stored object.
+#' @param value object to be stored.
+#'
+#' @returns \value{None} for `store()` and the stored object for `retrieve()`.
+#' @export
+#'
+#' @examples
+#' parse_nr <- function(line) {
+#'   m <- stringr::str_extract(line, "number=(\\d+)", group=1)
+#'   if (is.na(m)) list()
+#'   else store("nr", as.numeric(m))
+#' }
+#'
+#' p <- function() {
+#'   match_s(parse_nr) %then%
+#'     exactly(retrieve("nr"), literal("A"))
+#' }
+#'
+#' p()(c("number=3", "A", "A", "A")) # success
+#' p()(c("number=2", "A", "A", "A")) # failure
+store <- function(name, value) {
+  .parcr[[name]] <- value
+}
+
+#' Retrieve an object
+#'
+#' @export
+#' @rdname store
+#'
+retrieve <- function(name) {
+  .parcr[[name]]
+}

tests/testthat/test-store.R

@@ -0,0 +1,17 @@
+test_that("storage and retrieval basics work", {
+  expect_no_error(store("a", 1))
+  expect_equal(retrieve("a"), 1)
+})
+
+test_that("storage and retrieval in a parser works", {
+  parse_nr <- function(line) {
+    m <- stringr::str_extract(line, "NUMBER=(\\d+)", group=1)
+    if (is.na(m)) list()
+    else {
+      N <- as.numeric(m)
+      store("N",N)
+    }
+  }
+  p <- function() {match_s(parse_nr) %then% exactly(retrieve("N"), literal("A"))}
+  expect_equal((p()(c("NUMBER=3", "A", "A", "A")))[["R"]], character(0))
+})